Effect of microstrip antenna substrate thickness and permittivity: comparison of theories with experiment

A set of measurements of patch antennas on substrates of varying thickness and permittivity, using three types of feeds, is presented. Resonant frequency and resonant resistance are compared with calculated values from the representative theories. The measurements show that erratic results may be obtained for substrates thicker than about 0.02 λ₀, where λ₀ is the free-space wavelength at the resonant frequency. These problems may be related to coax-to-microstripline transitions, but their cause is not yet definitely known. The theoretical models that are compared give reasonably good results for resonant frequency, but none give impedance results that are consistently reliable. It thus appears that further work is needed to develop analytical models that can adequately predict the impedance performance of microstrip antennas on thick and/or high dielectric constant substrates