Broadband Tightly Coupled Patch Antenna Using Characteristic Mode Analysis

This paper presents a $2 \times 2$ differential-fed antenna array with dimensions of $25.04 \mathrm{~mm} \times 19.48 \mathrm{~mm} \times 1.43 \mathrm{~mm}\left(0.81 \lambda_{0}\right.$ $\times 0.63 \lambda_{0} \times 0.046 \lambda_{0}$, where $\lambda_{0}$ is the wavelength at the center frequency). The design is low-profile, with element spacing less than $0.37 \lambda_{0}$, and features tightly coupled patch elements surrounded by parasitic mushroom-type structures. The antenna achieves an $\left|S_{11}\right|{\lt}-10 \mathrm{~dB}$ bandwidth from 8.0 to 11.35 GHz, covering the X-band (8 to 12 GHz) with a relative bandwidth of 34.6%-the in-band gain ranges from 5.76 dBi to 7.87 dBi. Characteristic Mode Analysis (CMA) was used to analyze and optimize performance, validating the design’s effectiveness.