Analysis of a probe-fed microstrip disk antenna

The input impedance of a probe-fed, annular-ring-loaded, circular-disk microstrip antenna is analysed using three methods of increasing complexity and sophistication. The first method uses a new variational formula which is more suitable for antennas driven by a voltage gap. The second method uses a mode-matching method where the field inside the coaxial line is expanded in terms of waveguide modes. The third method uses the mode-matching method with a diaphragm at the coaxial aperture. The mode-matching methods are identical to the variational formula under the single-mode approximation. However, when a diaphragm is present, the single-mode approximation is insufficient, implying that the variational formula is inaccurate when a diaphragm exists. The theory is used to study the effect of the gap capacitance at the coaxial aperture on the input impedance of the microstrip antenna. It is found that this gap capacitance is important when the dielectric substrate is thick.