Resonant resistance of probe- and microstrip-line-fed circular microstrip patches

The input resistance at resonance of the microstrip antenna is an important parameter to be determined to design the feed and its location for achieving the optimum performance. The authors propose an improved design formula for circular microstrip to meet this requirement for both probe- and microstrip-line-fed designs. A modified admittance boundary has been applied to improve the cavity model formulation available in the open literature and the computed results are verified with several measurements showing very close approximation compared to other methods. The generalised formula is applicable to designing circular microstrip antennas with and without an airgap between the substrate and the groundplane, where the feeding can be realised by a coaxial probe or by planar microstrip line fed at the edge.