and electric transverse planes and forms an equivalent
parallel-LC circuit across the waveguide. At the
resonant frequency, the iris acts as a high shunt
resistance. Above or below resonance, the iris acts
as a capacitive or inductive reactance.
POSTS and SCREWS made from conductive
material can be used for impedance-changing devices
in waveguides.
Views A and B of figure 3-45,
illustrate two basic methods of using posts and screws.
A post or screw that only partially penetrates into the
waveguide acts as a shunt capacitive reactance. When
the post or screw extends completely through the
waveguide, making contact with the top and bottom
walls, it acts as an inductive reactance. Note that when
screws are used, the amount of reactance can be varied.
Figure 3-45.—Conducting posts and screws.
WAVEGUIDE TERMINATIONS
Electromagnetic energy is often passed through
a waveguide to transfer the energy from a source into
space. As previously mentioned, the impedance of
a waveguide does not match the impedance of space,
and without proper impedance matching standing waves
cause a large decrease in the efficiency of the
waveguide.
Any abrupt change in impedance causes standing
waves, but when the change in impedance at the end
of a waveguide is gradual, almost no standing waves
are formed. Gradual changes in impedance can be
obtained by terminating the waveguide with a
funnel-shaped HORN, such as the three types illustrated
in figure 3-46. The type of horn used depends upon
the frequency and the desired radiation pattern.
Figure 3-46.—Waveguide horns.
As you may have noticed, horns are really simple
antennas. They have several advantages over other
impedance-matching devices, such as their large
bandwidth and simple construction.
A waveguide may also be terminated in a resistive
load that is matched to the characteristic impedance
of the waveguide. The resistive load is most often
called a DUMMY LOAD, because its only purpose
is to absorb all the energy in a waveguide without
causing standing waves.
There is no place on a waveguide to connect a
fixed termination resistor; therefore, several special
arrangements are used to terminate waveguides. One
method is to fill the end of the waveguide with a
graphite and sand mixture, as illustrated in figure 3-47,
view A.
When the fields enter the mixture, they
induce a current flow in the mixture that dissipates
the energy as heat. Another method (view B) is to
use a high-resistance rod placed at the center of the
E field. The E field causes current to flow in the rod,
and the high resistance of the rod dissipates the energy
as a power loss, again in the form of heat.
Still another method for terminating a waveguide
is the use of a wedge of highly resistive material, as
shown in view C of figure 3-47. The plane of the
wedge is placed perpendicular to the magnetic lines
3-21
