Due to Ohmic losses on the antenna surface a part of the power supplied to the antenna terminals is lost in heating of the antenna. The full power supplied to the antenna is then not radiated. The efficiency of an antenna is

Effective Aperture

It can be shown by the reciprocity theorem that an antenna has same radiation characteristics while transmitting and receiving. That is

The direction in which an antenna radiates maximally, and the direction from which the antenna receives maximum power are the same.

The antenna does not receive any power from the radiation arriving from the direction of a null.

An antenna maximally responds to that polarization which it generates while transmitting.

Effective aperture of an antenna is a parameter defined for the receiving antenna. It tells the capability of an antenna to tap power from a radiation arriving from certain direction.

If an antenna is placed in a radiation field with power flow density S, and if the power transferred to a matched load connected to the terminals of a receiving antenna under matched polarization condition is , the effective aperture is given as

Effective aperture in general is direction dependent, however when there is no specific mention, it is its maximum value.

The effective aperture has dimensions of area and generally has units .

The Effective aperture and the Directivity of an antenna are related through a relation,

The effective aperture is directly proportional to the directivity. Higher the directivity higher is the effective aperture.

The effective aperture has no direct relation to the physical aperture of an antenna. However, for aperture type antenna like horns, parabolic dishes, the effective aperture is equal to the physical aperture weighted by the aperture field distribution.

Since the directivity of the Hertz dipole is 1.5, its effective aperture is