A novel triple-mode resonant circular sector patch antenna with azimuthal null frequency scanning functionality is advanced in this article. The implicit periodicity of the roots of the zero-order Bessel function is revealed and then employed to design multi-mode resonant, azimuthal null frequency scanning antennas (ANFSAs). As is proven, when a parasitic radiator is properly incorporated, a circular sector patch radiator under resonances of multiple TM _0 m ( m = 1, 2, …) modes can yield a unilateral null frequency scanning functionality in its azimuth plane. A prototype with a flared angle of α = 15° is designed and fabricated on a single-layered air substrate to validate the design approach. As numerically and experimentally verified, the proposed unilateral ANFSA exhibits a null frequency scanning functionality from ϕ = 150° to 285°, covering an operation bandwidth of about 19.1% with a null depth smaller than −10 dB.