Mode Theory Approach to Arrays February 1966

Attention is concentrated on the radiating and evanescent modes which must appear, both in free space and in the receiving transmission lines, when an infinite array of radiators is illuminated by a plane wave. If the radiators have a tendency to form a slow-wave structure (as all radiators must, even if to only a very slight degree), then for angles of incidence greater than a certain amount there can be propagating modes (surface waves capable of carrying power) supported by the radiators. These propagating modes are dubbed "internal grating lobes" because they can appear even if the spacing of the radiators is close enough to prohibit the possibility of grating lobes in free space. The main effect of the internal grating lobes is to cause mismatching of the array, thereby deteriorating its scanning performance. It was found possible to demonstrate the existence of internal grating lobes experimentally. The main conclusion is that the simplest types of radiators (slots in a flat sheet or dipoles above a ground plane) are probably best for phased arrays because, of all types considered both practical and imaginable, they have the least slow-wave character.