This paper proposes a monopolar microstrip antenna with symmetric ring-shaped trapezoid ground slots. The center patch facilitates gap-coupling feeding directly connected with a 50 Ohm coaxial line, while six gap-coupled radiators are arranged in a 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 an omnidirectional radiation pattern with broad bandwidth. From the optimization, we have obtained a smaller size and a thinner substrate of the proposed antenna.
