A compact, quasi-TEM horn antenna is presented. This planar-monopole-fed, surface-mounted, UWB antenna has nearly constant gain of 4.8 ± 0.65 dBi from 2.7512 GHz. It is λ/4 (45 mm) long at the lowest operating frequency. It also has nearly linear phase response in this ultrawideband. The radiation pattern in the azimuth plane is broad and the pattern in the elevation plane is relatively narrow.