Design of a Low Profile and Compact Omnidirectional Filtering Patch Antenna

A low profile omnidirectional patch antenna with filtering response is investigated in this paper. The triangular patch antenna is axially fed by a probe at its center, exciting both its TM₁₀ and TM₁₁ modes. Comparing with the traditional circular patch, the triangular patch can not only minimize the patch size but also can generate a radiation null at the upper band edge. A ring slot and a series of shorting vias are introduced into the patch to merge the two modes, enhancing the bandwidth of the passband. The combination of the two elements simultaneously generates a radiation null at the lower band edge. Consequently, a compact filtering patch antenna with quasi-elliptic bandpass response is obtained without involving specific filtering circuits. The prototype with profile of $0.03 λ₀ has a 10-dB impedance bandwidth of 8.9% (4.3-4.7 GHz), an average gain of 6.0 dBi within passband, an out-of-band suppression level of more than 30 dB within lower stopband (0-3.6 GHz), and more than 20 dB within upper stopband (5.1-6.3 GHz).