# Antenna Theory - Reciprocity

An antenna can be used as both transmitting antenna and receiving antenna. While using so, we may come across a question whether the properties of the antenna might change as its operating mode is changed. Fortunately, we need not worry about that. The properties of antenna being unchangeable is called as the property of reciprocity.

## Properties under Reciprocity

The properties of transmitting and receiving antenna that exhibit the reciprocity are −

• Equality of Directional patterns.
• Equality of Directivities.
• Equality of Effective lengths.
• Equality of Antenna impedances.

Let us see how these are implemented.

### Equality of Directional patterns

The radiation pattern of transmitting antenna1, which transmits to the receiving antenna2 is equal to the radiation pattern of antenna2, if it transmits and antenna1 receives the signal.

### Equality of Directivities

Directivity is same for both transmitting and receiving antennas, if the value of directivity is same for both the cases i.e. the directivities are same whether calculated from transmitting antenna’s power or receiving antenna’s power.

### Equality of Effective lengths

The value of maximum effective aperture is same for both transmitting and receiving antennas. Equality in the lengths of both transmitting and receiving antennas is maintained according to the value of the wavelength.

### Equality in Antenna Impedances

The output impedance of a transmitting antenna and the input impedance of a receiving antenna are equal in an effective communication.

These properties will not change though the same antenna is operated as a transmitter or as a receiver. Hence, the property of reciprocity is followed.