A Low-Profile Half-Mode Annular Microstrip Antenna with Pattern Diversity

A low-profile microstrip antenna with pattern diversity is proposed in this letter. By setting the inner boundary of an annular microstrip patch as short-circuit, a half-mode annular ring antenna is obtained, whose two resonant modes, i.e., TM11 and TM01 modes, with different radiation patterns can degenerate. Besides, the shorting boundary also contributes a low coupling between the antenna and the other circuits in the center of the annular ring. The antenna is designed based on characteristic equations of the cavity model and also verified by characteristic mode analysis. The final prototype has three ports, representing two orthogonal broadside patterns and a conical pattern. All the ports can cover the 5.8 GHz Industrial, Scientific, and Medical (ISM) band. Isolations between all the ports are higher than 15 dB, and the envelope correlation coefficients are lower than 0.08, which ensure an excellent diversity performance. Numerical and experimental results agree well and prove the function of the proposed design. Due to the simple topology and the competitive performance, the proposed antenna will be a suitable candidate for ISM applications in a wireless local area network.