Theory and Experiment on Stacked Circular Microstrip Patch Antennas for Low-Coupling Array Design

Abstract

Based on the cavity model and Greenberger&#x0027;s addition theorem, a theory is formulated to analyze stacked circular microstrip patch antennas (S-CMPAs) for low-coupling array design. Expressions for electric fields are given. In general, the calculated electric fields agree with those simulated. Our theoretical analysis demonstrates that if the electric fields <i>E_x</i> excited by the stacked circular patches completely cancel each other out, the S-CMPA array will exhibit the self-decoupling characteristic. The vector fitting technique is adopted to extract the zeros and poles of the <i>S</i>₂₁ curve to get a deeper insight into the mutual coupling between the two S-CMPA elements. The zeros are desirable and account for self-decoupling characteristics. For experimental validation, a two-element S-CMPA array was designed, fabricated, and measured. Results show that both elements achieve a wide impedance bandwidth of 14&#x0025; (3.32&#x2013;3.82 GHz) and a low coupling level of less than &#x2212;25 dB between the two elements for the free-space half-wavelength spacing at the center frequency. In addition, the designed antenna is extended into a four-element linear antenna array, which still maintains broad impedance bandwidth and low coupling levels.