A section of flexible waveguide with a pressurizable flange
ield Ex component of the TE31 mode inside an x-band hollow metal waveguide.
A waveguide is a structure that guides waves, such as electromagnetic
waves or sound waves. They enable a signal to propagate with minimal loss of
energy by restricting expansion to one dimension or two. This is a similar
effect to waves of water constrained within a canal, or why guns have barrels
that restrict hot gas expansion to maximize energy transfer to their bullets.
Without the physical constraint of a waveguide, signals will typically dissipate
according to the inverse square law as they expand into three dimensional space.
There are different types of waveguides for each type of wave. The original and
meaningis a hollow conductive metal pipe used to carry high frequency radio
waves, particularly microwaves.The geometry of a waveguide reflects its
function. Slab waveguides confine energy to travel only in one dimension, fiber
or channel waveguides for two dimensions. The frequency of the transmitted wave
also dictates the shape of a waveguide: an optical fiber guiding high-frequency
light will not guide microwaves of a much lower frequency. As a rule of thumb,
the width of a waveguide needs to be of the same order of magnitude as the
wavelength of the guided wave.Some naturally occurring structures can also act
as waveguides. The SOFAR channel layer in the ocean can guide the sound of whale
song across enormous distances.
Example of waveguides and a diplexer in an air traffic control radar
Waves propagate in all directions in open space as spherical waves. The power
of the wave falls with the distance R from the source as the square of the
distance (inverse square law). A waveguide confines the wave to propagate in one
dimension, so that, under ideal conditions, the wave loses no power while
propagating. Due to total reflection at the walls, waves are confined to the
interior of a waveguide. The propagation inside the waveguide, hence, can be
described approximately as a "zigzag" between the walls. This description is
exact for electromagnetic waves in a hollow metal tube with a rectangular or
The first structure for guiding waves was proposed by J. J. Thomson in 1893,
and was first experimentally tested by Oliver Lodge in 1894. The first
mathematical analysis of electromagnetic waves in a metal cylinder was performed
by Lord Rayleigh in 1897.
For sound waves, Lord Rayleigh published a full mathematical analysis of
propagation modes in his seminal work, “The Theory of Sound”.
The study of dielectric waveguides (such as optical fibers, see below) began
as early as the 1920s, by several people, most famous of which are Rayleigh,
Sommerfeld and Debye. Optical fiber began to receive special attention in the 1960s due to its importance to the communications industry.
Waveguide supplying power for the Argonne National Laboratory Advanced Photon
The uses of waveguides for transmitting signals were known even before the
term was coined. The phenomenon of sound waves guided through a taut wire have
been known for a long time, as well as sound through a hollow pipe such as a
cave or medical stethoscope. Other uses of waveguides are in transmitting power
between the components of a system such as radio, radar or optical devices.
Waveguides are the fundamental principle of guided wave testing (GWT), one of
the many methods of non-destructive evaluation.
A propagation mode in a waveguide is one solution of the wave
equations, or, in other words, the form of the wave. .
Due to the constraints of the boundary conditions, there are only limited
frequencies and forms for the wave function which can propagate in the
waveguide. The lowest frequency in which a certain mode can propagate is the
cutoff frequency of that mode. The mode with the lowest cutoff frequency is the
basic mode of the waveguide, and its cutoff frequency is the waveguide cutoff
In circuit theory, the impedance is a generalization of electrical
resistivity in the case of alternating current, and is measured in ohms ().
A waveguide in circuit theory is described by a transmission line having a
length and self-impedance. In other words, the impedance is the resistance of
the circuit component (in this case a waveguide) to the propagation of the wave.
This description of the waveguide was originally intended for alternating
current, but is also suitable for electromagnetic and sound waves, once the wave
and material properties (such as pressure, density, dielectric constant) are
properly converted into electrical terms (current and impedance for example).
Impedance matching is important when components of an electric circuit are
connected (waveguide to antenna for example): The impedance ratio determines how
much of the wave is transmitted forward and how much is reflected. In connecting
a waveguide to an antenna a complete transmission is usually required, so that
their impedances are matched.
The reflection coefficient can be calculated using:
is the reflection coefficient (0 denotes full transmission, 1 full
reflection, and 0.5 is a reflection of half the incoming voltage),
are the impedance of the first component (from which the wave enters) and the second component, respectively.
An impedance mismatch creates a reflected wave, which added to the incoming
waves creates a standing wave. An impedance mismatch can be also quantified with
the standing wave ratio (SWR or VSWR for voltage), which is connected to the impedance ratio and reflection coefficient by:
are the minimum
and maximum values of the voltage absolute value, and the VSWR is the voltage
standing wave ratio, which value of 1 denotes full transmission, without
reflection and thus no standing wave, while very large values mean high
reflection and standing wave pattern.
Waveguides can be constructed to carry waves over a wide portion of the
electromagnetic spectrum, but are especially useful in the microwave and optical
frequency ranges. Depending on the frequency, they can be constructed from
either conductive or dielectric materials. Waveguides are used for transferring
both power and communication signals.
In this military radar, microwave radiation is transmitted between the source
and the reflector by a waveguide. The figure suggests that microwaves leave the
box in a circularly symmetric mode (allowing the antenna to rotate), then they
are converted to a linear mode, and pass through a flexible stage. Their
polarisation is then rotated in a twisted stage and finally they irradiate the
Waveguides used at optical frequencies are typically dielectric waveguides,
structures in which a dielectric material with high permittivity, and thus high
index of refraction, is surrounded by a material with lower permittivity. The
structure guides optical waves by total internal reflection. An example of an
optical waveguide is optical fiber.
Other types of optical waveguide are also used, including photonic-crystal
fiber, which guides waves by any of several distinct mechanisms. Guides in the
form of a hollow tube with a highly reflective inner surface have also been used
as light pipes for illumination applications. The inner surfaces may be polished
metal, or may be covered with a multilayer film that guides light by Bragg
reflection (this is a special case of a photonic-crystal fiber). One can also
use small prisms around the pipe which reflect light via total
internal reflection —such
confinement is necessarily imperfect, however, since total internal reflection
can never truly guide light within a lower-index core (in the prism case, some
light leaks out at the prism corners).
An acoustic waveguide is a physical structure for guiding sound waves. A duct
for sound propagation also behaves like a transmission line. The duct contains
some medium, such as air, that supports sound propagation.
Sound synthesis uses digital delay lines as computational elements to
simulate wave propagation in tubes of wind instruments and the vibrating strings
of string instruments.
ATM manufactures a tremendous selection of Short, Low, Medium, and High Power
Terminations for Rectangular Waveguides that do not require Forced Air Cooling.
Double Ridge Waveguide Terminations are also available.
We offer a complete line of waveguide terminations in all waveguide sizes and
Please call us and discuss your needs with one of our design engineers.
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