Theory and experiment on microstrip antennas with airgaps

In a microstrip antenna, the resonant frequency of a particular mode is determined by the shape and size of the conducting patch, the relative permittivity of the substrate and, to some extent, the thickness of the substrate. If the resonant frequency is to be changed, a new antenna is usually needed. Recently, Lee and Dahele have shown that the resonant frequencies of microstrip antennas can be changed by introducing an adjustable airgap between the substrate and the ground plane. In addition to providing a means for tuning the resonant frequencies, the airgap also has the effect of increasing the bandwidth of the antenna. The purpose of the paper is to present a comprehensive report on our research on microstrip antennas with airgaps. First, experimental results obtained by introducing an airgap in (i) circular-discs, (ii) annular-rings and (iii) dual-frequency stacked-disc microstrip antennas are presented. Secondly, the theories that have been developed are described. Thirdly, comparison between theoretical and experimental results is given.