Near-field gain of aperture antennas

Formulas for the ratio of received to transmitted power are examined for microwave aperture antennas at any range. It is shown that with optimum aperture illuminations the farfield range equation continues to hold fairly well in the near field down to a distance at which it implies that nearly all the transmitted power is received! However, the aperture illuminations with maximum (nearly 100 per cent) are different from the uniform, constant phase illumination which is optimum in the far-field case. The optimum near-field illuminations not only have the phase variation associated with elliptic rather than parabolic reflectors but they also have some amplitude variations. Some simple illuminations which can be realized practically by lenses and dishes are shown to be sufficiently close to the optimum cases for most practical purposes. A formula for the power density in the near field of a transmitting aperture is also derived and it is shown how to maximize the power flow through any given area of space by design of the transmitting aperture illumination.