Semianalytical design approach to a triple-mode resonant, wideband null frequency scanning sectorial monopole antenna is proposed. As is demonstrated, TE, TE, and TE mode within a 150°, circumferentially resonant sectorial monopole are simultaneously excited to realize null frequency scanning functionality. The prototype antenna exhibits a wide impedance bandwidth of 75.3%, ranging from 1.40 to 3.09 GHz. Within the frequency band of 1.55–2.70 GHz, the radiation null linearly scans from θ = +90° to +8° with a null frequency scanning sensitivity of 12.9 MHz/°. Moreover, the antenna possesses maximum gain of 2.5 dBi in the backfire, -x-direction, which indicates that it should exhibit quasi-unidirectional radiation pattern in the azimuth plane.