4-8
Q5. What is the term used to identify the points of high current and high voltage on an antenna?
Q6. What is the term used to identify the points of minimum current and minimum voltage on an
antenna?
ANTENNA CHARACTERISTICS
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.
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
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 figure 4-7, 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.
