A novel design approach to triple-mode resonant null frequency scanning antenna (NFSA) with positive scanned slope is proposed. Firstly, a circular sector patch radiator with short-circuited circumference is introduced to generate multiple TM (m = 1, 2, …) mode resonances. Then, as theoretically predicted and numerically proved, the first three TM (m = 1, 2, 3) modes can be effectively combined to yield null frequency scanning functionality with positive scanned slopes. Next, a prototype antenna is designed, fabricated, and measured. As numerically demonstrated and experimentally validated, the proposed NFSA can exhibit a null frequency scanning functionality in xz-plane with positive scanned slope ranging from θ = −90° to +45° (for null depth smaller than −10 dB) within a fractional impedance bandwidth of about 34.3% (i.e., 2.97‑4.20 GHz).