The effect of feed on the input impedance of a microstrip antenna

The input impedance of a probe‐fed, annular‐ring‐loaded, circular‐disk microstrip antenna is analyzed using three methods of increasing complexity. The first method uses a new variational formula. The second method uses a mode‐matching method. 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. When a diaphragm is present, the singl‐mode approximation is insufficient, implying the variational formula is inaccurate when a diaphragm exists. The theory shows that the effect of the gap capacitance at the coaxial aperture on the input impedance of the microstrip antenna is important when the dielectric substrate is thick. The theories agree well with experiment data with no fudge factors.