The effect of the aperture ratio on the impedance bandwidth of an aperture coupled microstrip antenna (ACMA) is investigated according to the coupling strength from a microstrip feed line to a patch. Since the coupling strength between a feed line and a patch of an ACMA with a high permittivity (εr = 10) feed substrate is small, the impedance bandwidth of the ACMA increases as the aperture ratio increases beyond 0.1. As the feed substrate thickness increases, the aperture ratio for the maximum impedance bandwidth increases and the ratio of the maximum impedance bandwidth to the impedance bandwidth obtained at the aperture ratio of 0.1 increases. Since the coupling strength between a feed line and a patch of an ACMA with a low permittivity (εr = 2.2) feed substrate is large, the maximum impedance bandwidth of the ACMA is approximately the same as the impedance bandwidth obtained at the aperture ratio of 0.1.