2-21.
ISOLATORS
Isolators are used to minimize reflections in waveguides. One type of isolator is shown in figure 94. This
circulator-isolator is basically a four-port isolator, consisting of a folded magic tee, two 90 nonreciprocal phase
shifters, a 3-db short-slot hybrid, and two terminating loads.
a. Detail B of figure 94 illustrates the energy path from the transmitter. The energy enters the magic tee
at port 1 and is then split into two equal and in-phase components, A and B. The split signal is fed through the
two 90, nonreciprocal ferrite phase shifters, where component A is retarded 90 relative to component B. These
two components are recombined in the 3-db short-slot hybrid. Since the two components are of equal amplitude
and differ in phase by 90 (component A lagging), they add to give an output at port 2.
b. Detail C illustrates the reflected energy path. Reflected energy enters the isolator at the 3-db short-
slot hybrid at port 2 and is divided into two equal amplitude components, A and B. Component B, however, lags
A by 90,. These two components propagate through the 90 nonreciprocal ferrite phase shifters, where B is
delayed by an additional 90 (relative to A), thereby making a total differential phase shift of 180. The two
components are recombined in the folded magic tee. Since the two components are of equal amplitude and differ
in phase by 180, they add to give an output at port 3. The magic tee prevents the out-of-phase energy from being
propagated through port 1. The energy from port 3 is then absorbed in a load. Thus, it can be seen that the
circulator-isolator permits the transmitted energy to be propagated with very slight attenuation, but absorbs
antenna reflections in a load (where the reflected energy is dissipated as heat).
2-22.
POLARIZERS
A polarizer is an iris-loaded section of circular waveguide that is used to change the polarization of the
transmitted and received signals in certain microwave applications.
Section IV. ANTENNA FEED SYSTEMS
2-23.
GENERAL
An antenna is used either for sending electromagnetic energy into space or for collecting electromagnetic
energy from space. Fortunately, separate antennas are not required for communication equipment to transmit and
receive electromagnetic energy. Any antenna will receive energy from space with the same efficiency with which
it transfers energy into space. Because of this property, known as reciprocity, this discussion will treat antennas
from the viewpoint of the transmitting antenna. The same principles apply when the antennas are used for
receiving electromagnetic energy.
2-24.
WAVEGUIDE RADIATOR
a. When electromagnetic energy is to be radiated into space, the efficiency of the radiator is a major
consideration. Suppose the waveguide is left open on one end, as shown in figure 95. The energy propagated to
the open end
344 L2
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