Electromagnetic Radiation and Radio Waves
(Natural and Man-Made Miracles)
Electromagnetic radiation is a wonderful thing. It brings us heat and
lights up our day, it brings us radio and television and carries our
telephone conversations. It brings us the Sun's energy which is needed by
all plants for photosynthesis and growth. It brings warmth to the
inhabitants of the Earth's animal kingdom and to some of them to tan their
bodies. We also use it to see through solid bodies, to find our way around
the planet and to cook our food.
In a tremendous intellectual leap, in 1873 James Clerk
the existence of electromagnetic waves and worked out mathematically what
their properties might be before anybody had ever observed, or even thought
of, such a phenomenon. Since then, communications engineers have performed
miracles harnessing this radiation for a myriad of uses.
Electromagnetic radiation has the following interesting properties
- It can be found in nature or be man-made.
- It does not require a medium for propagation.
- It travels with the speed of light.
- It carries energy as it propagates. The higher the frequency, the
higher the energy associated with the wave.
- It can transfer its energy to the matter on which it impinges.
- Its transferred energy may be sufficient to break chemical bonds,
ionising the matter on which it impinges.
- Its propagation obeys the inverse square law.
- It can be used to carry information.
- It can be broadcast outwards to reach many locations or it can be
formed into beams to reach a particular spot.
- It can be be reflected or refracted.
- It can be split and recombined to form diffraction patterns.
- It can travel great distances. The radiation resulting from a
simple100 volt, 1 MHz sine wave fed into a suitable antenna can be
detected as far away as the next planet.
- It travels in straight lines.
- It can be bent around the Earth's circumference by reflection from the
- It can pass through walls.
- It can be captured by placing a metal rod, a loop, parabolic metal
dish or horn in its path and it can be launched into the atmosphere with
the same tools.
Radio waves are a specific example of electromagnetic radiation. Despite
all the communications benefits "electromagnetic radiation" makes possible,
the name has a sinister connotation. The alternative name, "radio waves",
does not seem nearly so threatening. But too much of a good thing, even
water, can be dangerous if present in excessive quantities at the wrong
place or time. So it is with electromagnetic radiation.
We are in fact swimming in an ocean of radio waves of various strengths.
home we have high frequency radiation coming from
- Hundreds of long wave, medium wave. short wave and UHF radio
- Dozens of terrestrial television signals
- Television signals at microwave frequencies beamed down by satellites
- UHF signals from hundreds of mobile phones and their local base
- VHF Private mobile radio signals used by the emergency services and
- Television remote controls
- Remote control toys (cars and planes)
- Microwave GPS satellite navigation location signals whether we use
them or not.
- Wi-Fi networks for communications between computers and computer
- Bluetooth connections between electronic appliances
- Laser light in CD players
- Infra red television remote controls
- Garage door openers
- Car door remote locking keys
- Infra red radiation from cookers and domestic heaters
- Leakage from microwave ovens
- Continuous unwanted radio frequency interference (RFI) generated by
computer circuit boards and oscillators in radio reception and
- Random RFI due to local electrostatic discharges from motor
commutators on household equipment and power tools as well as automobile
ignition systems (sparking plugs).
- Random RFI due to distant electrostatic discharges from lightning
strikes anywhere between the signal source and the
And at the other end of the spectrum we have
- Very low frequency radiation from power cables, electric motors,
domestic appliances, transformers and battery chargers.
Depending on where we live we may also be near enough to experience
signals from other sources even though we may not have the equipment to
- Air traffic control systems
- Aircraft instrument landing systems
- Radar surveillance
- Microwave repeater systems used for broadband communications links
- Speed cameras
- Very low frequency radiation from electric fields radiating from high
voltage electricity grid transmission lines, transformers and power cables.
Closer to home we submit ourselves to high levels of radiation from
- X Ray machines
- X Rays from CAT scanners
- Electromagnetic fields from MRI scanners
But curiously many hospitals ban the use of mobile phones because their
tiny transmitters might interfere with sensitive medical equipment.
Then we are all bathed in more general background sources of radiation
most of which we can not avoid and some we can.
- High frequency radiation from the Sun and other artificial light
sources at optical frequencies
- Infra red radiation (heat) from the Sun
- Man made heat and light sources
High energy, short wavelength electromagnetic radiation such as ultra-violet
rays, X rays and gamma rays can cause ionisation of other materials when
present at high enough energies and this can cause serious and permanent
damage to human tissue. This radiation may be found in nuclear installations
and may also be used in controlled medical treatments. Such radiation may be
found in the domestic environment but fortunately not at dangerous levels.
- Low level X rays from high voltage cathode ray tubes (CRT) formerly
used in colour televisions and monitors
- Ultra-violet lamps and tanning equipment
- Gamma rays not normally present in the home
The first man-made radio waves were created in 1888 by
three years after the world's first practical automobile was launched by
Karl Benz. Before
that, apart from light waves and the odd lightning discharge, there were
almost no radio waves in the atmosphere. The growth of radio waves in the
atmosphere in the last one and a half centuries has followed the growth of
industrial development, just like the concentration of carbon dioxide in the
atmosphere, but at least radio waves have not been blamed for global warming.
(Not yet anyway!)
Communications and Engineering Miracles
- With all these radio signals vying for our attention, amongst a
background of unwanted radiation sources, all whizzing by with a speed of
186,000 miles per second, thanks to communications engineers you can poke
your mobile phone or radio antenna into the air and select just the signal
that was intended just for you.
- The very limited bandwidth available within the electromagnetic
spectrum, which is suitable for radio communications, accommodates
millions of communications links with a collective bandwidth of many times
the available bandwidth by simultaneously using the same frequencies
without interfering with eachother. Another set of challenges answered by
- We might also expect that all the radio signals in the atmosphere
would be completely scrambled with each other. Fortunately by some natural
miracle the signals retain their integrity. They may be superimposed on
eachother or swamp eachother and they may pick up electrical noise during
their travels but they do not mix to form sum and difference frequencies
as they would in a non linear device and so no miraculous engineering
solution is needed to decode or operate upon the new frequency components
to reconstruct the original signal. They only need to be separated from
The Electromagnetic Wave
describe how electromagnetic radiation is propagated. He showed that a
varying magnetic field induces an associated varying electric field
perpendicular to the magnetic field and this varying electric field in turn
induces an associated varying magnetic field in the plane of the initial
magnetic field. Together these two varying fields form an electromagnetic
wave propagating at the speed of light in a direction perpendicular to both
the electric and magnetic fields as shown in the diagram below.
Source - G.R. Delpierre and B.T. Sewell,
University of Capetown (Modified)
Radiation Wavelength and Frequency
The frequency f (Hertz) of the wave is inversely
proportional to the wavelength λ (metres) and is given by the
f = c / λ
where c is the speed of light (m/sec).
Radiation and the Inverse Square Law
The rate at which energy emanating from a fixed, constant source of
electromagnetic radiation passes through a surface at a distance d
from the source is proportional to 1/d2. This is known as
the Inverse Square Law. It arises simply because the surface enclosing the
source is a sphere, centred on the source, through which all the energy must
pass and the surface area of this sphere increases as the square of the
distance d from the source. Thus the energy flow (measured in Watts
per square metre (W/m2)) falls off rapidly as the distance from the source
Radiation and Polarisation
The individual electric and magnetic fields in an electromagnetic wave
are orthogonal (at right angles) to eachother with the plane of oscillation
of the fields determined by the orientation of the radiating element such as
an antenna. By convention the polarisation refers to the plane of
oscillation of the electric field.. In the diagram above the polarisation is
vertical as represented by the direction of the electrical field E and is
said to be linear.
Electromagnetic waves may also be circularly polarised, in which case,
the tip of the electric field vector E, describes a helix along the
direction of propagation. Such waves may be generated from two crossed
dipoles fed with a 90° degree time-phase difference (phase quadrature) or by
a helical antenna radiating in the direction of its axis.
The Electromagnetic Wave Spectrum
Electromagnetic waves can typically be described by any of the following
three physical properties: the frequency f, wavelength λ,
or photon energy E. The diagram below shows all possible frequencies
of electromagnetic radiation and the corresponding photon energies and some
of the applications for which they are used. The spectrum covers an enormous
range with wavelengths ranging from the size of an atom to almost the size
of the universe, (Over 26 orders of magnitude). The corresponding photon
energies occupy a similar range, from the unmeasurable to the highly
Wave - Particle Duality
wave - particle duality theory showed that paradoxically,
electromagnetic radiation and particles of matter could exhibit both wave-like
and particle-like properties but not at the same time. In practice this
means that some properties of radiation can best be explained by wave theory
while others can better be explained by particle theory which describes
electromagnetic radiation as an energy flow carried by particles called
photons, each with a characteristic energy which depends on the
frequency of the radiation.
photon energy E of a single photon associated with the
electromagnetic wave increases with frequency and is given by the
E = h x f (Joules) or h x c /
where h is Planck's constant (6.63 X 10-34
Joule seconds or 4.14 X 10−15 eV seconds) and f is
the frequency of the wave and c is the velocity of light
(299.8 x 106 m/sec) .
- The spectrum above shows that the individual photons in visible light
have energies of a few electron Volts while the particles in cosmic rays
with an equivalent frequency of around 1025 Hertz have
relatively enormous energies of over 10 billion electron Volts (1.6
nanoJoules). Though a nanoJoule is very small, the total energy flow
associated with the radiation is many, many times greater due to the very
high number of photons making up the overall photon flux (See below).
- Below a frequency of around 100 GHz, which includes most of the
spectrum used for radio communications, the energy of individual photons
is almost negligible at less than 10−4 eV or 10−24
The photon flux Φ of a radiated wave, defined as the number n
of photons per second per unit area of the wave is given by
Φ = n/m2/sec
The energy E associated with the photons is given by
E = n x h x f(Joules)
The radiation intensity P or power density (radiated
power per unit area) associated the photon flux is given by
P = Φ x E = n x h x f / sec
/ m2 (Joules / sec / m2 or Watts / m2)
The number of photons n in E Joules of energy
at any frequency or wavelength is given by
n = E x h / f = E x h x λ /
The number of photons per Joule (setting E = 1Joule)
for light is given by
n = h x λ / c
Note that a the radiation intensity depends on BOTH the photon flux AND
the frequency of the radiation.
- Common Light Sources
- For visible green light with a wavelength λ = 500 nm (500 x
The photon energy E = h x c / λ = (6.63 X 10-34)
x (299.8 x 106) / (5 x 10-7) = 3.98 x 10-19
Joules or 2.48 eV
The number of photons per Joule of radiated energy is = 1 / ( 3.98 x
10-19) = 2.513 x 1018 (a very large number!)
- Making some gross assumptions we can calculate the rate at which
photons are emitted by a 100 Watt incandescent light bulb.
- The rate energy is supplied to the lamp = 100 Watts = 100 Joules
- But only about 2.25% of this energy is converted to visible light.
Energy Efficiency) Thus the lamp emits 2.25 Joules of radiant
light energy per second.
- The lamp actually emits a wide spectrum of radiation, most of
which is infra red radiation but we are only considering the visible
energy here which amounts to about 10% of the total radiated energy.
The visible energy is emitted over the spectrum from red to violet (wavelength
7.5 X 10-7m to 3.5 X 10-7m) with varying
intensity, but for the purposes of this calculation we can assume that
the average wavelength of the radiation is 5.0 X 10-7m
which is the wavelength of green light near the middle of the visible
spectrum. See the graph of
Radiation which has a similar spectrum and also the
Wave Spectrum above.
- From the above, the rate at which visible light photons are
radiated from a 100 Watt incandescent light is 2.25 x 2.517 x 1018
= 5.66 x 1018 photons per second.
- The total number of photons emitted per second over the full
radiation spectrum of the light source (heat and light) will depend on
the temperature of the source and will be about 10 times the number of
photons contained in the visible light.
- Cosmic Radiation
Cosmic radiation is not strictly electromagnetic radiation. Cosmic rays
are in fact streams of high energy particles originating from outside the
earth's atmosphere. They are not homogenious and may have different
constituent particles. Typically they consist mainly of protons, (positively
charged Hydrogen nuclei) which make up around 89% to 90% of the stream,
alpha particles (Helium nuclei) which make up around 9% of the stream, the
nuclei of other heavier elements which account for about 1% of the
particles and beta particles (electrons) make up the remaining 1%.
Similarly the cosmic ray particles may have different energy levels with
particles originating from the sun, the so called "solar wind" having
relatively low energy levels of around 106 eV, while particles
emanating from outside the solar system typically have energy levels
ranging from about 108 to1012 eV, though energy
levels of up to 1021 eV have been recorded. This is many orders
of magnitude greater than the 1013 eV which the best
terrestrial particle accelerator, CERN's
Large Hadron Collider (LHC) can produce.
Before the invention of particle accelerators such as the cyclotron and
the synchrotron, nuclear physics experimentors often used cosmic rays as
the source of high energy particles in their experiments.
See also Cosmic
Rays - History
Being composed of sub-atomic particles, cosmic rays do not propagate
with the speed of light, but at some speed
close to it. Their particle energy levels are close to the photon
energies in the higher frequency electromagnetic waves and simply for
convenience, cosmic rays are often included in graphical representations
of the electromagnetic spectrum with an equivalent wavelength or frequency
for their energy levels (as in the
Similar to nuclear radiation, the high energy cosmic ray particles can
cause ionisation of materials on which they impinge and as such can have
dangerous physiological effects. See
Physiological Effects of
Electromagnetic Radiation (below) and
Physiological Effects of Nuclear Radiation.
Fortunately the earth's magnetic field deflects much of the cosmic
radiation away from the earth and some of what get's through is absorbed
by the earth's atmosphere. Nevertheless, cosmic radiation accounts for
about 13% of all background radiation at the earths surface. The
at the earth's surface attributable to cosmic radiation amounts to about
3.6 milliSieverts (mSv) whereas the dosage from all sources of background
radiation (including the nuclear decay of the earth's elements) is around
3.0 mSv in the USA and 2.0 mSv in the UK. The cosmic energy dosage however
increases with altitude which can be a health hazard for airline crews and
frequent fliers and is positively dangerous for astronauts. It is estmated
that cosmic rays contribute to 100,000 cancer deaths per year.
- For cosmic radiation with an equivalent wavelength of 10-16
The photon energy is (6.63 X 10-34) x (299.8 x 106)
/ (10-16) = 1.99 x 10-9 Joules or 1.24 x 109
The number of photons per Joule is = 1 / (1.24 x 109) =
5.03 x 1010
Note that as a consequence of the shorter wavelength, each photon of
cosmic radiation contains 5 x 109 times as much energy as the
green light photons and can consequently be much more damaging. (See
By the same token, green light radiation needs correspondingly 5 x 109
more photons to make up one Joule of radiated energy than cosmic
radiation because of the lower energy level of the photons emitted by
Ionisation Effects of Electromagnetic Radiation
Ionisation is the breaking of chemical bonds holding matter together,
releasing ions or electrons from the molecules or atoms, leaving two charged
particles or ions: molecules with a net positive charge, and the free
electrons with a negative charge. This can occur naturally by
when salts are dissolved in aqueous solutions causing their constituent
elements to separate into ions.
In the case of electromagnetic radiation ionisation occurs in a more
forcible manner when matter is bombarded with high energy photons. If the
photon energy is high enough it can knock electrons out of molecules or
atoms leaving positively charged ions and negatively charged electrons.
radiation spectrum diagram above shows how the photon energy increases
with frequency and that at frequencies above the visible light spectrum, the
photon energy of the radiation is sufficient to cause ionisation of the
matter on which it impinges. Below the frequency of visible light, and this
includes the emissions from microwave ovens and all the frequencies used for
radio communications, the radiation is non-ionising since the photon energy
of the radiation is so small that ionisation is not normally possible unless
the intensity is exceptionally high.
Long distance radio communications depend on ionisation of the upper
layers of the earth's atmosphere by cosmic rays. The resulting free ions
form a conductive blanket, known as the
reflects radio waves enabling radio signals to reach beyond the horizon by
bending around the curvature of the earth.
Physiological Effects of Electromagnetic Radiation
Ionising radiation is particularly hazardous to living organisms because
its effects are painless, cumulative and latent : you can't sense that
radiation damage is happening and symptoms may take up to several weeks to
At frequencies above the upper end of the visible light spectrum,
starting with ultra violet (UV) radiation, the photon energy becomes
sufficient to cause ionisation damage to human body tissue. Overexposure can
cause burns due to the heating effect of the radiation but prolonged
exposure can result in chemical changes to the skin tissue. Ionisation can
cause DNA mutation leading to tissue damage and the possible formation of
cancerous tumours. At progressively higher frequencies, such as X-rays and
above, the greater photon energy of the radiation not only causes increased
damage but it penetrates deeper into the body with even more serious
Higher energy (gamma) radiation is still more dangerous. Its properies
together with those of other ionising radiation are outlined in the section
Conducting Gas Plasmas
The physiological effect on the body of non-ionising radiation, (frequencies
below the visible light spectrum) is the heating of the exposed tissue,
often referred to as its "thermal" effect. For short exposures this is not
dangerous but damage can be caused by prolonged exposure to high levels of
It is important to distinguish between electromagnetic radiation and
- Electromagnetic radiation is the propagation of energy by means
of electromagnetic waves (interlinked, varying electric and magnetic
fields) such as heat, light, radio waves, X rays and gamma rays, all
travelling with the speed of light. It is relatively harmless below the
frequency of X rays, but at X ray frequencies and above, the
electomagnetic wave carries sufficient energy to cause ionisation of the
materials on which it impinges and hence can be hazardous to humans and
- Nuclear radiation is the flow of diiscrete, high energy sub-atomic
matter particles, not waves, resulting from the natural decay of nuclear
materials or from nuclear reactions such as fission and fusion. The
velocity of the particles may approach, but can never reach, the speed of
light. The ever present background radiation on earth is due to the decay
of earthly nuclear materials found in the earth's crust but also due to
debris from the extra-terrestrial fission and fusion reactions taking
place on the sun and the stars in the cosmos which result in the constant
bombarding of the earth by cosmic rays. Fortunately the level of
background radiation is so low that the human race is able to live with
it. Evolution has not however equipped us to live with high levels of
nuclear radiation which could possibly occur from man made nuclear
reactions here on earth. Every attempt is made to contain the radiation
produced in controlled nuclear reactions employed in the electrical power
industry, but very rarely things may go wrong. On the other hand, nuclear
weapons depend on unfettered, runaway nuclear reactions which spread
nuclear radiation indiscriminately.
The Eye - A Biological Miracle
The eye is essentially a very sensitive radio receiver and
- It has a wide band tuner, the retina, with a bandwidth of 390
THz (TeraHertz = 1012Hz) which can detect electromagnetic
radiation in the frequency range from 400 to 790 THz, (200,000 times
higher than microwaves).
In more detail:
- It has an automatic gain control system, the iris, which
protects against signal overload.
- It has a broadband, narrow beamwidth, directional, variable focus
antenna, the lens, which captures the radiation.
- It has an automatic focusing system, accommodation by cilary
muscles, which optimises the reception for different distances, from
close-ups to infinity, by controlling the shape of the lens.
- It has a rangefinder function, as well as 3D vision, provided by the
eyes taken in pairs, parallax between the images.
- It has an image scanning system, the rods and cones, with a
resolution of 150,000 pixels/ sq. mm. which enables the relative spatial
position of the sources to be identified.
- It has signal amplitude sensors, the rods, which measure
pixel luminance (brightness) with a dynamic range of more than 10
million to 1
- It can detect amazingly low photon fluxes of 5 to 9 photons per
Photon Energy above)
- It has signal frequency sensors, the cones, which identify
the pixel chrominance (colour) with a frequency range of 390 TeraHertz..
- It has a spectrum analyser display mechanism, colour. The
received radiation itself has no colour. Colour is the way the eye
perceives and represents the frequency of the radiation.
- It has a self cleaning and protection mechanism, the eyelid.
- It has an expected lifetime of 70 years or more.
There is no electronic equipment which comes anywhere near to this level
We could also consider that some people think it's a biological miracle
that we don't all die from exposure to all the electromagnetic radiation in
Radio Frequency Safety Limits
Specific Absorption Rate (SAR)
The magnitude of the effect of radio frequency radiation on the body
depends on the intensity and duration of the radiation. The specific
absorption rate (SAR) is commonly used to measure the power absorbed by the
body from microwave ovens, mobile phones and
It is a measure of the potential thermal effects on the patient's tissue due
to exposure of the body to electromagnetic radiation and is defined as the
power absorbed per mass of tissue in Watts per kilogram. It is not the power
emitted by the source. The actual energy absorbed by the body depends on its
distance from the source as well as the shapes of the source and the body
and their relative exposure and orientation towards each other
The tolerance of the body to radio frequency radiation depends on which
part is involved, vital organs being much more susceptible to damage than
the body's extremities. The SAR may be averaged either over the whole body,
or over a small sample volume weighing a few grams.
- For mobile phones, for which absorption of RF energy by the
body is an unwanted consequence, the safe SAR limit is specified by the
FCC in the USA as 1.6 W/kg (averaged over 1 gram of tissue) whereas in
Europe the IEC specifies 2.0 W/kg (averaged over 10 grams)
- For MRI scans, whose function depends on the absorption of
electromagnetic energy by the body, the US, FDA limits are:
- 4 W/kg averaged over the whole body for any 15-minute period
- 3 W/kg averaged over the head for any 10-minute period; or
- 8 W/kg in any gram of tissue in the extremities for any period of 5
For reference an SAR of 2 W/kg would take 2 days to melt a kilogram of
ice. (Since the latent heat of fusion of water is 334 kJ/Kg, it will require
334,000 Watt seconds of energy to melt. With a 2 Watt source it will take
Nuclear Radiation Effects and Safety Limits
A Word About Microwave Ovens
operate in the same 2.4 GHz frequency band as Wi-Fi, Bluetooth and ZigBee
wireless communications systems but at a much higher power. (See
power level comparisons
below). Since the frequencies and the associated quantum energies used by
all of these applications, including microwave ovens, are a million times
lower than those of x-rays (see the
above), they cannot produce the damaging ionisation associated with high
frequency electromagnetic radiation.
The microwave energy used in the oven does not actually transform or
oxidise the organic compounds which make up the ingredients in the food as
in normal cooking. It merely excites dipole molecules, mainly water and fats,
contained in the food increasing their kinetic energy. Dipole molecules are
those with a positive charge at one end and a negative charge at the other.
The alternating electric field of the microwaves causes the molecules to
rotate with each cycle as they try to align themselves with the field. As
the oscillating molecules become involved in collisions with other molecules,
putting them also into motion, the agitation causes the molecules to heat up.
This heat is passed on by conduction to everything in contact with the
dipole molecules so that the heat spreads through the food finally heating
up the container or plate holding the food.
At 2,450 MHz, the frequency of the microwave radiation is in the non
ionising region of the electromagnetic spectrum and hence the radiation does
not have the energy to cause tissue damage by ionising and breaking down the
molecules or atoms in the food. Though human tissue also contains dipole
molecules, a short exposure to the microwave radiation produced by a
microwave oven is likely to be much less damaging than momentarily putting
your hand on a hot stove. To make doubly sure of safety, microwave ovens
have safety interlocks which switch off the magnetron completely if the oven
door is open and in addition they incorporate shielding to ensure that the
maximum leakage of radiation from the oven when the door is closed is
limited to agreed national standards. The United States FDA requirement
states that new ovens may not leak microwave radiation in excess of 1 mW/cm2
at 5 cm (2 inches) from the oven surface and that, once placed into service,
the maximum permissible microwave radiation is 5 mW/cm2 at 5 cm
from the oven surface.
Some Facts to Put the Power Levels of Received RF Radiation into
- The magnetrons used in the microwave oven produce between 600 and 1000
Watts of microwave power a frequency of 2,450 MHz but the energy is
confined in a shielded compartment.
- Inside a typical 800 Watt microwave oven with a food plate diameter of
27 cm (10.5 inches), assuming all the magnetron output power is
concentrated on the plate, the radiated power density on the plate will be
1,400 mW/cm2, or about 14 times the solar radiation at the
Earth's surface. The
radiated power from the Sun is 100 mW/cm2 (normally quoted
as 1.0 kW/m2) at the surface of the Earth.
- The U.S. FDA safety limit for radiation leaked from a microwave oven
is 5 mW/cm2 maximum at 5 cm (2 inches) from the surface of the
oven. This is just one twentieth of the radiation from sunlight.
- Because the frequency of the radiation used in microwave ovens is less
than one thousandth of the frequencies of solar radiation in the visible
light spectrum, the potential damage from microwave radiation is less than
one thousandth of the damage which could be caused by the more ionising
radiation from the Sun. See the
Radiation Spectrum above.
- The user's exposure to microwave energy leaked from a microwave oven
follows the inverse square law, as is the case with all omnidirectional
radiation, falling off rapidly as the distance of the user from the source
(the oven) increases. On the other hand the radiation from direct sunlight
will be the same no matter where the user stands because the distance to
the Sun will not change appreciably.
- The acceptable radiation leakage level from microwave ovens is much
lower than the radiation exposure from mobile phones.
- With mobile and cordless phones the antenna transmitting the radio
frequency power will be very close to the user's brain, causing the
maximum potential hazard for the level of radiated power involved. These
devices however generally have omnidirectional antennas so that less than
half the radiated power will be directed towards the user and some phones
may also have shielding to reduce the radiation towards the user's head
- Sunlight can be much more dangerous than leakage from microwave ovens
or radiated power from mobile phones. Sun burn, sun stroke and skin cancer
are well known and common consequences of over-exposure to sunlight.
Similar damage from the use, or misuse, of mobile phones and microwave
ovens is almost unknown.
- The cross sectional area of a human head is about 300 cm2 (
0.03 m2). The amount of solar radiation impinging on the top of
an unprotected human head at noon will be 1.0 kW/m2 x 0.03 m2
= 30 Watts or 15 times the total radiation emitted by a
typical mobile phone.
- Furthermore, staring directly at the mid day Sun for 30 minutes
without sunglasses will do immediate and serious damage to your eyes, much
worse than any damage likely to result from talking for 30 minutes on a
mobile phone while sitting next to a microwave oven cooking your lunch at
Characteristics of waves
Wavelength (λ): the distance for one complete
vibration. Once you go past one wavelength the pattern starts to
Amplitude (A): the height of the wave above (or below)
the rest position. It is related to the energy of the wave. For
example, a louder sound will have a greater amplitude. For a
transverse wave it will be the height of the wave from its rest
Crest: high point of a wave
Trough: low point of a wave
Propagation: the traveling movement of the wave
Phase: The position and direction of a point of a
Since many waves are sine waves, they can be described using the
degrees of a circle.
Using A (below) as a starting point:
F is 360˚ away from A: it is “in phase”
C is 180˚ away from A: it is “out of phase”
B is 90˚ out of phase from A
|1. Which pairs of points are in phase with each other?
2. Which pairs of points are 180° out of phase of each other?
3. Which points are 90 ° out of phase of E?
4. Which points are 180 ° out of phase of C?
1: BF, DH, CG ; 2: BD, DF,
FH, CE, EG ; 3: D & F ; 4:B & D
“how many waves per second”
cycles/second or vibrations per second (Hz)
Hz = “waves per second” (1/sec)
For sound frequency is the pitch
how long it takes for one wave
f and Hz are inverses of each other.
What does that mean!?
Frequency and period are said to be “inverses of each
You’ll often get it delivered this way: “Frequency and
Period are inverses of each other”. And that would be that. No
explanation. No clarification. Not even an indication of why it is
important that they are inverses of each other.
This means that as one goes up, the other goes down.
Another way of thinking of it would be that as the frequency goes down
it takes longer for each wave (less waves per second means more time for
If you look at the units for frequency (Hz means “cycles
per second”) it also helps to clear it up a bit:
If you flip cycles per second (f) it becomes seconds for each
cycle (the period).
Probably the best way to keep these relationships clear
is to always pronounce Hz as “cycles per second” and period as “how long
it takes for one wave.”
|1. Which wave has the largest λ?
2. Which wave has the highest f?
3. Which wave has the highest amplitude?
4. Which waves have the same λ?
1:B ; 2:D ; 3:A ; 4:A&C
|Since waves are usually shown as sine waves, the phase of a wave is
often described in terms of degrees of a circle.
- If you start the cycle at A, one complete cycle will be 360°. Half
a cycle will be 180° and a quarter will be 90°.
- If two points are 360° away from each other (A & F; B & G; E & H))
they are in the same part of the cycle and are said to be “in phase”.
- Any other point will be “out of phase”.
- A and C are 180° out of phase (opposite phases)
- A and B are 90° out of phase.
The Wave Equation
The “wave equation” gives us the
relationship between speed, frequency and wavelength:
v = fλ
(Which I think is one of the prettier equations!) If the
velocity of a wave is to stay constant, then the frequency must go up as
the wavelength goes down- and vice versa.
For example: Light in a vacuum must always be the same
c = 3.00 x 108m/s
If you increase the frequency of the light, say from red
light to blue light, then the wavelength must go down.
Since f is also equal to 1/T
then the equation can be written as
v = λ/T
(Not as pretty)
|1. What is the frequency of a wave if 4.0 waves pass a fixed
point in 10 seconds?
2. What is its period?
3. What will happen to the period of a wave if the frequency is
4. What is the frequency of a wave if its period is 0.25 second?
1: 4/10 or .4Hz ; 2: 1/.4
or 2.5sec ; 3: halved ; 4: 1/.25 or 4Hz
Material through which a wave passes.
Mechanical waves such as sound and water waves need a
Electromagnetic waves (light) do not need a medium although they CAN
travel through media.
Ex.: Light can travel through glass and water.
Resonance: sympathetic vibration
The vibration of a body at its natural frequency caused by a vibrating
source at the same frequency.
Another interpretation of this could be to give a series of well-timed
pushes to get something going. You will be pushing at the natural
frequency of the object.
Examples of resonance:
Pushing someone on a swing.
That annoying guy next to you at the red light with the speakers so
loud you can feel it with your windows. His rattling fenders are
resonating from the music.
Rocking a stuck car to get it out of the snow.
Something vibrating in your car only when you are at a certain speed.
Water molecules when microwaves them.
Swirling a cup of liquid to get it to spin.
The classic opera-star-hits-the-high-note-and-shatters-the-wine-glass
effect. That is, of course, if she can somehow find the exact unique
natural frequency of that specific glass.
The effect of two or more waves passing simultaneously
through the same region of a medium
Superposition is where two waves are in the same place
at the same time.
Two waves in superposition will interfere with each other.
Constructive- two waves that are "in
phase" at the same place and same time will add energy to each other
making the wave stronger. (Adding +2 and +2)
Destructive- two waves that are "out of
phase" at the same place and same time will take away energy from each
other making the wave weaker. (Adding +2 and -2)
Bose ™ noise cancelling headphones. The headphones have
a microphone on the outside to pick up outside noise- specifically the
steady drone of machines and aircraft engines for example. The sound
waves are then reproduced but in the opposite phase to cancel out the
Similar to the Bose™ noise cancelling headphones but
much cooler! Microphones mounted on the outside of a helicopter pick up
the engine and rotor noise. Speakers on the outside of the craft produce
the exact sound but, again, 180 º out of phase to cancel out the noise
making it almost silent.
(This was used on the SEAL Team Six raid into Osama Bin
Laden’s stronghold on May 2, 2011.)
All EM waves move at the speed of light but each type
of wave is a different size. We call it a
is the distance from one crest to the crest of the next wave. ( it works
with troughs too)
The WAVELENGTH determines the type
number of waves that pass by a fixed point
One wave per second is called 1
1,000 waves per second is 1
1,000,000 waves per second is 1
1,000,000,000 waves per second is
1 GigaHertz (cell phones)
Explore electromagnetic (EM) waves, their features and how they differ
from other waves. Learn how EM waves are organized on a spectrum based on
the amount of energy they produce, from radio waves to gamma rays.
Heinrich Hertz and Electromagnetic Waves
Who is Heinrich Hertz? If you guessed that he was the founder of the popular
American car rental company with a similar name, you're not alone. But
Heinrich Hertz wasn't a car rental entrepreneur. Instead, he was a German
scientist who performed experiments with electricity when electricity was
still a fancy new thing that scientists had a lot to learn about.
In 1888, when Hertz was 30, he made an electric spark jump from one terminal
to another and noticed a second spark at the same time between two terminals
a couple of yards away. Exciting stuff, I know, but this was 1888, and what
Hertz noticed was a different kind of electromagnetic wave that eventually
came to be known as Hertzian waves.
A few years later, in 1896, a young Italian scientist named Guglielmo
Marconi built on Hertz's discovery and created the first radio transmitter,
sending radio signals for a mile. (A mile!) Hertzian waves are now called
radio waves and are used every day, from listening to the radio to watching
What Are Electromagnetic Waves?
We are surrounded by waves we can see and hear, from ocean waves to sound
waves. A wave shows the transfer of energy, from the wind that starts an
ocean wave to the sound that moves through the air to your ear drum. Waves
that pass through a physical object or medium are called mechanical waves.
Unlike mechanical waves, electromagnetic waves do not need a medium to
travel or propagate. Electric and magnetic fields both produce vibrations
and, together, the two types of energy create electromagnetic waves.
Waves take different shapes, but electromagnetic waves all have a snake-like
shape, which makes them transverse waves. Transverse waves are measured by
their height, or amplitude, and by their wavelength, or the distance between
the highest point of one wave, the crest, to the crest of the next wave. The
lowest point of a wave is called a trough. Trough to trough can be measured,
too. When analyzing an electromagnetic wave, both the amplitude and distance
between waves is measured.
We measure both the amplitude, or height of a wave (a), and the distance
between waves (b). Diagram of a wavelength
One whole wave, from crest to crest, or trough to trough, is called a cycle.
The number of cycles that occur per second is the wave's frequency. In honor
of Heinrich Hertz, we measure frequency in hertz or Hz.
Types of Electromagnetic Waves
Electromagnetic waves are ordered on the electromagnetic spectrum, by
frequency. They range from radio waves with fewer cycles per second to the
extraordinarily fast and harmful high frequency of gamma rays.
Radio waves have the lowest frequency of the seven bands of waves on the
electromagnetic spectrum, which also means they have the least amount of
energy. Radio waves have wavelengths measuring from miles to the length of a
football, or around 11 inches.
It is common to talk about the frequency of radio waves, or the number of
waves per second. When tuning in to a radio station, a person is listening
to a specific frequency of radio waves. AM stations are numbered from 520 to
1610, with each number representing the frequency of the station at
thousands of hertz per second, or kilohertz, abbreviated kHz. FM station
frequencies range from 87.0 to 107.9 million hertz per second, called
megahertz or MHz.
Sound is converted into EM waves and sent through radio dishes like this one.
Your radio then receives these radio waves and changes them back into sound
waves. Image of a radio satellite
Next on the spectrum are microwaves, a type of radio wave that are less than
11.8 inches long. The microwaves people use to heat food have waves
measuring about five inches. Microwaves aren't just for heating leftovers or
cups of coffee, though. Microwaves are also used for radar, television and
Microwaves occur at higher frequencies, with billions or even trillions of
cycles occurring per second. Since writing out 4,000,000 hertz is kind of
clunky, it would be written as 4 gigahertz or 4 GHz. Digital radio is
broadcast at a frequency of 2.5 billion hertz per second, or 2.5 GHz.
Infrared waves occur at an even higher frequency than microwaves. Infrared
waves are used to power television remote controls and for thermal imaging,
like when using a pair of night vision goggles. When you feel warmed by the
sunlight, the energy you feel is infrared radiation from the sun. Since
infrared waves have such high frequencies, their wavelengths are so tiny
they are only hundredths or thousandths of an inch.
All electromagnetic waves are light, but the band of the electromagnetic
spectrum that people and animals can see is called visible light. When a
beam of light passes through a prism, a person can see each color of the
rainbow separated into their individual wavelengths. Red, the longest of the
wavelengths, measures around 700 nanometers; yellow is around 600 nanometers;
and violet, the shortest, is around 400 nanometers in length.
This diagram breaks down the electromagnetic spectrum by frequency and size
of wavelengths. Notice the rainbow-colored section of visible light. Diagram
of the electromagnetic spectrum
Electromagnetic radiation -- electromagnetic spectrum
We are bombarded by rays of energy all the time. This is electromagnetic
radiation. Your eyes can detect some of these rays, but most of the
radiation is invisible. Although some are harmful, all of the rays can be
useful to us. They are waves of energy that can travel through space and
matter. Electromagnetic radiation comes from the Sun, stars and galaxies,
traveling through space to reach us. It can also be made artificially. It
consists of electromagnetic waves with a wide range of frequencies and
wavelengths.In order of increasing frequency (or decreasing wavelength),
some of these are: radio waves, microwaves, infrared rays, light rays,
ultraviolet rays, X-rays, and gamma rays. All electromagnetic radiation
travels at the speed of light, and the waves or rays can penetrate materials.
The complete range of frequencies of electromagnetic radiation is the
Wavelength, amplitude, frequency
The figure below depicts the most important characteristics of an
electromagnetic wave: the wave crest and the wave
trough. Both undulation conditions, called phases,
are repeated periodically. The smallest distance between two points of the
same phase, e.g. between two wave troughs, is called the wavelength.
The peak of an electromagnetic wave is referred to as its amplitude.
of electromagnetic waves.
The frequency of wave troughs and wave crests per time unit is called
frequency. The shorter a wavelength, the higher the
frequency as well as the energy of an electromagnetic wave.
Electromagnetic waves with high and low frequencies.
The electromagnetic spectrum
In nature, there are more kinds of electromagnetic waves such as radio
waves and microwaves or the so-called gamma radiation, roentgen (or X-)
radiation or thermal radiation. All these waves can be classified within the
electromagnetic spectrum according to their wavelengths. For example, radio
waves have longer wavelengths than microwaves.
The electromagnetic spectrum arranged
according to wavelength ranging from shorter (left) to longer (right)
The image above shows the different waves of the electromagnetic spectrum
according to wavelength ranging from shorter (left) to longer (right)
wavelengths. Leftmost, you can see the short-wavelength and high-energy,
dangerous gamma radiation. In the middle, there is the
visible light and rightmost you can see the long-wavelength
radio waves. Wavelengths range from the size of an atom (several
billionth millimetres) to the size of a city (several kilometres).
If the wavelength of the visible light had to be compared with the
diameter of a hair, the hair would have to be split about a hundred times
for it to be as extensive as the wavelength of the visible light.
What is Light?
Light travels in the form of waves. A wave is a traveling disturbance.
Examples of waves are waves on a rope and waves in a slinky (recall
the demonstrations in class). In the first example the
disturbance is the displacement of the rope from its usual position. The
pattern of the displacement travels along the rope, but the material that
makes up the rope does not go very far. Waves on a rope are called
transverse waves because the displacement of the rope is in a
direction perpendicular to the direction of propagation of the wave.
Similarly, in the second example, the disturbance is the crowding of the
rings of the slinky. This disturbance (i.e., the pattern) travels along
the slinky while the rings themselves do not go very far away from their
usual positions. The wave in the slinky is a longitudinal wave,
i.e. the displacement of the rings is in the same direction as the
dierection of propagation of the wave. Light
resembles waves on a rope: it is a traveling disturbance of the electric
and magnetic fields in space. It is
thus called an electromgnetic wave. Sound, which is also a wave,
resembles waves on a slinky : it has the form of compressions and
rarefactions in air.
Here are the main properties of a wave and their significance (see
also Fig.2.3. in Ch.2 of the textbook):
Wavelength and Amplitude
||how high are the peaks
relative to the valleys
wave propagates, the frequency is the number of peaks that pass a given
point in 1 second.. For example, the figure below shows how a wave moves
if it has a frequency of 2 cycles per second, or 2 Hz (Hertz):
Relationship Between Wavelength, Frequency, and
Speed of a Wave.
The frequency and wavelength of a wave are
inversely proportional to each other. As the wavelength gets longer, the
frequency gets lower and vice versa. The relationship between them can be
written mathematically as follows
- In vacuum, light always travels at a speed of: 3 x 105
km/s = 300,000 km/s, which we have encountered before, in Lecture 2. The
is the universal speed limit, in the sense that nothing can
travel faster than light. The above equation also means that
wevelngth and frequency are inversely proportional to each other. In
other words, the higher the frequency, the shorter the wavelength has to
be so that their product stays the same and equal to teh speed of light.
Saying that a wave (or light) has a high frequency is equivalent to
saying that it has a short wavelength.
Wave-Particle Duality of Light
Light does not consist of continuous waves (waves that go from their
origin to their destination uninterrupted). Rather it consists of wave
packets, which can be thought of as small pieces of a wave that travel
together in bundles. This property is caled the wave-particle duality of
light (and of all electromagnetic waves in general): light behaves both as
a wave and as a stream of particles.
Each wave packet is called a photon
and it carries a fixed amount of
energy, which is proportional to its frequency. Photons are often
refered to as particles as well. Putting all of these principles together
we can sumarize the relationship between wavelength, frequency, and energy
of a photon as follows:
lower energy in a photon =
longer wavelength = lower frequency
higher energy in a photon =
shorter wavelength = higher frequency
The Significance of the Wavelength/Frequency/Energy: The
Photons travel through space, reach our eyes and interact with the
atoms there, depositing their energy. The amount of energy they deposit in
the atoms in our eyes depends on the wavelengths of the photons and it
determines the color we perceive. This is how we see things. Our eyes are
sensitive to photons in a very particular range of wavelengths, which is
call visible light.
Range of wavelengths of visible light (note that 1 cm = 0.01 m = 10-2
m and 1 nm = 10-9 m):
| 400 nm
|| 700 nm
4 x 10-5 cm
7 x 10-5 cm
|| 0.00007 cm
Note that blue light is made up of higher
photons (i.e., shorter wavelength, higher frequency), than
red light. White light is the combination of light of all wavelengths in
the above range (i.e., all colors: violet, indigo, blue, green, yellow,
You are probably familiar with other types of electromagnetic radiation
(i.e., electromagnetic waves) which are not visible to our eyes, such as
radio waves, microwaves, infra-red light, ultraviolet light, X-rays, and
gamma rays. These types of radiation are very similar to light in nature,
with the only difference that they have a different wavelength.
See the illustration of the electromagnetic
spectrum in Fig.2.8 in Ch.2. of the textbook.
||Very very long wavelength (a few meters).
Used for communications. They pass through the atmosphere without
||Much longer wavelength than visible light
(typically about 10-3 cm). They are absorbed and emitted by
molecules in the atmosphere.
||Somewhat longer wavelength than red light.
Emitted by objects at room temperature, such as human bodies. Absorbed
by water vapor in the Earth's atmosphere.
|Visible or Optical Light:
||Corresponds to the range of wavelengths listed above.
Most of the light from the Sun is emitted in the form of visible light.
It can pass through the Earth's atmosphere without being absorbed.
||Short wavelength compared to the blue.
Causes tanning or sunburn. Dissociates molecules. Causes mutations in
living cells. Absorbed by ozone molecules in the upper atmosphere.
||Much shorter wavelength than visible
light (high energy). Emitted by very hot gases (plasma). Cause
mutations in living cells. Absorbed by the upper layers of the Earth's
||Even shorter wavelength than X-rays (much
higher energy). Indicative of very high energy processes, such as
nuclear reactions or energetic particles gyrating in a magnetic field.
Cause mutations in living cells. Absorbed by the upper layers of the
longitudinal and transverse waves
Here we explain the difference between longitudinal and transverse
waves and how we measure the amplitude, wavelength and frequency. The
equation velocity = frequency x wavelength is explained
It was James Clerk Maxwell who showed in the 1800s that
light is an electromagnetic wave that travels through space at the
speed of light. The frequency of light is related to its wavelength
Let's look at an example calculation.
The light blue glow given off by mercury street lamps has a
wavelength of λ = 436nm. What is its frequency?
The unit s-1 is so common when talking about waves that it
was given the name Hertz. That is, 1 s-1 = 1 Hz. Thus, we would
say that light with a wavelength of 436 nm corresponds to a frequency of
6.88 × 1014 Hertz.
The region from λ ≈ 400-750 nm is visible to the human eye
and is therefore called the visible region of the electromagnetic
radiation. As we saw in the example above, blue light is near the high
frequency limit of our eyes. Red light, with wavelengths near 750 nm are
at the low frequency limit of our eyes. Light that contains all
frequencies in the visible region will appear as white light.
More generally, the different regions of the electromagnetic
spectrum are given different names. Below are the names given to the
different regions (frequency ranges) of light according to their frequency
The most basic concepts about a wave are
wavelength ( ), frequency (f),
velocity (v), and
The first three quantities are related by the equation
wave speed = wavelength x frequency
v = x f
|(1) Imagine ocean waves crashing onto the
beach. Think of reasonable numbers for the following:
(a) What is the wavelength of the waves? That is, what do you
think is the distance separating the crest of one wave from the crest of
the next wave?
(b) What is the frequency of the waves? (Hint: use the formula:
wave frequency = 1/wave period. If a wave comes once a minute,
wave period is 1 minute; if a wave comes once an hour, wave period is 1
(c) Fiigure out how fast the waves must be traveling. (Calculate
Waves in What?
The key concepts in this section are:
Electromagnetic radiation comprises varying electric and magnetic
fields that can be thought of either as waves or as light particles -
All electromagnetic radiation travels at the speed of light where
speed of light = wavelength x frequency
Low frequency = large wavelength = low energy. High frequency = small
wavelength = high energy.
|(2) The speed of light is 3.00 x 108
m/sec = 3 x 105 km/sec. What is the speed of light in miles/hour?
Wavelength = l ~ 600 nanometers
= 6 x 102 x 10-9 meters
= 6 x 10-7 m
(1 nanometer = 10-9 meter - TINY!)
So, what is the frequency of visible light?
frequency = speed of light / wavelength
= 3 x 108 m/s / 6 x 10-7 m
= 0.5 x 1015 Hz
- yes! VERY high frequency!
Radio - pick your favorite station!
Say, 106 FM
Frequency = 106 Megahertz ("mega" = 1 million)
= 1.06 x 102 x 106 Hz
= 1.06 x 108 Hz
Re-arranging speed of light = wavelength x frquency we can work
out the wavelength for radio waves from the 106 FM radio station:
wavelength = speed of light / frequency
= 3 x 108 m/s / 1.06 x 108 Hz
= 3 meters - about 10 feet.
The Electromagnetic Spectrum
The next two figures are very important. Make sure you
really understand them.
|(3) The figure above shows the visible part
of the electromagnetic spectrum--the rainbow of colors that is produced
when white light is spread out according to wavelength.
(a) Which has a longer wavelength, blue or red light?
(b) Note the units--nanometers (nm)--i.e. 10-9
meter. Green light has a wavelength of 500 nm. How many wavelengths of
green light are there in a meter?
|(4) This next figure (above) shows the
electromagnetic spectrum from gamma rays to radio. Note that the range
of wavelengths covers ten factors of 10, from 10-14 meter to
104 meters (or 10 km).
(a) Infrared radiation is the energy you feel from a fire.
The wavlength of infrared light is about 1 "micro-meter" = 1 micron.
How many infrared wavelengths are there in a meter?
(b) Microwave radiation is easily absorbed by water and
allows us to heat up food quickly. How many microwave wavelengths,
each 1mm long, are there in a meter?
We have all seen waves (water waves, flags rippling in the wind, vibrations
along ropes or strings), so we know what they are when we see them. They
are a disturbance propagating though a medium in such a way that the
disturbance moves, but the medium itself does not. Waves come in two
varieties: transverse and longitudinal. For a transverse
wave, the medium vibrates at right angles to the wave motion (example:
waves on a rope). For a longitudinal wave, the medium vibrates in the same
direction as the wave motion (example: waves along a slinky).
Physicists characterize waves by three parameters: amplitude, frequency,
and wavelength. The amplitude is the "height" of the wave, or in other
words, a measure of the energy in the wave. You find the frequency of a
wave by counting how many wave crests pass a fixed point in a certain
interval of time. Frequency is usually measured in cycles per second, also
known as hertz (Hz). (That is, a 100 Hz wave has a frequency of 100 cycles
per second.) The wavelength is the distance from one wave crest to the next.
For sound waves, amplitude is related to the loudness of the sound, and
frequency is related to the pitch. The higher the frequency of the sound
wave, the higher the pitch. The speed of a sound wave through air at room
temperature and pressure is about 343 m/s. You can relate the speed of the
sound wave to its wave parameters by:
v = f l
where: v = the speed of the wave, f = the wave frequency,
l = the wavelength.
For many types of waves, including sound waves, the speed of the wave
through a medium does not depend too much on the frequency. Sound waves of
high frequency and low frequency move at pretty much the same speed through
air -- that is, the sound from a tuba or from a dog whistle
both travel at about 343 m/s. Since the speed of sound is essentially
constant, the equation above means that high frequency always implies a
short wavelength for sound, and vice versa.
All waves carry energy and momentum, just like particles. They can interact
with matter, and transfer momentum, and cause heating to occur, and so forth.
Waves have several properties which are distinctly different from those of
particles, however. The most important ones are interference and
What is Wave Frequency?
We know that disturbance causing energy transfer from one point to another
is called a wave. Let us consider a wave traveling from point. So let us
count how many oscillations passes through that point in sometime time say 1
second. This is called the frequency of that wave with respect to that point.
Thus in general, we can say wave frequency is the number of oscillations
made by the wave per unit time. The unit for wave frequency is Hertz or Hz.
Wave Frequency Definition
The Wave frequency is defined as "The total number of vibrations or
oscillations made by the particles per unit time is called the Wave
Frequency and is denoted by f.
The formula for the Wave Frequency is:
f = Number of Oscillation / Time taken
The inverse or the reciprocal of the time period is the frequency of the
The frequency is the quantity obtained when we divide velocity of the wave
by its wavelength.
f = 1/T
where T = time period.
The figure depicts the different types of waves as classified according to
Formula for Frequency of a Wave
Here are some of the formulas for wave frequency:
If the wave equation is
y = A sin (ωt + ϕ)
where ω = Angular frequency,
ϕ = phase difference,
t = time period.
The frequency is related to angular frequency by the formula:
f = ω2π
The formula for the frequency to the time period in a wave is:
f = 1/T
where T = time period
ω= 2 π f = dθ /dt
Unit : Radian Per Second.
Velocity of the wave is related to the frequency by the formula:
f = vλ
where f = frequency,
if we consider the wave (electromagnetic wave) to be moving through vacuum
then v = c or the speed of light. Hence the formula reduces to:
f = C/λ
Here C = 3 × 108 m/s.
The Frequency of a Wave is?
The total number of vibrational cycle or the oscillations that are made per
second by the particles is called frequency of the wave. or The total number
of distinct cycles that are completed in unit time.
The frequency is dependent on both wavelength and velocity of the wave. The
mathematical relation to wavelength and velocity by the following formula:
V = f .λ
where V = Velocity of the wave,
f = frequency of the wave,
λ= wavelength of the wave.
The Frequency of a Sound Wave Determines?
The total number of complete back-and-forth particle vibrations of the
medium per unit time in sound wave is called the Sound Wave Frequency.
For the sound wave we use Hertz as the unit of measurement where 1 Hertz = 1
Conceptually whenever a wave passes the medium it makes the first particle
to which it comes in contact, vibrate. Then this particle vibrates the
nearby particle at the same frequency. This is how energy is propagated.
This is clear that the particles vibrate at the same frequency.
Waves can be of two types:
1.High frequency wave
2.Low frequency wave.
In a High frequency wave the numbers of vibrations per unit time are far
more than that of a low frequency wave.
The sound moreover depends on pitch, loudness and quality where pitch is
related to frequency of the sound wave.
Radio Waves Frequency
1.These are the waves which are having the lowest frequency in the
electromagnetic spectrum. They are given out by transmitter.
2.They are formed as a result of thunder, lightning etc.
3.They are used in communication mostly.
4.They are of four types of radio waves namely:
-Ultra high frequency (UHF)
-Very High frequency (VHF).
The Prolonged exposure to these frequency rays is known to cause cancer.
1.They are Used by antennas.
2.They are used for data transmission via modulation.
3.The frequency ranges from 3 KHZ to 3000 GHz. This is also called radio
High Frequency Waves
The High frequency waves are the waves with extremely less wave length.They
have a high frequency. Hence they pass through a given point many number of
times every second. These are utilized in communication over long distances.
Ultrasonic waves and gamma waves are the examples of such waves. Greater the
frequency greater would be the pitch.
They can be used for communication to moon.
They Can also be used for various other scientific functions and research.
Sine Wave Frequency
Sine wave is a mathematical function. It basically tells us about the smooth
oscillation that is repetitive.
The sine wave is expressed as:
y = A sin (ωt + ϕ)
Here A = amplitude of the sine wave
Φ= phase of the wave
ω= angular frequency of the wave
Also ω= 2 πf
Here f is the frequency of the wave.
How to Calculate Frequency?
Using above formulas we can calculate the frequency. Below are given some
problems on frequency:
Question 1: Frequency of a wave motion is 250 Hz. what is its time period?
Frequency f = 250 Hz.
Time period T = 1/f
Question 2: What is the frequency of a wave with a time period of 0.05
given Time period, T = 0.05 s
The Frequency is given by
f = 1/T
f = 1 / 0.05s
= 20 Hertz.
Frequency, f = 20 Hertz.
Question 3: A Sound wave traveling in air has a wavelength of 1.6 ×
10-2 m. if the Velocity of sound is 320 m/s. Calculate the frequency of the
Wavelength of the sound λ= 1.6 × 10^-2 m,
Velocity of sound (V) = 320 m/s
Velocity of sound is given by V = f. λ
Frequency f = V/λ
= 320m/s / 1.6×10−2m
= 320×10^3 / 16s
= 20 ×10^3 Hz
Frequency f = 2 ×10^4 Hz.
Electromagnetic waves consist of electric (E) and magnetic (B) fields
propagating through space. These fields are orthogonal (at right angles to
each other), in phase (reach same peak at same time), and fluctuate
perpendicular to the direction of motion.
There you see an EM wave propagating outwardly from a metal rod (antenna)
given a high frequency signal. The electric field and current oscillate
vertically within the antenna, radiating off a vertically polarized electric
field. Because fluctuating electric fields induce fluctuating magnetic
fields at right angles and vice versa, electromagnetic waves consist of both
An easier way to understand such waves is to visualize them in terms of
the vector potential rather than magnetic or electric field. The vector
potential is a more fundamental field, analogous to the momentum carried by
flowing water. If a thick rope is dragged through water, some of the water
surrounding it will be dragged along. Likewise with a wire or antenna
through which current flows. The current (I) drags some “ether” along with
it, and that flow is the vector potential (A).
The antenna shown earlier is just a vertical wire with an oscillating
rather than steady current. So let’s look at the vector potential field
around the antenna:
In this diagram, only a slice of the right side of the field is shown.
Here you see the vector potentials varying over distance. If this were
animated you would see each arrow oscillate vertically, and the train of
these would move out and away from the antenna. The electric field is also
oriented vertically since it arises from changes in the vector potential,
but with a 90 degree phase lag.
As mentioned earlier, a current-carrying wire is surrounded by a circular
magnetic field due to differences between adjacent parts of the vector
potential field creating vorticity. Same holds true for the antenna:
The preceding section depicted electromagnetic waves in terms of the more
elementary “vector potential” field. Thinking in terms of fundamental rather
than derivative phenomena is the key to understanding almost anything. Here
I will use vector potentials to explain longitudinal
To recap, transverse waves are undulations whose orientation of
fluctuation is perpendicular to the direction of travel. An antenna given a
high frequency electrical signal will radiate a transverse electromagnetic
wave. The electric component may be illustrated like so:
The magnetic field is not shown in this diagram but would look similar
except being horizontal rather than vertical. Since the electric field
derives from changes in the vector potential (A), the wave can be shown in
its more fundamental A-field configuration:
The A-field is oriented in the same direction as the E-field but with a
phase difference. Notice that there is only one field shown, and
that this field is complete in itself; there is no need to draw separate
electric and magnetic fields at right angles to each other, because the
latter are just two derivative phenomena stemming from this single A-field.
In contrast to transverse waves, longitudinal waves fluctuate in the
direction of propagation. A common example would be sound waves, which
consist of an alternating series of displacements in air where the
displacement points in the direction that sound travels. So for longitudinal
EM waves, the vector potential fluctuates in the direction of travel rather
than perpendicular to it.
How Antennas Work
We can't see them but radio and television
waves are just another form of light. They have a much longer wavelength
than visible light but both are electromagnetic radiation.
To generate radio and TV waves we typically
make electrons oscillate up and down on an antenna. This is done by applying
a variable voltage or alternating current to the antenna. Antennas are
generally made of metals and metals act like containers filled with a liquid
made of electrons. Metal atoms have one or
more weakly held electrons in their outer shells which can "float"
from atom to atom.
Electric Field Around a Positive Charge
||When a negatively charged
electron moves it leaves behind what is generally referred to as a
positively charged hole. The hole is simply an atom with more positive
protons than negative electrons.
The electrical fields for the two types
of charges are shown in Figures 1and 2. These are ray diagrams. The
arrows show the direction of the force that would be exerted on a unit
of positive charge.
Unlike a vector diagram, the length of a
ray does not indicate the magnitude of the force. Instead, the space
between rays indicates magnitude. Both diagrams in Figures 1 and 2 show
that the magnitude of the field decreases with increasing distance from
the charge because the space between the rays increases.
Electric Field Around a Negative Charge
We can use a simple analogy to help
understand how electromagnetic waves are produced by moving charges. Imagine
for a minute that the rays or electric field lines shown in Figures 1 and 2
are like very long springs attached to a circular frame with the charge at
the center, almost like a trampoline. If the charge is bounced up and down
waves will propagate outward along the springs. Yes, the world of
electromagnetic radiation is far more complex than our simple analogy but
hopefully it gives you some idea of how a moving charge could create a wave.
|The waves and
variation in the electric field account for the "electro" part of the
term electromagnetic waves.
A moving charge is essentially a current
and currents create circular magnetic fields. In Figure 3, a positive
charge moving straight out of the page would produce a magnetic field
represented by the blue dashed line. The direction of the field can be
determined using the right hand thumb rule. The thumb is pointed in the
direction of the current and the fingers of the right hand wrapped into
a loose fist. The fingers point in the direction of the magnetic field.
Note that the magnetic field lines are
perpendicular to the electric field lines. This is one of the famous
characteristics of electromagnetic waves.
Magnetic Field (shown in blue) Created by a positive Charge Moving
Straight out of the Plane of the Page
Okay, you're probably wondering why we use an
example of a positive charge when we just got finished saying that it's the
electrons which move. It turns out that all the conventions in electricity
and magnetism are set up for positive charges. Much of this can be traced
back to the work of Benjamin Franklin. Unfortunately, the electron had not
even been discovered in Franklin's time.
When we talk about current we pretend the
positive holes are actually moving in the opposite direction as the
electrons. It may seem pretty silly but it does work as a concept and so
we're sticking with the tradition.
If a variable voltage is applied, it will
send an electrical wave up an antenna. Free electrons in the antenna act as
the media for propagating the wave. The situation is similar to longitudinal
sound waves propagated in a metal rod. The sound wave is carried by
alternating regions of tension and compression. In the compressed areas the
rod's molecules are pushed a little closer together. In the tension areas
they are pulled a little further apart. Although the molecules barely move,
the sound wave can be transmitted great distances.
The very slight motion of electrons up and
down an antenna is enough to cause electromagnetic waves to radiate out the
sides of the antenna at the same frequency as the variable voltage applied
to it. These are used for transmitting radio and television signals as well
as other forms of wireless communication.
Like sound, when electrical waves at a
defined frequency hit the end of an antenna they are reflected backwards and
form a standing wave in the antenna. Antenna waves move at the speed of
light (3 x 10 8 m/s) and so the travel time from one end of the
antenna to the other is pretty quick.
The electrical waves created on antennas
typically have a fixed wavelength. If the length of the antenna is wisely
chosen it's possible to make it resonate. The free end of an antenna acts
like an open circuit. Voltage drop is maximum across an open circuit and
zero across a short circuit. Hence the end of an antenna forms an anti-node
or area of maximum voltage or e-field strength. A node is a point which has
zero e-field. The distance between an anti-node and node is a quarter of a
The wavelength of an electromagnetic wave is
calculated as follows:
||C = speed of
light (3 x 108 m/s)
||f = frequency
|Figure 4 shows a dipole
antenna which is generally considered the simplest form of antenna. In
this case each half of the antenna is roughly 1/4 wavelength long with
the antenna fed from its center. Hence, the total antenna is 1/2
wavelength long. The ends of the antenna correspond to anti-nodes and
the center to nodes. This configuration causes the antenna to resonate.
An antenna will still transmit even if the
length is not ideal for
resonance. However, less of the power input to the transmitter
will actually show up as useful output signal. In other words, the
efficiency of the system will be significantly lower.
Dipole antennas are considered balance
devices because they are symmetrical and work best when they are fed with a
balanced current. In other words, the current has to be of equal size on
both halves. This is usually accomplished with a balun when the antenna is
fed with a coaxial
cable. Coaxial cable is considered unbalanced, hence the word balun
is formed from parts of the words BALanced and UNbalanced. A balun is
basically a small transformer.
The optimum size of a dipole antenna is
slightly different than would be expected based on wavelength alone. This is
due to the interaction of the balun and antenna. However, the predicted
resonance length is usually very
close to the length for optimum broadcast efficiency.
Electromagnetic waves emitted from an antenna
are generally modeled as transverse waves. Since the waves have both
electric and magnetic field components and are emitted in three dimensional
space, the transverse wave model drawn in text books is a bit over
simplified but the full picture is almost impossible to draw.
Waves emitted from simple monopole and dipole
antennas tend to be polarized. In other words, if the emitting antenna is
vertical the receiving antenna also has to be vertical for best reception.
If the receiving antenna is horizontal the signal it picks up will be
Antenna design is very complex and requires a
lot of time and study to master. However, any antenna will have to oscillate
charged particles in order to transmit radio signals and will tend to do
this best if the antenna is resonating.
Electromagnetic radiation is described as a cyclic repeating wave having
electrical and magnetic fields with amplitude (peak value from the average)
and period (time between repeating portions of the wave). Frequency equals
the number of cycles per second, and the wavelength is the distance between
repeating points as determined from the frequency and velocity (see text for
relationship between velocity, wavelength, and frequency).
Path of EM wave propagation in a circuit wire
The image is my visualization of drift velocity and electromagnetic (EM)
propagation of charge wave in a closed circuit. The slow drift velocity of
the electrons follows the path of the circuit (a circle wire). Does the the
EM wave follow the same path of that of the drift velocity?
Since textbooks and online resources I found offer no understandable
description/differentiation, I assume they take the same path (of the
But I cannot understand why:
(1) If the wave is induced by and propagation from the voltage source (battery),
then it should take the vector path of the magnetic field created by the
battery, instead of the circuit path.
(2) If the electromagnetic wave is caused by some ballistic effect (electron
“pressuring” the next electron like water molecules in a tube), then
shouldn’t the wave left tangent to the wire and shoot to outer space? (similar
in sound wave, when
To rephrase my question with a better picture, when the battery apply a
electric potential to an closed circuit wire, there are two currents - the
very slow drift current from electrons, and the current in form of EM wave
traveling near the speed of light. What is causing the EM wave the bend and
turn along the wire?
Perhaps I should elaborate that I am not asking about the radiation or
antenna effect. I am curious on the actual "electricity/energy/signal"
current (not the drift current by electrons) going in the path of the
circuit wire instead of radiating outwards. I have amend the picture so it
looks more like a current going through a bulb rather than looking like an
antenna. (sorry for the bad drawing..I did my best job)
Introduction to EM Wave Propagation
Antenna is a set of conducting wires that allow electric current to pass.
When the electric current fluctuates, the lectromagnetic wave radiation
occurs. The antenna radiates the wave energy into space or receive energy
from the space.
The radiation ability depends the wire’s length and shape. For example, if
the two wires are very close, the electric and magnetic field are trapped
between them and the radiation is very weak (figure 1a). As the two wires
are apart further, the radiation becomes stronger, meaning more energy is
radiated into the space (figure1b, figure1c)
The Discovery of Radio Waves - 1888
Heinrich Rudolf Hertz
Heinrich Hertz was
the first to send and receive radio waves. James Clerk Maxwell had
mathematically predicted their existence in 1864. Between 1885 and 1889, as
a professor of physics at Karlsruhe Polytechnic, he produced electromagnetic
waves in the laboratory and measured their wavelength and velocity. He
showed that the nature of their reflection and refraction was the same as
those of light, confirming that light waves are electromagnetic radiation
obeying the Maxwell equations.
All of these findings were
first published in the journal Annalen der Physik,(see below right)
then in Hertz's first book, Untersuchungen Ueber Die Ausbreitung Der
Elektrischen Kraft (Investigations on the Propagation of Electrical
Energy), shown at right. His book is considered to be one of the most
important works of science. This is where he first describes his
confirmation of the existence of electromagnetic waves.
Annalen der Physik und
Chemie is one of the oldest physics journals worldwide. The journal,
still in publication today, publishes original papers in the areas of experimental,
theoretical, applied and mathematical physics and related areas.
Early experimental Hertz
radiator and resonator for creating and detecting Hertzian waves
Simple spark gap apparatus similar to this was the first ever built to
produce and detect radio waves
There are 12 complete volumes of
Annalen der Physik und Chemie in my collection. Included are Hertz's
many papers proving the Maxwell hypothesis on the propagation of
electromagnetic waves. These papers laid the foundation for the development
of radio and electromagnetic wave transmission applications. Also included
are more Hertz papers plus others by Roentgen, Planck, Boltzmann, Angstrom,
Hertz's electric-wave generator consisted of a spark gap to which
was attached a pair of outwardly extending conductors, corresponding in a
miniature way to the aerial and earth wires of a modern radio transmitter.
His receiver was a wire ring having a minute opening across which, when
electro-magnetic waves arrived, tiny sparks would pass. This wire ring was
in some respects like the loop receiver of today; with it Hertz was able not
only to indicate the receipt of waves, but also to determine their intensity
and direction of travel. Heinrich Hertz, despite the fact that his work was
limited to laboratory distances and that he did not suggest the use of his
waves for telegraphy, is the pioneer whose experiments laid the foundation
for radio as we now know it.
A few years after Hertz's first work with invisible electro-magnetic waves,
Elihu Thomson, of Lynn, Massachusetts, proposed (1889) their use for
signaling through fogs or even through solid bodies that would shut off
light waves. Sir William Crookes in 1892 made a startling prophecy of
electric-wave telegraphy and telephony. Meanwhile, Hertz's experiments had
been taken up and extended by a number of scientists, chief among whom were
Professor Edouard Branly, of Paris; Sir Oliver Lodge, of London; and
Professor Augusto Righi, of Bologna, Italy. Branly and Lodge devised
numerous forms of “radio conductors”, or receivers utilizing some of the
phenomena also discovered by Hughes, for the delicate reception of electric
waves; Righi invented various types of wave producers and con-firmed and
added to Hertz's observations.
Calculate the theoretical length for a "half-wave" antenna, assuming a
transmitter "carrier" frequency of 105.3 MHz:
Shown here is a simple quarter-wave antenna, comprised of a single
wire projecting vertically from one terminal of an RF voltage source, the
other terminal connected to earth ground:
We know at this point that any circuit comprised of inductance (L) and
capacitance (C) is capable of resonating: attaining large values of
AC voltage and current if ëxcited" at the proper frequency. The so-called
tank circuit is the simplest example of this:
A spark-gap transmitter for generating radio frequency electromagnetic
waves. Such devices served as the transmitters for most early wireless
Electro-magnetic waves wireless transmission and
reception system in the beginning of XX century. The spark gap in the
induction coil can be switched on and off by a telegraphic key. Oscillatory
currents from the spark gap excite the coherer, which becomes conductive.
When the coherer is placed in series with a battery and a telephone receiver,
it switch the currents on and off in synchronization with the telegraphic
key in the transmitter.
a) Telegraphic key
b) Spark gap
d) Telegraphic receiver
e) Telephone receiver
Spark gap transmitter
For example, in the case of the nuclear power plant, the
receptor was readily identified. The turbine control valves were
malfunctioning. The source and the coupling path were originally unknown;
however an investigation revealed that wireless handsets used by the plant
employees were the source. Although at this point the coupling path was not
known, the problem was solved by eliminating the source (e.g. restricting
the use of low‑power radio transmitters in certain areas). A
more thorough and perhaps more secure approach would be to identify
the coupling path and take steps to eliminate it. For example, suppose it
was determined that radiated emissions from a wireless handset were inducing
currents on a cable that was connected to a printed circuit card that
contained a circuit that controlled the turbine valves. If the operation of
the circuit was found to be adversely affected by these induced currents, a
possible coupling path would be identified. Shielding, filtering, or
rerouting the cable, and filtering or redesigning the circuit would then be
possible methods of attenuating the coupling path to the point where the
problem is non‑existent.
The source of the tramway problem was thought to be
transients on the tramway's power. The coupling path was presumably through
the power supply to the speed control circuit, although investigators were
unable to reproduce the failure so the source and coupling path were never
identified conclusively. The receptor, on the other hand, was clearly shown
to be the speed control circuit and this circuit was modified to keep it
from becoming confused by unintentional random inputs. In other words, the
solution was to eliminate the receptor by making the speed control circuit
immune to the electromagnetic phenomenon produced by the source.
Potential sources of electromagnetic compatibility
problems include radio transmitters, power lines, electronic circuits,
lightning, lamp dimmers, electric motors, arc welders, solar flares and just
about anything that utilizes or creates electromagnetic energy. Potential
receptors include radio receivers, electronic circuits, appliances, people,
and just about anything that utilizes or can detect electromagnetic energy.
Methods of coupling electromagnetic energy from a source
to a receptor fall into one of four categories.
- Conducted (electric current)
- Inductively coupled (magnetic field)
- Capacitively coupled (electric field)
- Radiated (electromagnetic field)
Coupling paths often utilize a complex combination of
these methods making the path difficult to identify even when the source and
receptor are known. There may be multiple coupling paths and steps taken to
attenuate one path may enhance another.
A Brief History of EMC
In the late 1880's, the German physicist Heinrich Hertz performed
experiments that demonstrated the phenomenon of radio wave propagation, thus
confirming the theory published by James Clerk Maxwell two decades earlier.
Hertz developed a spark in a small gap between two metal rods that were
connected at the other end to metal plates as shown in Figure 2. The spark
excitation created an oscillating current on the rods resulting in
electromagnetic radiation near the resonant frequency of the antenna. The
receiving antenna was a loop of wire with a very thin gap. A spark in the
gap indicated the presence of a time‑varying field and the maximum spark gap
length provided a measurement of the received field's strength.
Figure 2. Early antennas constructed by Heinrich Hertz.
Guglielmo Marconi learned of Hertz's experiments and improved upon them. In
1895, he developed the wireless telegraph, the first communications device
to convey information using radio waves. Although the significance of his
invention was not initially appreciated, the U.S. Navy took an interest due
to the potential of this device to enhance communication with ships at sea.
In 1899, the Navy initiated the first shipboard tests of the wireless
telegraph. While the tests were successful in many ways, the Navy was unable
to operate two transmitters simultaneously. The reason for this problem was
that the operating frequency and bandwidth of the early wireless telegraph
was primarily determined by the size, shape and construction of the antenna.
Receiving antennas were always "tuned" (experimentally) to the same
operating frequency as the transmitting antenna, however the bandwidth was
difficult to control. Therefore when two transmitters were operating
simultaneously, receivers detected the fields from both of them to some
extent and the received signal was generally unintelligible. This early
electromagnetic compatibility problem came to be referred to as Radio
Frequency Interference (RFI). As the popularity of the wireless telegraph
grew, so did the concern about RFI.
In 1904, Theodore Roosevelt signed an executive order empowering the
Department of Commerce to regulate all private radio stations and the Navy
to regulate all government stations (and all radio stations in times of war).
Different types of radio transmitters were assigned different frequency
allocations and often were only allowed to operate at certain times in order
to reduce the potential for RFI.
By 1906, various spark‑quenching schemes and tuning circuits were being
employed to reduce the bandwidth of wireless transmitters and receivers
significantly. However, it was the invention of the vacuum tube oscillator
in 1912 and the super heterodyne receiver in 1918 that made truly narrow
band transmission and reception possible. These developments also made it
possible to transmit reasonably clear human speech, which paved the way for
commercial radio broadcasts.
The period from about 1925 to 1950 is known as the golden age of
broadcasting. During this period the popularity of radio soared. As the
number of radios proliferated, so did the electromagnetic compatibility
problems. RFI was a common problem because the regulations governing
intentional or unintentional interference with a commercial radio broadcast
were lax and more people had access to radio equipment. In order to
alleviate this problem, the Federal Communications Commission (FCC) was
established in 1934 as an independent agency of the U.S. Government. It was
empowered to regulate U.S. interstate and foreign communication by radio,
wire, and cable. FCC regulations and licensing requirements significantly
reduced the number of radio frequency interference problems.
However, due to the increasing number of radio receivers being located in
homes, the general public was introduced to a variety of new EMC problems.
Unintentional electromagnetic radiation sources such as thunderstorms,
gasoline engines, and electric appliances often created bigger interference
problems than intentional radio transmitters.
Intrasystem interference was also a growing concern. Super heterodyne
receivers contain their own local oscillator, which had to be isolated from
other parts of the radio's own circuit. Radios and phonographs were lumped
together in home entertainment systems. Radios were installed in automobiles,
elevators, tractors, and airplanes. The developers and manufacturers of
these systems found it necessary to develop better grounding, shielding, and
filtering techniques in order to make their products function.
In the 1940's many new types of radio transmitters and receivers were
developed for use during World War II. Radio signals were not only used for
communication, but also to locate ships and planes (RADAR) and to jam enemy
radio communications. Because of the immediate need, this equipment was
hurriedly installed on ships and planes resulting in severe EMC problems.
Experiences with electromagnetic compatibility problems during the war
prompted the development of the first joint Army‑Navy RFI standard, JAN‑I‑225,
"Radio Interference Measurement," published in 1945. Much more attention was
devoted to RFI problems in general, and techniques for grounding, shielding
and filtering in particular. Electromagnetic compatibility became an
engineering specialization in a manner similar to antenna design or
In 1954, the first Armour Research Foundation Conference on Radio Frequency
Interference was held. This annual conference was sponsored by both
government and industry. Three years later, the Professional Group on Radio
Frequency Interference was established as the newest of several professional
groups of the Institute of Radio Engineers. Today, this group is known as
the Electromagnetic Compatibility Society of the Institute of Electrical and
Electronics Engineers (IEEE).
During the 1960's, electronic devices and systems became an increasingly
important part of our society and were crucial to our national defense. A
typical aircraft carrier, for example, employed 35 radio transmitters, 56
radio receivers, 5 radars, 7 navigational aid systems, and well over 100
antennas . During the Vietnam War, Navy ships were often forced to shut
down critical systems in order to allow other systems to function. This
alarming situation focused even more attention on the issue of
electromagnetic compatibility. Outside the military, an increasing
dependence on computers, satellites, telephones, radio and television made
potential susceptibility to electromagnetic phenomena a very serious concern.
The 1970's witnessed the development of the microprocessor and the
proliferation of small, low‑cost, low‑power semiconductor devices. Circuits
utilizing these devices were much more sensitive to weak electromagnetic
fields than the older vacuum tube circuits. As a result, more attention was
directed toward solving an increasing number of electromagnetic
susceptibility problems that occurred with these circuits.
In addition to traditional radiated electromagnetic susceptibility (RES)
problems due to intentional and unintentional radio frequency transmitters,
three additional classes of electromagnetic susceptibility problems gained
prominence in the '70s. Perhaps the most familiar of these, outside the
military, is electrostatic discharge (ESD). An electrostatic discharge
occurs whenever two objects with a significantly different electric
potential come together. The "shock" that is felt when a person reaches for
a door knob after walking across a carpet on a dry day is a common example.
Even discharges too weak to be felt however, are capable of destroying
Another electromagnetic susceptibility problem that gained notoriety during
the '70s was referred to as EMP or ElectroMagnetic Pulse. The military
realized that a high‑altitude detonation of a nuclear warhead would generate
an extremely intense pulse of electromagnetic energy over a very wide area.
This pulse could easily damage or disable critical electronic systems. To
address this concern, a significant effort was initiated to develop
shielding and surge protection techniques that would protect critical
systems in this very severe environment.
The emergence of a third electromagnetic susceptibility problem, power line
transient susceptibility (PLT), was also a direct consequence of the
increased use of semiconductor devices. Vacuum tube circuits generally
required huge power supplies that tended to isolate the electronics from
noise on the power line. High‑speed, low‑power semiconductor devices on the
other hand were much more sensitive to transients and their modest power
requirements often resulted in the use of relatively small low‑cost supplies
that did not provide much isolation from the power line. In addition, the
low cost of these devices meant that more of them were being located in
homes and offices where the power distribution is generally not well
regulated and relatively noisy.
The emphasis on electromagnetic susceptibility during the 1970's is
exemplified by the number of task groups, test procedures, and product
standards dealing with susceptibility that emerged during this decade. One
organization established in the late 70's known as the EOS/ESD Association (EOS
stands for Electrical Over Stress) deals exclusively with the susceptibility
problems mentioned above.
Another change that occurred during the 60's and 70's was the gradual
displacement of the term RFI by the more general term EMI or Electromagnetic
Interference. Since not all interference problems occurred at radio
frequencies, this was considered to be a more descriptive nomenclature. EMI
is often categorized as radiated EMI or conducted EMI depending on the
Two events in the 1980's had significant, wide‑ranging effects on the field
of electromagnetic compatibility.
The introduction and proliferation of low priced personal computers and
Revisions to Part 15 of the FCC Rules and Regulations that placed limits on
the electromagnetic emissions from computing devices.
The proliferation of low priced computers was important for two reasons.
First, a large number of consumers and manufacturers were introduced to a
product that was both a significant source and receptor of electromagnetic
compatibility problems. Secondly, the availability of low cost, high speed
computation spurred the development of a variety of numerical analysis
techniques that have had an overwhelming influence on the ability of
engineers to analyze and solve EMC problems.
The FCC regulations governing EMI from computing devices were phased in
between 1980 and 1982. They required all electronic devices operating at
frequencies of 9 kHz or greater and employing "digital techniques" to meet
stringent limits regulating the electromagnetic emissions radiated by the
device or coupled to the power lines. Virtually all computers and computer
peripherals sold or advertised for sale in the U.S. have to meet these
requirements. Many other countries established similar requirements.
In the 1990's, the European Union adopted EMC regulations that went well
beyond the FCC requirements. The European regulations limited unintentional
emissions from appliances, medical equipment and a wide variety of
electronic devices that were exempt from the FCC requirements. In addition,
the European Union established requirements for the electromagnetic immunity
of these devices and defined procedures for testing the susceptibility of
electronic systems to radiated electromagnetic fields, conducted power and
signal line noise, and electrostatic discharge.
The impact of these regulations was overwhelming because for the first time
engineers, managers, and corporate presidents were made painfully aware of
what it means to have an EMC problem. At a time when the market for
computers was mushrooming, many of the latest, most advanced designs were
being held back because they were unable to meet government EMI requirements.
Companies formed EMC departments and advertised for EMC engineers. An entire
industry emerged to supply shielding materials, ferrites, and filters to
computer companies. EMC short courses, test labs, magazines, and consultants
began appearing throughout the world. The international attention focused on
EMC encouraged additional research. Significant progress was made toward the
development of more comprehensive test procedures and meaningful standards.
Today these trends are continuing. Computing devices are getting denser,
faster, and more complex creating new challenges for the EMC engineer while
advances in numerical electromagnetics are revolutionizing the state‑of‑the‑art
in EMC analysis. Regulations limiting electromagnetic emissions continue to
be upgraded and new regulations concerning the susceptibility of electronic
devices are being developed and introduced.
The Future of Electromagnetic Compatibility
If you were to listen to a computer company executive explaining corporate
strategy for dealing with EMC problems in the future, you would very likely
hear something like "We are striving to make EMC an integral part of the
product design process, rather than attempting to solve problems by 'patching'
a design that is nearly complete." Of course this is not a new idea. Since
the early RFI problems with the wireless telegraph, engineers have realized
that it is cheaper, easier and more effective to design a product that is
compatible than it is to "fix" an existing design that has an EMC problem.
To some extent, early EMC involvement has been a goal all along and steady
progress has been made.
For example, radio circuit designers are keenly aware of bandwidth
requirements and out‑of‑band radiation is rarely a problem anymore even with
prototype designs. When digital circuits first appeared, interference
between the digital and analog portions of a device was common, however
eventually this became less of a problem as circuit designers learned to
isolate analog and digital grounds. Today, computers are routinely designed
with some degree of shielding, filtering, and special grounding techniques.
The reason that "early EMC involvement" continues to be extolled as an idea
whose time has come is that the scope and complexity of EMC problems is
steadily increasing. New technologies create unique situations rendering
existing EMC "fixes" and design rules obsolete. Engineers who are familiar
with fundamental EMC concepts and analysis techniques can readily apply this
knowledge to emerging technologies and anticipate potential EMC problems
during a product's design phase. In the past however, the emphasis has been
on communicating the design rules and fixes themselves rather than "burdening"
the circuit designers with fundamental EMC concepts. As a result, EMC
problems have kept one step ahead of the circuit designers and the call for
"early EMC involvement" continues.
Fortunately, this situation is beginning to change. The tips and tricks that
caused many engineers to view EMC as a black art are being examined more
closely and used with greater caution. More significantly, the importance of
many fundamental principles drawn from electromagnetics and circuit theory
is being recognized. These principles are essential to an understanding of
how a circuit interacts with its electromagnetic environment.
In the years to come, as EMC continues to evolve from an engineering art to
an engineering science, the need to make the principles of EMC part of the
electrical engineering curriculum will become more apparent. Advances in
computer hardware and numerical modeling techniques will enable the
efficient application of these principles to the analysis of complex
circuits and systems. Once circuit and system designers are familiar with
these concepts and techniques, "early EMC involvement" will be the rule
rather than just the goal.
Around every electrically charged object is a force field that can be
detected and measured. This force field can cause electric charges to move
in the field. When an object is charged electrically, there is either a
greater or a smaller concentration of
electrons than normal. Thus, a difference of
potential exists between a charged
object and an uncharged object. An electric field is, therefore, associated
with a difference of potential, or a voltage.
This invisible field of force is commonly represented by lines that are
drawn to show the paths along which the force acts. The lines representing
the electric field are drawn in the direction that a single positive charge
would normally move under the influence of that field. A large electric
force is shown by a large concentration of lines; a weak force is indicated
by a few lines.
When a capacitor is connected across a source of voltage, such as a
battery, it is charged by a particular amount, depending on the voltage and
the value of capacitance. (See figure 1-25.) Because of the emf (electromotive
force) of the battery, negative charges flow to the lower plate, leaving the
upper plate positively charged. Along with the growth of charge, the
electric field is also building up. The flux lines are directed from the
positive to the negative charges and at right angles to the plates. When the
capacitor is fully charged, the voltage of the capacitor is equal to the
voltage of the source and opposite in polarity. The charged capacitor stores
the energy in the form of an electric field. It can be said, therefore, that
an electric field indicates voltage.
Figure 1-25. - Electric fields between plates.
If the two plates of the capacitor are spread farther apart, the
electric field must curve to meet the plates at right angles (fig.
1-26). The straight lines in view A
of figure 1-26 become arcs in view B, and approximately semicircles in view
C, where the plates are in a straight line.
Instead of flat metal plates, as in the capacitor, the two elements can take
the form of metal rods or wires and form the basic antenna.
Figure 1-26. - Electric fields between plates at different angles.
In figure 1-27, two rods replace the plates of the capacitor, and the
battery is replaced by an ac source generating a 60-hertz
signal. On the positive alternation
of the 60-hertz generator, the electric field extends from the positively
charged rod to the negatively charged rod, as shown. On the negative
alternation, the charge is reversed. The previous explanation of electrons
moving from one plate to the other of the capacitor in figure 1-25 can also
be applied to the rods in figure 1-27.
Figure 1-27. - Electric fields between elements.
The polarity of charges and the direction of the electric fields will
reverse polarity and direction periodically at the frequency of the voltage
source. The electric field will build up from zero to maximum in one
direction and then collapse back to zero. Next, the field will build up to
maximum in the opposite direction and then collapse back to zero. This
complete reversal occurs during a single cycle of the source voltage. The
HALF-WAVE DIPOLE ANTENNA (two separate rods in
line as illustrated in figure 1-27) is the fundamental element
normally used as a starting point of reference in any discussion concerning
the radiation of electromagnetic
energy into space. If rf energy from the ac generator (or transmitter) is
supplied to the element of an antenna, the voltage across the antenna lags
the current by 90 degrees. The antenna acts as if it were a capacitor.
When current flows through a conductor, a magnetic field is set up in the
area surrounding the conductor. In fact, any moving electrical charge will
create a magnetic field. The magnetic field is a region in space where a
magnetic force can be detected and measured. There are two other fields
involved - an INDUCTION FIELD, which exists
close to the conductor carrying the current, and the RADIATION FIELD,
which becomes detached from the current-carrying rod and travels through
To represent the magnetic field, lines of force are again used to
illustrate the energy. Magnetic lines are not drawn between the rods, nor
between high- and low-potential points, as the E lines that were discussed
earlier. Magnetic lines are created by the flow of current rather than the
force of voltage. The magnetic lines of force, therefore, are drawn at right
angles to the direction of current flow.
The magnetic fields that are set up around two parallel rods, as shown in
figure 1-28 view A, are in maximum opposition. Rod 1 contains a current
flowing from the generator, while rod 2 contains a current flowing toward
the generator. As a result, the direction of the magnetic field surrounding
rod 1 is opposite the direction of the magnetic field surrounding rod 2.
This will cause cancellation of part or all of both magnetic fields with a
resultant decrease in radiation of the electromagnetic energy. View B
illustrates the fact that if the far ends of rods 1 and 2 are separated from
each other while the rods are still connected to the generator at the near
more space, and consequently less opposition, will occur between the
magnetic fields of the two rods. View C illustrates the fact that placing
the rods in line makes the currents through both rods flow in the same
direction. Therefore, the two magnetic fields are in the same direction;
thus, maximum electromagnetic radiation into space can be obtained.
Figure 1-28. - Magnetic fields around elements.
Magnetic lines of force are indicated by the letter H and are called H
lines. The direction of the magnetic lines may be determined by use of the
left-hand rule for a conductor: If you grasp the conductor in your left hand
with the thumb extended in the direction of the current flow, your fingers
will point in the direction of the magnetic lines of force. In view C of
figure 1-28, the direction of current flow is upward along both halves of
the elements (conductors). The lines of magnetic force (flux) form
concentric loops that are perpendicular to the direction of current flow.
The arrowheads on the loops indicate the direction of the field. The left-hand
rule is used to determine the direction of the magnetic field and is
illustrated in figure 1-29. If the thumb of the left hand is extended in the
direction of current flow and the fingers clenched, then the rough circles
formed by the fingers indicate the direction of the magnetic field.
Figure 1-29. - Left-hand rule for conducting elements.
Q.46 What do we call the field that is created between two rods when a
voltage is applied to them?
Q.47 When current flows through a conductor, a field is created around the
conductor. What do we call this field?
Simple Marconi Radiator. (Transmitter ) (Left Picture) = B, battery; I,
induction coil; K, signaling key; S, spark gap; A, aerial wire; E, earth
THE APPARATUS FOR WIRELESS TELEGRAPHY.
With this apparatus Heinrich Hertz proved that an electric spark produced
impulses which travel through the air. A spark leaped across contacts on the
left, inducing current in the ring on the right.
“Swift as wireless message my wishes rush
through space to greet you”. These were the very words from the
discoverer of the modern wireless communication—–Marconi, Guglielmo. It was
a spark on how the wireless communication had been born to this world, and
like a blink on how it is able to grow. However, this would become
impossible for us without the people who contributed their ideas to the
concept of so called “wireless communication”. So as we venture to our
yesterday’s yesterdays, just relax and enjoy.
From time to time, the discovery of “wireless
communication” ignited from the past years.
From Telegraph to “The Birth of Radio”,
- 1867 — Maxwell predicts existence of electromagnetic (EM) waves
- 1886 – A German physicist, Heinrich Hertz performed
an experiment which made a way to the revolutionized communication. His
experiment consists of two circuits with a small gap. When he connected it
to the high voltage source, a spark occurred even without
contact. Somehow energy from the first spark was received by the
second circuit. This energy was carried through EM waves or
Figure 1.1 — Hertz experiment, consisting of a transmitter (1) and a
- 1887 — Hertz proves existence of EM waves; first spark transmitter
generates a spark in a receiver several meters away
- 1890 — Branly develops coherer for detecting radio waves
- 1896 — Guglielmo Marconi demonstrates wireless
telegraph to English telegraph office
“The Birth of Radio”
- 1895 – Hertz didn’t think that this is a new means of transmitting
information without wires attached that’s why Guglielmo Marconi opened the
way for modern wireless communications by transmitting the three-dot Morse
code for the letter ‘S’ over a distance of three kilometers using
electromagnetic waves. From this beginning, wireless communications has
developed into a key element of modern society. From satellite
transmission, radio and television broadcasting to the now ubiquitous
mobile telephone, wireless communications has revolutionized the way
- 1897 — “The Birth of Radio” – Marconi awarded patent for wireless
- 1897 — First “Marconi station” established on Needles island to
communicate with English coast
- 1898 — Marconi awarded English patent no. 7777 for tuned communication
- 1898 — Wireless telegraphic connection between England and France
Hertz Experiment for Production and Detection Electromagnetic Wave
Em Waves are generated and detected using electrical sources. An induction
coil is connected to two spherical electrodes with a narrow gap between
them (the transmitter). The coil provides short voltage surges to the
sphere charging once positive, the other negative. A spark is generated
between the spheres when the voltage between them reaches the breakdown
voltage for air. As the air in the gap is ionized, it conducts, more
readily and the discharge between the spheres becomes oscillatory. From an
electrical circuit viewpoint, this is equivalent to an LC circuit, where
the inductance is that of the loop and the capacitance is due to the
For an LC circuit, frequency 1/2π √LC. Since L and C are quite small, the
frequency of oscillation is very high = 100Mhz. Em waves are radiated at
this frequency as a result of the oscillation (and hence acceleration) of
free charges in the loop. Hertz was able to detect these waves using a
single loop of wire with its own spark gap (the receiver) this loop,
placed several meters from the transmitter, has its own effective
inductance, capacitance, and natural frequency of oscillation. Sparks were
induced across the gap of the receiving electrodes when the frequency of
the receiver was adjusted to match that of the transmitter. Thus, Hertz
demonstrated that the oscillating current induced in the receiver was
produced by em waves radiated by the transmitter.
For more help in Hertz Experiment for Production and Detection
Electromagnetic Wave please click the button below to submit your
MAXWELL’S WAVES DISCOVERED
In 1865 James Clerk Maxwell predicted the existence of electromagnetic waves.
He suggested that an accelerated charge would produce a non-uniformly
changing electric field that would in turn produce a changing magnetic field.
By Faraday’s Law, this non-uniformly changing magnetic field would in turn
produce a changing electric field and so on. He showed mathematically that
such fields would propagate through space as a wave motion with a speed of 3
x 108 m/s. This speed agreed so closely with values of the speed of light
measured by Fizeau in 1849 and Foucault in 1862 that Maxwell became
convinced that light was a form of electromagnetic wave.
Heinrich Hertz, a German physicist, achieved the first experimental
demonstration of electromagnetic waves in 1887. Hertz used an induction coil
to produce oscillating electric sparks between two brass balls connected to
two brass plates. The brass plates acted as an aerial system. He used a
small loop of wire with a tiny gap in it as the receiver. See diagram below.
As sparks jumped across the gap between the balls, sparks were also observed
jumping the gap in the receiver. Hertz reasoned that the spark discharge
oscillating backwards and forwards between the brass balls set up changing
electric and magnetic fields that propagated as an electromagnetic wave, as
postulated by Maxwell. When these waves arrived at the receiver, the
changing electric field component caused charges in the loop to oscillate,
thus producing the spark across the gap in the receiver.
Hertz carried out a thorough investigation of these waves and showed that
they did indeed possess properties similar to light – reflection, refraction,
interference, diffraction and polarisation. By setting up an experiment in
which he allowed the waves to reflect from a metal sheet and interfere with
themselves to produce standing waves, Hertz was able to determine their
wavelength. He calculated the frequency of oscillation of the sparks in his
transmitter from knowledge of the parameters of the circuit. Then using
v = f l
he calculated the speed of the waves as 3 x 108 m/s, as predicted by Maxwell.
Thus, Hertz’s experiment confirmed Maxwell’s prediction of EM waves and
provided strong experimental support for the idea that light was a form of
transverse EM wave.
The waves produced by Hertz eventually became known as radio waves and his
research led to the development of radio communications. As Hertz suspected
it was indeed oscillating charges that produced the EM waves. Today we know
that radio waves are produced when an oscillating voltage applied to an
antenna causes free electrons to oscillate along that antenna. This
generates an EM wave that spreads out from the transmitter at 3 x 108 m/s.
When the EM wave strikes a receiving antenna it forces charges in the
antenna to oscillate at the frequency of the wave. This oscillating
electrical signal is then converted into an audio-frequency signal by diodes
in appropriately tuned electronic circuits.
Applications of the production of EM waves by oscillating electric charges
in radio antennae started with the demonstration of “wireless” telegraphy by
Sir Oliver Lodge in 1894. Marconi accomplished the first trans-Atlantic
transmission in 1901. The invention of the triode valve amplifier in 1906
enabled radio transmission of speech and music over long distances. The
invention of the transistor in 1948 eventually resulted in further
improvements in radio transmission and reception and decrease in size of
transmitters and receivers. Today, radio communications networks, citizen-band
radio, mobile phone networks and television image transmission are examples
of applications of EM wave production. (This information in this last
paragraph is no longer required by the Syllabus.)
The Discovery of Electromagnetic Radiation
The most dramatic prediction of Maxwell's theory of electromagnetism,
published in 1865, was the existence of electromagnetic waves moving at the
speed of light, and the conclusion that light itself was just such a wave.
This challenged experimentalists to generate and detect electromagnetic
radiation using some form of electrical apparatus.
The first clearly successful attempt was made by Heinrich Hertz in 1886. For
his radio wave transmitter he used a high voltage induction coil, a
condenser (capacitor, Leyden jar) and a spark gap - whose poles on either
side are formed by spheres of 2 cm radius - to cause a spark discharge
between the spark gap’s poles oscillating at a frequency determined by the
values of the capacitor and the induction coil.
This first radio waves transmitter is basically, what we call today, an LC
oscillator. For an animated explanation of this device click here. More
information about this subject could be found in basic electronics text
To prove there really was radiation emitted, it had to be detected. Hertz
used a piece of copper wire, 1 mm thick, bent into a circle of a diameter of
7.5 cm, with a small brass sphere on one end, and the other end of the wire
was pointed, with the point near the sphere. He added a screw mechanism so
that the point could be moved very close to the sphere in a controlled
fashion. This "receiver" was designed so that current oscillating back and
forth in the wire would have a natural period close to that of the "transmitter"
described above. The presence of oscillating charge in the receiver would be
signaled by sparks across the (tiny) gap between the point and the sphere (typically,
this gap was hundredths of a millimeter).
Conceptual Schematic of Hertz's Experiment
In this experiment Hertz confirmed Maxwell’s theories about the existence of
In more advanced experiments, Hertz measured the velocity of electromagnetic
radiation and found it to be the same as the light’s velocity. He also
showed that the nature of radio waves’ reflection and refraction was the
same as those of light, and established beyond any doubt that light is a
form of electromagnetic radiation obeying the Maxwell equations.
Summing up Hertz's importance: his experiments would soon trigger the
invention of the wireless telegraph and radio by Marconi and others and TV.
In recognition of his work, the unit of frequency - one cycle per second -
is named the “hertz”, in honor of Heinrich Hertz.
Repeat Hertz’s Experiments
Warning: experiments with electricity should
be performed under the supervision of teachers or adults familiar with
electricity safety procedures. Especially, take in account that experiments
with induction coils and capacitors can produce high voltage shocks.
Hertz first experiment – creating, sending and detecting radio waves – is
relatively simple, not beyond the abilities of middle school students. In
order to begin, read carefully the experiment links and ensure that you
understand the basic principals. Brows further the web and consult your
local library, your teacher and other knowledgeable adults and experts.
Hertz’s more advanced experiments, mentioned above, require some extra
ability and knowledge, and in order to perform these experiments
successfully the students are also required to be able to read and
understand a few books by Hertz or about Hertz listed in the resource
- Perform an investigation to demonstrate the production and
reception of radio waves
Hertz demonstrated the production of radio waves and confirmed Maxwell’s
prediction that there were EM waves with frequencies outside the visible
Hertz wanted to produce EM waves with frequencies and wavelengths other
than visible light.
Setup and Method
An induction coil was used to create a rapidly oscillating B-field which
caused a rapid sparking across a gap between spherical electrodes in a
This circuit formed the transmitter and a receiving loop also with a gap
in it, was placed some distance from the transmitter.
The high voltage induction coil connected to the transmitter was switched
on and changes observed
Changes in the receiving electrodes were observed
When the power was on, sparking occurred between the electrodes at the
transmitter and this also resulted in sparks at the receiver.
- The high voltage AC produced sparking and a rapidly oscillating
electric field which gave rise to a magnetic field and so on.
- Thus, EM radiation (radio waves) were produced and traversed the
distance to the receiver
- The EM radiation travelling towards the receiver struck the electrodes
of the receiver, energising electrons in the conducting
surface and caused them to jump across the gap as a spark.
- Note: there were no electrical connections between the transmitter and
Other properties observed
- Reflection – Hertz reflected waves off a zinc plate
and they still reached receiver to cause sparking
- Refraction – Radio waves were refracted through a
- Polarisation – He rotated the receiver’s plane
relative to the transmitter
- The receiver’s intensity and length of sparking at the receiver was
a maximum when the plane was parallel and a minimum when perpendicular.
- Interference – he observed that waves reaching the
receiver from 2 different paths interfered constructively and
destructively to produce interference pattern of light and dark patches.
- Distance – the length and intensity of sparking at
the receiver was not affected by the distance between transmitter and
receiver. Suggested that light waves are self-propagating
- Speed – The speed was accurately measured to be “c” (see
next dot point)
These observations strongly supported Maxwell’s prediction of EM
radiation and model of light: self-propagating, transverse waves of
alternating electric and magnetic fields that are perpendicular to one
another. Hertz concluded that radio waves were able to cause sparking at the
- Describe Hertz’s observation of the effect of a radio wave on a
receiver and the photoelectric effect he produced by failed to investigate
The photo electric effect is the emission of electrons from the surface
of a conductor when it is subject to EMR.
- This was first observed by Hertz in 1887 but he did not create the
- He enclosed the receiver in a dark box to create a dark environment
for making observations.
- In doing so, he observed that the length and intensity of sparking
- On removing the various walls of the box in succession, he found that
only the portion of the case which shielded the receiver from the
transmitter had this effect on sparking.
- Hertz knew that radio waves produced by the transmitter would not be
blocked by the box so he reasoned that the box must be blocking other type
of EM waves.
This led onto further investigations and observations.
- The length and intensity of sparking at the receiver was diminished
when glass (blocks UV) was used as a shield between transmitter and
- Then quartz was used (quartz does not block UV) and no change to the
sparking was observed
- When a mercury vapour lamp (emits UV) was shone onto the receiver,
sparking was increased.
He named these effects the photo electric effect but did not investigate
experiment to measure the speed of radio waves
- Outline qualitatively Hertz’s experiments in measuring the
speed of radio waves and how they relate to light waves
In order to prove that these radio waves were EM waves, he showed they
had similar properties to light, namely its speed.
Measuring the Speed
- In order to determine velocity
Hertz needed to measure frequency and wavelength.
- The frequency of the waves was already known, as Hertz had used an RLC
(resistor inductor capacitor) in his set up. This produced a sinusoidal
current of constant frequency.
- To measure the wavelength, radio waves were allowed to reach the
receiver from 2 different paths – one directly and one following
reflection off a metallic surface.
- These two waves met at the receiver and created an interference
pattern of light and dark patches corresponding to relative max and min.
- By moving the receiver back and forth, the interference pattern of the
waves could be analysed
- The difference between successive maxima or successive minima gave
Conclusion: Hertz then found the speed of radio waves
Microwave Energy - the next part of my
Making Electromagnetic Weapons series. For the Electromagnetic Pulse
Generator, check out the last three articles
I'm sure almost all of you have used a microwave at some point in your
lives. As a child, I always found microwaves fascinating; the idea of
heating food with invisible energy, and even creating lightning should the
user accidentally microwave metal! However, microwaves are not only used for
heating food. Microwave energy generally falls under the 2.4 GHz (Gigahertz
band). This same band is used by many wireless technologies such as
Bluetooth and Wi-Fi. Microwaves consist of any wavelength between 300 MHz
(0.3 GHz) and 300 GHz. The range (energy) depends on the "strength" of the
Here's a visual representation of the electromagnetic spectrum:
Simple Cooking Appliance or Lethal Weapon?
Well, it really depends. In this article, I'll be going over the simple
basics of a microwave weapon, since microwave energy is a huge topic. In its
simplest form, any waveform transfer of energy starts with excited particles
and ends with excited particles.
Inside a microwave, you'll find a large transformer (called a MOT or
Microwave Oven Transformer), a large capacitor (rated around 1-2 kV; 1-100
uF), some high voltage diodes (for rectifying the alternating current from
the transformer), a magnetron (the microwave emitter—I'll go into this later),
and other electrical components for operating the main electronics.
In a Microwave Weapon (MW), the components can be as simple as a
magnetron, a transformer, a diode, and a capacitor. Of course, the magnetron
is certainly not that simple, consisting of several finely tuned "antennas"
and other components. A basic illustration of how a magnetron works is
The round "1" is an electron source, the area between the power source
and the antenna is the electron "accelerator", and the antenna itself is a
simple way of "amplifying" and broadcasting the electron energy at a
specific frequency. When these "tuned electrons" hit an object (specifically
water or metal), they excite the molecules and generate heat, or in the case
of metal, electrical energy. This is why microwaves are so dangerous
compared to EMPs. Microwaves not only wreck havoc on electronics, but also
can harm living beings.
This is where I must issue a WARNING!!! Microwaves are extremely
dangerous. They can PERMANENTLY HARM YOU! If you feel even the slightest
uncertainty towards the physics, dangers, and overall understanding of
microwaves, DO NOT construct a microwave weapon.
The best way to create a homemade microwave weapon is with an old
microwave. If you want to upgrade to a
more powerful, long range device, it's practically impossible unless
you have a physics lab with extensive measuring equipment. However, an
average microwave puts out 1,000-2,000 watts of energy, quite enough for
Microwaves tend to "fly in all directions" unless they are directed.
However, this is what the antenna does—directs the microwaves. In my
experimentation, I discovered that a slight cone-shaped metal funnel has the
best microwave-focusing ability. I was able to fry an old cell phone from up
to 10 feet using three magnetrons and one funnel. This constitutes to about
6,000 watts (W) of directed energy, quite an accomplishment for 15 bucks
spent at a thrift store. The circuit diagram for each individual magnetron
looked something like this:
On a basic level, the circuit consists of a transformer, a voltage
doubler (diode and capacitor) and a magnetron. The three MOTs draw lots of
power, so I had to hook everything into a thick, direct mains line. The
magnetron itself looks like this:
There are two large magnets that "direct" the electrons as they pass
through the antenna. Also, the device has a heat sink to cool off. There are
many other components and function aspects of the magnetron that are very
complicated, but interesting. If you're curious, check out the information
in this article.
Once finished, the entire apparatus should look something like this:
The waveguide (or metal funnel cone) guides the microwaves in a linear
direction, and allows them to be focused in a specific direction. Once
directed, the microwaves can generate electrical current in any conductive
metal they encounter. How much electricity they generate is determined by
the distance from the magnetron and the power of the output. The microwave
gun will also disrupt wireless communications (depending on their
frequencies) and excite water molecules.
- MICROWAVES ARE VERY VERY DANGEROUS. DO NOT attempt to build this
device unless you are very very confident in your understanding of the
dangers, correct practice of safety, and legal concerns.
- HIGH VOLTAGE! Microwave Transformers can easily kill you! Treat then
with respect! Remember... Fear of Lightning.
- DO NOT use this device on anything or anywhere where it violates FCC
rules or any other legal constraint!
- I am not responsible for any damage, harm, or legal trouble you get
An antenna (plural
antennae or antennas), or aerial, is an electrical device which converts
electric power into radio waves, and vice versa. It is usually used with a
radio transmitter or radio receiver. In transmission, a radio transmitter
supplies an electric current oscillating at radio frequency (i.e. a high
frequency alternating current (AC)) to the antenna's terminals, and the
antenna radiates the energy from the current as electromagnetic waves (radio
waves). In reception, an antenna intercepts some of the power of an
electromagnetic wave in order to produce a tiny voltage at its terminals,
that is applied to a receiver to be amplified.
Antennas are essential components of all equipment that uses radio. They are
used in systems such as radio broadcasting, broadcast television, two-way
radio, communications receivers, radar, cell phones, and satellite
communications, as well as other devices such as garage door openers,
wireless microphones, Bluetooth-enabled devices, wireless computer networks,
baby monitors, and RFID tags on merchandise.
Typically an antenna consists of an arrangement of metallic conductors
(elements), electrically connected (often through a transmission line) to
the receiver or transmitter. An oscillating current of electrons forced
through the antenna by a transmitter will create an oscillating magnetic
field around the antenna elements, while the charge of the electrons also
creates an oscillating electric field along the elements. These time-varying
fields radiate away from the antenna into space as a moving transverse
electromagnetic field wave. Conversely, during reception, the oscillating
electric and magnetic fields of an incoming radio wave exert force on the
electrons in the antenna elements, causing them to move back and forth,
creating oscillating currents in the antenna.
Antennas can be designed to transmit and receive radio waves in all
horizontal directions equally (omnidirectional antennas), or preferentially
in a particular direction (directional or high gain antennas). In the latter
case, an antenna may also include additional elements or surfaces with no
electrical connection to the transmitter or receiver, such as parasitic
elements, parabolic reflectors or horns, which serve to direct the radio
waves into a beam or other desired radiation pattern.
The first antennas were built in 1888 by German physicist Heinrich Hertz in
his pioneering experiments to prove the existence of electromagnetic waves
predicted by the theory of James Clerk Maxwell. Hertz placed dipole antennas
at the focal point of parabolic reflectors for both transmitting and
receiving. He published his work in Annalen der Physik und Chemie (vol. 36,
Animation of a half-wave dipole antenna transmitting radio waves, showing
the electric field lines. The antenna in the center is two vertical metal
rods, with an alternating current applied at its center from a radio
transmitter (not shown). The voltage charges the two sides of the antenna
alternately positive (+) and negative (−). Loops of electric field (black
lines) leave the antenna and travel away at the speed of light; these are
the radio waves.
Animated diagram of a half-wave dipole antenna receiving energy from a
radio wave. The antenna consists of two metal rods connected to a receiver
R. The electric field (E, green arrows) of the incoming wave pushes the
electrons in the rods back and forth, charging the ends alternately positive
(+) and negative (−). Since the length of the antenna is one half the
wavelength of the wave, the oscillating field induces standing waves of
voltage (V, represented by red band) and current in the rods. The
oscillating currents (black arrows) flow down the transmission line and
through the receiver (represented by the resistance R).
Standing waves on a half wave dipole driven at its resonant frequency.
The waves are shown graphically by bars of color (red for voltage, V and
blue for current, I) whose width is proportional to the amplitude of the
quantity at that point on the antenna.
Diagram of the electric fields (blue) and magnetic fields (red) radiated
by a dipole antenna (black rods) during transmission.
1- Quarter-wave whip antenna on an FM radio for 88-108 MHz
2- Rubber Ducky antenna on UHF 446 MHz walkie talkie with rubber cover
3- Rabbit ears half-wave dipole television antenna for VHF channels 54-217
4- Yagi-Uda television antenna for analog channels 2-4, 47-68 MHz
5- Log-periodic antenna covering 140-470 MHz
6- Two-element turnstile antenna for reception of weather satellite data,
137 MHz. Has circular polarization.
7- 108 MHz reflective array antenna of AN-270 radar used during WW2.
8- Reflective array UHF TV antenna, with bowtie dipoles to cover the UHF
470-890 MHz band
9- Ferrite rod receiving antenna from AM radio, 550 - 1600 KHz. The antenna
also serves as the inductor in the tuned circuit for the receiver.
- Quadrant antenna, similar to rhombic, at an Austrian shortwave
broadcast station. Radiates horizontal beam at 5-9 MHz, 100 kW
10- Loop direction finding antenna covers 1.75 - 30 MHz, 6 ft diameter
11-NASA Cassegrain parabolic spacecraft communication antenna, Australia.
Uses X band, 8 – 12 GHz. Extremely high gain ~70 dBi.
12- Microwave horn antenna bandwidth 0.8–18 GHz
13- X band marine radar slot antenna on ship, 8 – 12 GHz.
We know that electric current produces a
magnetic field. We also know that when a conducting loop is moved
through a magnetic field, we have electric current induced in the loop.
Thus time varying electric and magnetic fields produce each other. This
symmetry is very interesting and is one of the most fundamental
observations in physics.
James Clark Maxwell (1831 – 1879) formulated a set of equations to
explain these effects. There are four equations known as Maxwell′s
equations that deal with electric and magnetic fields and their sources
(charge and current densities). Together with the Lorentz force
equation, the Maxwell′s equations give mathematically all the basic laws
The most important outcome of Maxwell′s equation is the presence of
electromagnetic wave. Electromagnetic wave propagates in medium when
there is a time varying electric and magnetic field present and the
speed of propagation is close to the speed of light.
Far reaching conclusion was drawn from this observation – that light
itself is an electromagnetic wave. At the heart of production of
electromagnetic waves is an oscillating electric charge. These
oscillating charges produce an oscillating magnetic field (or flux) and
an oscillating magnetic field, in turn, produces an oscillating electric
Charge density (ρ)
Charge density is a measure of electric charge per unit volume of space,
in one, two or three dimensions. The linear, surface, or volume charge
density is the amount of electric charge per unit length, surface area,
Current density (J)
Current density is a measure of the density of flow of a conserved
charge, in other words flux of the charge.
Electric displacement field (D)
In a dielectric material the presence of an electric field E
causes the bound charges in the material to slightly separate,
inducing a local electric dipole moment. The electric displacement field
D is defined as
D = ε0 E + P
where ε0 is the permittivity of free space, and P
is the (macroscopic) density of the permanent and induced electric
dipole moments in the material, called the polarization density.
Separating the total volume charge density into free and bound charges.
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