In electromagnetics and antenna theory, the aperture of an antenna is defined as "A surface, near or on an antenna, on which it is convenient to make assumptions regarding the field values for the purpose of computing fields at external points. The aperture is often taken as that portion of a plane surface near the antenna, perpendicular to the direction of maximum radiation, through which the major part of the radiation passes."
Effective area
The effective area of an antenna is defined as "In a given direction, the ratio of the available power at the terminals of a receiving antenna to the power flux density of a plane wave incident on the antenna from that direction, the wave being polarization matched to the antenna." Of particular note in this definition is that both effective area and power flux density are functions of incident angle of a plane wave. Assume a plane wave from a particular direction , which are the azimuth and elevation angles relative to the array normal, has a power flux density ; this is the amount of power passing through a unit area normal to the direction of the plane wave of one square meter.
By definition, if an antenna delivers watts to the transmission line connected to its output terminals when irradiated by a uniform field of power density watts per square meter, the antenna's effective area for the direction of that plane wave is given by
The power accepted by the antenna (the power at the antenna terminals) is less than the power received by an antenna by the radiation efficiency of the antenna. is equal to the power density of the electromagnetic energy , where is the unit vector normal to the array aperture, multiplied by the physical aperture area . The incoming radiation is assumed to have the same polarization as the antenna. Therefore,
and
The effective area of an antenna or aperture is based upon a receiving antenna. However, due to reciprocity, an antenna's directivity in receiving and transmitting are identical, so the power transmitted by an antenna in different directions (the radiation pattern) is also proportional to the effective area . When no direction is specified, is understood to refer to its maximal value.
Effective length
Most antenna designs are not defined by a physical area but consist of wires or thin rods; then the effective aperture bears no clear relation to the size or area of the antenna. An alternate measure of antenna response that has a greater relationship to the physical length of such antennas is effective length measured in metres, which is defined for a receiving antenna as
where
is the open-circuit voltage appearing across the antenna's terminals, is the electric field strength of the radio signal, in volts per metre, at the antenna.
The longer the effective length, the greater is the voltage appearing at its terminals. However, the actual power implied by that voltage depends on the antenna's feedpoint impedance, so this cannot be directly related to antenna gain, which is a measure of received power (but does not directly specify voltage or current). For instance, a half-wave dipole has a much longer effective length than a short dipole. However the effective area of the short dipole is almost as great as it is for the half-wave antenna, since (ideally), given an ideal impedance-matching network, it can receive almost as much power from that wave. Note that for a given antenna feedpoint impedance, an antenna's gain or increases according to the square of , so that the effective length for an antenna relative to different wave directions follows the square root of the gain in those directions. But since changing the physical size of an antenna inevitably changes the impedance (often by a great factor), the effective length is not by itself a useful figure of merit for describing an antenna's peak directivity and is more of theoretical importance.