A wideband, compact monopolar patch antenna with an inherent filtering response is presented in this paper. By employing a slot-assisted mode-tailoring technique, the field distributions of several higher-order modes at the patch center are engineered to produce a stable monopolar radiation pattern with enhanced bandwidth. The closed-form analysis indicates that increasing the current intensity at the central region can transform the conventional TM22 mode into one that exhibits a monopolar pattern. This concept is realized by incorporating four slanted slots at the corners of the patch, which constrain the surface current toward the patch center. Besides, these slots also excite an additional slot mode, which serves as a second resonance. At higher frequencies, the slots continue to confine the current distribution of the original TM33 mode to a small central region and thereby forming a quasi-TM01 monopolar radiation. By combining the three engineered resonances, the proposed single-layer patch achieves a significantly improved bandwidth. (The simulated and measured impedance bandwidths (S11 < -10 dB) are 26.9% and 27.1% respectively). More importantly, the proposed antenna preserves a stable radiation pattern even when the ground-plane size changes. This robustness arises from the slot-assisted mode-tailoring scheme, which strongly confines the current distribution to the central region of the patch. The small-footprint prototype realizes peak gains of 6.92 dBi in simulation and 6.42 dBi in measurement. To ensure the application of the proposed antenna on a vehicle, a large-scale simulation has been performed to verify the wide impedance bandwidth, realized gain and consistent radiation patterns when the antenna is positioned on the roof of a car. While good in-band results are obtained. Radiation nulls are observed near the band edges, resulting from the mutual cancellation of radiated fields. With its broad bandwidth, stable monopolar radiation, and built-in filtering (stopband gain not exceeding 0 dBi), the proposed antenna offers an attractive solution for modern wireless platforms demanding compact form factors and robust interference immunity.
Li et al. (Thu,) studied this question.