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‫ﺗﻤﺮﯾﻦ ﺳﺮي ﭼﻬﺎرم درس زﺑﺎن ﺗﺨﺼﺼﯽ‬
Waveguides
Any system of wires may be used as a transmission line, but the
simplest arrangements are invariably preferred in practice. Thus
parallel-wire and coaxial lines are by far the most common. In a
similar way, a pipe with any sort of cross section could be used as a
waveguide, but the simplest cross sections are preferred.
Accordingly, waveguides with constant rectangular or circular cross
sections are normally employed, although other shapes may be used
from time to time for special purposes. As with regular transmission
lines, so in waveguides, the simplest shapes are the ones easiest to
manufacture, and the ones whose properties are simplest to evaluate.
A rectangular waveguide is shown in Figure 1, as is a circular
waveguide for comparison. In a typical setup, there may be an
antenna at one end of a waveguide and some form of load at the
other end. The antenna generates electromagnetic waves, which
travel down the waveguide to be eventually received by the load. It
is seen that the waves are truly guided.
Figure 1. Waveguides: (a) Rectangular; (b) Circular.
The walls of the guide are conductors, and therefore
reflections from them take place. It is of the utmost importance
to realize that conduction of energy takes place not through the
walls, whose function is only to confine this energy, but through
the dielectric filling the waveguide, which is usually air. In
discussing the behavior and properties of waveguides, it is
necessary to speak of electric and magnetic fields, as in wave
propagation, instead of voltages and currents, as in transmission
lines. This is the only possible approach, but it does make the
behavior of waveguides more complex to grasp.
Because the cross-sectional dimensions of a waveguide must be
or the same order as those of a wavelength, use at frequencies
below about 1 GHz is not normally considered, unless special
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circumstances warrant it.
Both waveguides and transmission lines can pass several
signals simultaneously, but in waveguides it is sufficient for
them to be propagated in different modes to be separated. They
do not have to be of different frequencies. Again, a number of
waveguide components are similar if not identical to their
coaxial counterparts. These components include stubs, quarterwave transformers, directional couplers and taper sections.
Indeed, the operation of a very large number of waveguide
components may best be understood by first looking at their
transmission-line equivalents.
A major problem with twin-lead transmission lines at higher
frequencies is that the amount of direct radiation from such
lines increases with the frequency of the signal being
transmitted. The result is that a twin-lead transmission line
radiates virtually all of the energy it is carrying, and transmits
little, if any, to a load at frequencies above several hundred
megahertz. The problem of energy loss due to radiation is
almost totally eliminated with coaxial lines and waveguides
because these forms of transmission lines 'enclose' the signal
and prevent its radiation.
A second source of energy loss for both parallel-lead and
coaxial transmission lines, is in the dielectric that supports the
separation of conductors. This is called dielectric loss.
Although, theoretically, there is no current flow in an insulator,
a dielectric, there is some current flow in actual, practical
dielectrics, and there is dissipation. Of course, this dissipation is
extremely small. However, it increases with frequency. Again,
at very high frequencies, an energy loss becomes consequential.
Because waveguides are completely hollow and in most cases
filled with air, dielectric loss is virtually nil.
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A third form of energy loss in transmission lines is in the I R
heating of the conductors of the line. Heating or 'copper' loss is
directly proportional to the resistance of a conductor, for a
given current. And the resistance of conductors of RF energy
increases with frequency! This is the result of the phenomenon
called 'skin effect'. As the frequency of current increases, it
'travels' more and more on the surface of a conductor. The
penetration of the disturbance of electron movement becomes
shallower. This means that a smaller cross section of a
conductor is utilized for current flow. And the consequence of
that, in turn, is an increase in the resistance of the conductor
since resistance is inversely proportional to cross-sectional area.
Coaxial lines represent some improvement over parallel-wire lines
in the matter of heating loss since the conduction area of the outer
conductor is significantly larger than that of the inner conductor.
However, a waveguide has a major advantage in this regard: the
inner (or one) conductor is completely eliminated; and the
conduction area of the inner surface of the guide is significantly
larger than that of the coaxial line.
The use of waveguides is not all gravy. In comparison with
other forms of transmission lines, they are difficult and expensive
to install. The skills required for installation are more like those of
a plumber than of an electronics technician, or even of an
electrician. Waveguides, in most instances, are rigid devices. Their
routing must be carefully planned. Joints or connection points must
be carefully made to avoid discontinuities in the inner, reflecting
surfaces and the consequent creation of standing waves. Other
forms of transmission lines are relatively flexible and can simply
be unrolled and positioned to conform to almost any surface
contour.
Questions:
1- Why are waveguides not normally used at frequencies below 1 GHZ?
2- What similarities are there between transmission lines and
waveguides?
3- What are the disadvantages of waveguides over other transmission
lines?
4- What are the skills required for waveguide installation similar to?
5- What difficulty might arise from rigid-waveguide installation?