Design of Omnidirectional Antennas Using TM₂₂ and Quasi-TM₁₁ Modes with Characteristic Mode Analysis

This study presents the design of two high-gain omnidirectional antennas with minimal pattern ripple. Antenna I is based on a conventional microstrip patch structure, while Antenna II integrates a modified design with four metal probes. Characteristic mode theory (CMT) was applied to analyze the far-field radiation patterns of both antennas, with a focus on the distinct radiation modes. The analysis revealed that Antenna I operates in the TM₂₂ mode and Antenna II in the quasi-TM₁₁ mode, both exhibiting omnidirectional radiation characteristics. A comparative investigation of four different feeding techniques was conducted to ensure equal amplitude and phase excitation at each port, resulting in a low pattern ripple for both designs. A 1:4 power divider was implemented to validate the designs, and the performance of Antennas I and II was experimentally assessed. The measurement results showed that the −10 dB operating bandwidths of Antennas I and II spanned 2.42–2.50 GHz and 2.34–2.57 GHz, respectively, with corresponding peak gains of 8.0 dBi and 4.55 dBi at a frequency of 2.45 GHz.