A novel monopolar microstrip antenna with symmetric ring-shaped trapezoid ground slots is proposed in this article. The center patch facilitates gap-coupling feeding directly connected with 50 Ohm coaxial line, while six gap-coupled radiators arranged in quasi-circle configuration. The trapezoid ground-slots beneath the six radiators serve to adjust the impedance bandwidth and reduce the overall antenna size. The proposed antenna exhibits good omni-directional radiation pattern with broad bandwidth. Consequently, the antenna supports degenerated non-fundamental TM02 and TM31 modes and demonstrates an impressive impedance bandwidth of 760 MHz from 5.44 GHz to 6.2 GHz with respect to 13% fractional bandwidth. Compared to recent monopolar microstrip antenna research, the proposed antenna shows a smaller size and a thinner substrate. This outcome makes the proposed antenna as ultra-low-profile, and our design stands out by providing a comparable bandwidth.
