Ultrathin Multimode Fabry–Perot Cavity With Wide Phase Resonance Bandwidth Enabled by Janus Partially Reflective Surface

Fabry–Perot cavity (FPC) of less than λ/2 height often suffers from a narrow phase resonance bandwidth. This is mainly due to the violated cavity phase condition originating from the single intracavity resonance mode. By enriching the mode, the phase resonance relationship can be expanded and made easier to meet within a broader band. The enriched intracavity resonance mode is accomplished by an anisotropic Janus partially reflective surface, which offers three types of reflection phases. By combining the resonance bandwidth of each mode, an FPC with a wide phase bandwidth is obtained. The ultrathin cavity is implemented by an isotropic artificial magnetic conductor. Finally, based on this FPC, a reflection-type FPC antenna adopting an external feed antenna is designed. The FPC antenna achieves a 3 dB gain bandwidth of 20.8% (5.47–6.74 GHz) with a peak gain of 14.0 dBi under a 0.3 mm (λ/180) cavity, which reduces the profile of the reflection-type FPC antenna. The
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performance of the antenna keeps less than −10 dB within 5.50–6.67 GHz.