TC 9-64 _________________________________________________________________________
4-30. You can define an antenna as a conductor or group of conductors used
either for radiating electromagnetic energy into space or for collecting it from
space. Electrical energy from the transmitter is converted into
electromagnetic energy by the antenna and radiated into space. On the
receiving end, electromagnetic energy is converted into electrical energy by
the antenna and is fed into the receiver.
4-31. Fortunately, separate antennas seldom are required for both
transmitting and receiving RF energy. Any antenna can transfer energy from
space to its input receiver with the same efficiency that it transfers energy
from the transmitter into space. Of course, this is assuming that the same
frequency is used in both cases. This property of interchangeability of the
same antenna for transmitting and receiving is known as antenna
reciprocity. Antenna reciprocity is possible because antenna characteristics
are essentially the same for sending and receiving electromagnetic energy.
RECIPROCITY OF ANTENNAS
4-32. In general, the various properties of an antenna apply equally,
regardless of whether you use the antenna for transmitting or receiving. The
more efficient a certain antenna is for transmitting, the more efficient it will
be for receiving on the same frequency. Likewise, the directive properties of a
given antenna also will be the same whether it is used for transmitting or
receiving.
4-33. Assume, for example, that a certain antenna used with a transmitter
radiates a maximum amount of energy at right angles to the axis of the
antenna, as shown in figure 4-8, view A. Note the minimum amount of
radiation along the axis of the antenna. Now, if this same antenna were used
as a receiving antenna, as shown in view B, it would receive best in the same
directions in which it produced maximum radiation; that is, at right angles to
the axis of the antenna.
4-10
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