Experimental comparison of the radiation efficiency for conventional and cavity backed microstrip antennas

The radiation efficiency of conventional microstrip antennas generally decreases when the substrate thickness or permittivity is increased because of loss to surface waves. However, constructing a metal cavity around the microstrip antenna prevents the surface wave propagation. Thus, the cavity backed microstrip antenna has been predicted to have increased radiation efficiency (Zavosh and Aberle 1995). In this paper, we compare conventional and cavity backed microstrip patch antennas on substrates with an electrical thickness of 0.067 /spl lambda//sub 0/ and dielectric constants of /spl epsi/r=2.94, 6.15, and 10.2. As one would expect, the radiation efficiency of the conventional patch decreases with increasing dielectric constant, but the efficiency remains relatively constant for the cavity backed patch. In this work, three different methods are used to measure the radiation efficiencies: a far field gain comparison, a Wheeler cap method and an input admittance method.