Design, fabrication, and measurement of a compact filtering microstrip antenna with second-order quasi-elliptic broadside antenna gain response are presented. A U-shape radiating patch is excited by a T-shape resonator through an inset coupling structure. The U-shape patch acts as a radiator as well as the last stage of the filter, and the inset coupling structure can be treated as the admittance inverter in filter design. The design procedure follows the circuit approach-synthesis of bandpass filters. The broadside gain of the filtering antenna has two poles in passband and two broadside radiation nulls (zeros) at the band edges for improving selectivity. Compared to the conventional inset-fed microstrip antenna, with a little extra circuit area, the proposed filtering antenna has a flatter passband response, better frequency skirt selectivity, and almost twice wider bandwidth. The measurement result shows a good agreement with the simulations.