A Novel Method to Design Small Wideband Half Mode Cavity Antenna Under Resonances of TM₁,₁ and TM₃,₁ Modes

In this study, we propose a novel method for designing a compact and wideband half-mode cavity antenna (HMCA) that simultaneously radiates in the TM1,1 and TM3,1 modes. This advancement leverages an intrinsic property of HMCAs that has not been addressed in previously reported research. It is investigated that for a conventional HMCA with proper dimensions, resonant frequencies of TM1,1 ( $f_{1,1}$ ) and TM3,1 ( $f_{3,1}$ ) modes converge closely, whereas the resonant frequency of TM2,2 ( $f_{2,2}$ ) mode shifts out of the range between these two modes. So, unlike conventional methods, a wide frequency bandwidth can be realized without employing any extra components such as shorting pins and slots which add complexity and cost to the design and deteriorate cross polarization. However, since this modes reallocation results in a low input admittance, a stepped probe feed is used for excitation to enhance input reflection coefficient (S11). Consequently, the dual resonance operation achieves a broad frequency bandwidth, characterized by low cross-polarization, a stable radiation pattern, and minimal gain fluctuation. Antenna exhibits more than 21% size reduction compared with the wideband HMCAs proposed in the recently published articles. The proposed HMCA is implemented and tested to verify the design approach.