You can define antenna characteristics 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.
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 characteristics 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
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.
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 the picture below 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.
Reciprocity of antennas.
characteristic of a given antenna that remains the same whether the
antenna is used for transmitting or receiving is GAIN. Some antennas are
highly directional that is, more energy is propagated in certain
directions than in others. The ratio between the amount of energy
propagated in these directions compared to the energy that would be
propagated if the antenna were not directional is known as its gain.
When a transmitting antenna with a certain gain is used as a receiving
antenna, it will also have the same gain for receiving.