A Broadband Impedance-Matching Method for Microstrip Patch Antennas Based on the Bode-Fano Theory

Considering the narrow bandwidth of microstrip antennas, but also their applicability in upcoming technologies, this paper addresses the problem of wide-band matching, the theoretical bounds on the matching bandwidth and low-cost and low-complexity matching strategies. In this context the Bode-Fano bounds of single mode, linearly polarized aperture-coupled microstrip antennas is evaluated, optimized and compared to the theoretical bounds on matching bandwidth of other common feeding technologies. A detailed study of the input impedance of aperture-coupled patch antennas shows how to widen the Fano bounds. Based on this, a straight-forward and effective method to optimize the Fano bound is given. After optimization of the antennas input impedance, basic matching techniques can be applied, to exploit the enlarged bandwidth potential. As an example a -transformer as matching element is proposed. Design equations and simulation and measurement results of X-band prototypes are given as verification.