Continuous Null Frequency Scanning Microstrip Annular Sector Patch Antenna under Triple-Mode Resonance

A continuous null frequency scanning microstrip annular sector patch antenna (MASPA) is proposed. The available resonant modes of a MASPA were calculated using a 2.5-wavelength prototype magnetic dipole. As theoretically predicted, the resonant $\mathrm{TM}_{120 / 41,1}, \mathrm{TM}_{240 / 41,1}$, and $\mathrm{TM}_{360 / 41,1}$ modes in a 123° annular sector patch antenna are excited and perturbed, resulting in a continuous null frequency scanning function. The prototype antenna is then fabricated on a single-layer air substrate. Experiments show that the proposed MASPA can achieve continuous null scanning from $\theta=+22^{\circ}$ to -32° with a fractional impedance bandwidth of about $23.1 \%$ (i.e., 3.01-3.76 GHz) with null depth higher than $\mathbf{1 0} \mathbf{d B}$. Therefore, the proposed approach can effectively realize continuous null frequency scanning without incorporating additional parasitic patches.