Application of the minimum scattering antenna theory to mismatched antennas

The mutual impedance between canonical minimum scattering (CMS) antennas can be calculated directly from their far-field radiation patterns. While having the potential to greatly simplify array analysis and design, the theory has limited practical applicability because, by definition, CMS antennas are matched to their terminations. It is shown here that the theoretical basis of the mutual coupling calculation does not require the antennas to be matched and that the theory is simply extended to mismatched antennas. The only requirement is that the antennas are invisible, i.e., do not scatter, when open-circuited. This does not impose a greater restriction on the practical applicability of the method, since characteristic mode theory indicates that dipoles and loops, and perhaps other antennas, can be made very nearly invisible by a suitable reactive termination if the frequency is near or below first resonance. A new antenna, consisting of the original antenna plus a particular length of transmission line, will then be almost invisible on open circuit. The theory is applied to calculating the active impedance in an array of small monopole-like elements in a parallel plate waveguide and the results compared with experiment.