Mutual Coupling and Chassis-Mode Coupling in Small Phased Array on a Small Ground Plane

Feed point impedance and mutual coupling of monopole array elements on a small ground plane are found to depend critically on the ground plane size when one length dimension comes near to half-wavelength. This is demonstrated to be due to the excitation of the lowest order chassis mode, which acts as an additional radiator element parasitically coupled to the array elements. A model is given in terms of impedance matrix description of the array including the effect of excited chassis mode. Important consequences of the chassis mode excitation and coupling are formulated for the characteristics of phased arrays and for diversity - and MIMO concepts.