Waveguides
A waveguide is a special form of
transmission line consisting of a hollow, metal tube. The tube wall
provides distributed inductance, while the empty space between the tube
walls provide distributed capacitance:
Waveguides are practical only for signals
of extremely high frequency, where the wavelength approaches the
cross-sectional dimensions of the waveguide. Below such frequencies,
waveguides are useless as electrical transmission lines.
When functioning as transmission lines,
though, waveguides are considerably simpler than two-conductor cables --
especially coaxial cables -- in their manufacture and maintenance. With
only a single conductor (the waveguide's "shell"), there are no concerns
with proper conductor-to-conductor spacing, or of the consistency of the
dielectric material, since the only dielectric in a waveguide is air.
Moisture is not as severe a problem in waveguides as it is within
coaxial cables, either, and so waveguides are often spared the necessity
of gas "filling."
Waveguides may be thought of as conduits
for electromagnetic energy, the waveguide itself acting as nothing more
than a "director" of the energy rather than as a signal conductor in the
normal sense of the word. In a sense, all transmission lines function as
conduits of electromagnetic energy when transporting pulses or
high-frequency waves, directing the waves as the banks of a river direct
a tidal wave. However, because waveguides are single-conductor elements,
the propagation of electrical energy down a waveguide is of a very
different nature than the propagation of electrical energy down a
two-conductor transmission line.
All electromagnetic waves consist of
electric and magnetic fields propagating in the same direction of
travel, but perpendicular to each other. Along the length of a normal
transmission line, both electric and magnetic fields are perpendicular
(transverse) to the direction of wave travel. This is known as the
principal mode, or TEM (Transverse Electric and
Magnetic) mode. This mode of wave propagation can exist only
where there are two conductors, and it is the dominant mode of wave
propagation where the cross-sectional dimensions of the transmission
line are small compared to the wavelength of the signal.
At microwave signal frequencies
(between 100 MHz and 300 GHz), two-conductor transmission lines of any
substantial length operating in standard TEM mode become impractical.
Lines small enough in cross-sectional dimension to maintain TEM mode
signal propagation for microwave signals tend to have low voltage
ratings, and suffer from large, parasitic power losses due to conductor
"skin" and dielectric effects. Fortunately, though, at these short
wavelengths there exist other modes of propagation that are not as "lossy,"
if a conductive tube is used rather than two parallel conductors. It is
at these high frequencies that waveguides become practical.
When an electromagnetic wave propagates
down a hollow tube, only one of the fields -- either electric or
magnetic -- will actually be transverse to the wave's direction of
travel. The other field will "loop" longitudinally to the direction of
travel, but still be perpendicular to the other field. Whichever field
remains transverse to the direction of travel determines whether the
wave propagates in TE mode (Transverse Electric) or
TM (Transverse Magnetic) mode.
Many variations of each mode exist for a
given waveguide, and a full discussion of this is subject well beyond
the scope of this book.
Signals are typically introduced to and
extracted from waveguides by means of small antenna-like coupling
devices inserted into the waveguide. Sometimes these coupling elements
take the form of a dipole, which is nothing more than two open-ended
stub wires of appropriate length. Other times, the coupler is a single
stub (a half-dipole, similar in principle to a "whip" antenna, 1/4λ in
physical length), or a short loop of wire terminated on the inside
surface of the waveguide:
In some cases, such as a class of vacuum
tube devices called inductive output tubes (the so-called
klystron tube falls into this category), a "cavity" formed of
conductive material may intercept electromagnetic energy from a
modulated beam of electrons, having no contact with the beam itself:
Just as transmission lines are able to
function as resonant elements in a circuit, especially when terminated
by a short-circuit or an open-circuit, a dead-ended waveguide may also
resonate at particular frequencies. When used as such, the device is
called a cavity resonator. Inductive output tubes use toroid-shaped
cavity resonators to maximize the power transfer efficiency between the
electron beam and the output cable.
A cavity's resonant frequency may be
altered by changing its physical dimensions. To this end, cavities with
movable plates, screws, and other mechanical elements for tuning are
manufactured to provide coarse resonant frequency adjustment.
If a resonant cavity is made open on one
end, it functions as a unidirectional antenna. The following photograph
shows a home-made waveguide formed from a tin can, used as an antenna
for a 2.4 GHz signal in an "802.11b" computer communication network. The
coupling element is a quarter-wave stub: nothing more than a piece of
solid copper wire about 1-1/4 inches in length extending from the center
of a coaxial cable connector penetrating the side of the can:
A few more tin-can antennae may be seen
in the background, one of them a "Pringles" potato chip can. Although
this can is of cardboard (paper) construction, its metallic inner lining
provides the necessary conductivity to function as a waveguide. Some of
the cans in the background still have their plastic lids in place. The
plastic, being nonconductive, does not interfere with the RF signal, but
functions as a physical barrier to prevent rain, snow, dust, and other
physical contaminants from entering the waveguide. "Real" waveguide
antennae use similar barriers to physically enclose the tube, yet allow
electromagnetic energy to pass unimpeded.
- REVIEW:
- Waveguides
are metal tubes functioning as "conduits" for carrying electromagnetic
waves. They are practical only for signals of extremely high
frequency, where the signal wavelength approaches the cross-sectional
dimensions of the waveguide.
- Wave propagation through a waveguide
may be classified into two broad categories: TE (Transverse
Electric), or TM (Transverse Magnetic), depending on which
field (electric or magnetic) is perpendicular (transverse) to the
direction of wave travel. Wave travel along a standard, two-conductor
transmission line is of the TEM (Transverse Electric and
Magnetic) mode, where both fields are oriented perpendicular to the
direction of travel. TEM mode is only possible with two conductors and
cannot exist in a waveguide.
- A dead-ended waveguide serving as a
resonant element in a microwave circuit is called a cavity
resonator.
- A cavity resonator with an open end
functions as a unidirectional antenna, sending or receiving RF energy
to/from the direction of the open end.
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