Practical_Antenna_Handbook_0071639586

C h a p t e r 2 0 : M i c r o w a v e W a v e g u i d e s a n d A n t e n n a s 463
and the TM-mode impedance:
Z
TM
f
377 1 ⎛ ⎝ ⎜ ⎞
c
= – ⎟
f ⎠
2
(20.18)
Waveguide Terminations
When an electromagnetic wave propagates along a waveguide, it must eventually
reach the end of the guide. If the end is open, then the wave will propagate into free
space. The horn radiator is an example of an unterminated waveguide. If the waveguide
terminates in a metallic wall, then the wave reflects back down the waveguide,
from whence it came. The interference between incident and reflected waves forms
standing waves (see Chap. 4). Such waves are stationary in space but vary in the time
domain.
In order to prevent standing waves or, more properly, the reflections that give rise
to standing waves, the waveguide must be terminated in a matching impedance. When
a properly designed antenna is used to terminate the waveguide, it forms the matched
load required to prevent reflections. Otherwise, a dummy load must be provided. Figure
20.11 shows several types of dummy load.
The classic termination is shown in Fig. 20.11A. The “resistor” making up the
dummy load is a mixture of sand and graphite. When the fields of the propagated wave
enter the load, they cause currents to flow, which in turn cause heating. Thus, the RF
power dissipates in the sand-graphite mixture rather than being reflected back down
the waveguide.
A second type of dummy load is shown in Fig. 20.11B. The resistor element is a
carbonized rod critically placed at the center of the electric field. The E-field causes currents
to flow, resulting in I 2 R losses that dissipate the power.
Bulk loads, similar to the graphite-sand chamber, are shown in Fig. 20.11C, D, and
E. Using bulk material such as graphite or a carbonized synthetic material, these loads
are used in much the same way as the sand load (i.e., currents set up, and I 2 R losses dissipate
the power).
The resistive vane load is shown in Fig. 20.11F. The plane of the element is orthogonal
to the magnetic lines of force. When the magnetic lines cut across the vane, currents
are induced, which gives rise to the I 2 R losses. Very little RF energy reaches the metallic
end of the waveguide, so there is little reflected energy and a low VSWR.
There are situations where it isn’t desirable to terminate the waveguide in a dummy
load. Several reflective terminations are shown in Fig. 20.12. Perhaps the simplest form
is the permanent end plate shown in Fig. 20.12A. The metal cover must be welded or
otherwise affixed through a very low resistance joint. At the substantial power levels
typically handled in transmitter waveguides, even small resistances can be important.
The end plate (shown in Fig. 20.12B) uses a quarter-wavelength cup to reduce the
effect of joint resistances. The cup places the contact joint at a point that is a quarterwavelength
from the end. This point is a minimum-current node, so I 2 R losses in the
contact resistance become less important.
The adjustable short circuit is shown in Fig. 20.12C. The walls of the waveguide and
the surface of the plunger form a half-wavelength channel. Because the metallic end of
the channel is a short circuit, the impedance reflected back to the front of the plunger is