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