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A wideband omnidirectional patch antenna with filtering response is investigated in this paper. The circular patch antenna is centrally fed by a probe, with its TM₀₁, TM₀₂, and TM₀₃ modes simultaneously excited. Two ring slots are meticulously introduced into the patch and ground plane to merge the three modes of a kind, realizing a wide omnidirectionally radiating passband. The introduction of the slots also generates two radiation minimums at the lower and upper band-edges, leading to an enhanced suppression level in the stopbands. Hence, a compact wideband filtering patch antenna with quasi-elliptic bandpass response is obtained, without using any extra circuit. The proposed antenna has a low profile of 0.027 λ₀, a 10-dB impedance bandwidth of 19.5%, an average gain of 7.5 dBi within passband, and an out-of-band suppression level of over 23 dB within stopband.