SECTION II. ANTENNA FEED SYSTEMS
LEARNING EVENT 6: ANTENNAS
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 communications 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.
LEARNING EVENT 7: WAVEGUIDE RADIATOR
1. 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 2-6. The energy propagated
to the open end will encounter an impedance mismatch between the waveguide and space. Part of the energy
will be radiated into space and part will be reflected back into the waveguide because of the impedance
mismatch. The reflected energy will cause standing waves in the waveguide.
Figure 2-6. Waveguide radiator.
2. Despite this loss of efficiency, the waveguide radiator is sometimes used to radiate energy into space. The
waveguide opening is an aperture, and the size and shape of this aperture determines the polar distribution and
gain of the radiator. Because the waveguide radiator is the open end of the waveguide, the aperture dimensions
are the waveguide dimensions.
2-7
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