Improved bandwidth of microstrip antennas using parasitic elements

A method is described of doubling the bandwidth of rectangular microstrip patch antennas by locating capacitively excited ¿¿m/4 short circuit parasitic elements at their radiating edges. The antenna characteristics are explained in terms of an antiphase mode of a pair of coupled resonators, and it is shown that the bandwidth improvement is independent of the coupling capacitance. Experimental results are presented for both linearly and circularly polarised antennas which support the theoretical predictions.