Antenna Handbook MCRP 3-40.3C (Formerly MCRP 6-22D)

Antenna Handbook

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2-13

If an antenna is located within a congested urban environment or within a building, there are many objects which will scatter or reraditate the energy in a manner that can be detrimental to reception.

For example, the electric wiring inside a building can strongly reradiate RF energy. If a receiving antenna is in close proximity to wires, it is possible for the reflected energy to cancel the energy received directly from the desired signal path. When this condition exists, the receiving antenna should be moved to another location within the room where the reflected and direct signals may reinforce rather than cancel each other.

RECIPROCITY

The various properties of an antenna apply equally, regardless of whether the antenna is used for transmitting or receiving. This is what is meant by reciprocity of antennas. For example, the more efficient a certain antenna is for transmitting, the more efficient it will be for receiving the same frequency. The directive properties of a given antenna will be the same whether it is used for transmission or reception.

For example, figure 2-6 on page 2-14 shows a particular antenna used with a transmitter radiating a maximum amount of energy at right angles to the antenna wire. There is a minimum amount of radiation along the axis of the antenna. If this same antenna is used as a receiving antenna, it receives best in the same directions in which it produced maximum radiation (i.e., at right angles to the axis of the antenna). There is a minimum amount of signal received from transmitters located in line with the antenna wire.