Definition of unifying decoupling efficiency of different array antennas—Case study of dense focal plane array feed for parabolic reflector

This paper emphasizes the importance of recognizing the major role played by mutual coupling effects in array antennas, especially for dense arrays with element spacing within half a wavelength. For array operation in transmit mode, backward waves emanate both from element ports that are excited by forward waves, and from non-excited ports. This overall and excitation-dependent mismatch is herein characterized in terms of a decoupling efficiency. It is shown how this decoupling efficiency reduces to the well-known impedance mismatch factor for a conventional phase-steered array (all elements excited), and to the embedded element efficiency (only one element excited) used to characterize multiple input and multiple output (MIMO) arrays. Generally, this decoupling efficiency can be used to characterize the combined effect of mismatch and coupling for any excitation of an array, for instance, the Airy pattern excitation of focal plane array (FPA) feeds for reflectors, as showcased here. The abovementioned forms of decoupling efficiencies are evaluated and discussed for an example array of idealized rectangular apertures with special emphasis on the Airy excitation.