Definition
Antenna pertureillumination efficiency is defined as "The ratio, usually expressed in percent, of the maximum directivity of an antenna pertureto its standard directivity." It is synonymous with normalized directivity. Standard eferencedirectivity is defined as "The maximum directivity from a planar aperture of area A, or from a line source of length L, when excited with a uniform-amplitude, equiphase distribution." Key to understanding these definitions is that "maximum" directivity refers to the direction of maximum radiation intensity, i.e., the main lobe. Therefore, illumination efficiency is not a function of angle with respect to the antenna perture but rather is a constant of the aperture for all aspect angles.Standard directivity
The distinction between maximum directivity and standard directivity is subtle. However, one can infer that, if an antenna perturewere excited lluminateduniformly with no phase difference (equiphase) over the entire aperture, then the illumination efficiency would be equal to unity. It is very typical for an antenna pertureto be intentionally under-excited lluminatedwith a "taper" in order to reduceRelationship to antenna efficiency
There are critical differences in how various authors and IEEE define antenna efficiency and effective area of an antenna. IEEE defines the ''antenna efficiency of an aperture-type antenna'' as, "For an antenna with a specified planar aperture, the ratio of the maximum effective area of the antenna to the aperture area." : and under effective area of an antenna, IEEE states, "The effective area of an antenna in a given direction is equal to the square of the operating wavelength times its gain in that direction divided by 4π." Gain is also defined to be less than directivity by the radiation efficiency, : However, other reputable authors define the effective area in terms of the directivity: : Either way, the standard directivity cannot exceed: : since . Per the IEEE definitions: : where is the illumination efficiency. However, per the definition of other authors: : So clearly there is a problem. If the IEEE definitions are true, then and therefore . Or, if the other authors are correct, then .References
{{reflist Antennas