A thorough investigation of the effects of the ground plane size and the cavity structure on the characteristics of microstrip patch antennas built on a thin, low-permittivity substrate with an air gap was studied. Two microstrip patch antenna were chosen to study: the square microstrip patch antenna (narrow bandwidth) and the half E-shaped patch antenna (broad bandwidth). The studied antenna’s characteristics included the input impedance, gain, HPBW and the radiation patterns of the antennas. The results show that the gain in general was a periodic function of ground plane size, and the HPBW and radiation patterns of the antennas change in accordance with the variation in gain. The addition of the cavity structure can suppress the propagation of the surfaces waves and improve the gain of the antennas. The gain enhancement of 3 dB was accomplished and the maximum gain of 12 dB can be reached. The antennas with a folded cavity structure were also studied. It was demonstrated that by folding the metal plate of the cavity structure, the characteristics of the microstrip patch antennas did not change much whereas the height of the microstrip patch antennas can be reduced much.
