Capacitive matching of microstrip patch antennas

The input impedance of a thick microstrip patch antenna is inherently inductive. The patch can be matched by using a capacitive annular gap round the feed point. By varying the amount of capacitance the input resistance is also varied. It is shown that input resistances in the range 40 to 120 ohms can be chosen by varying the radius of the annulus. A return loss of -20 dB over a bandwidth of 13% was achieved. An equivalent circuit model is described which can be used as a design aid.