Dual-layer microstrip monopolar patch antenna with characteristic mode analysis

A monopole-like stacked microstrip antenna with dual-layer patches is proposed for ultra-wideband applications. The characteristic mode analysis (CMA) is utilized and contributes to the multi-mode behavior investigation, guiding the design process. The dual-layer patches both consist of two concentric-coupled rings. The patch configuration is optimized according to the desired mode behaviors at different frequencies. Correspondingly, the three merged resonant modes, containing the modes TM01 and TM02 of the middle layer and TM01 in the top layer, effectively broaden the operation band. The measurements demon-strate that the impedance bandwidth (IBW) of the antenna reaches 4.70-10.20 GHz (73.8%) with the return loss of 10 dB and a maximum gain of 8.7 dBi. The cross-polarization level is better than -28 dB. The proposed antenna possesses a small radius of 30 mm (0.47?(L)), and a low-profile of 5.78 mm (0.09?(L)). ?(L) is the free-space wavelength at the lowest operation frequency.