Single-Layer Evanescent Wave Absorbers in Rectangular Waveguides Based on Evanescent Mode Coupling and Attenuation

A microwave absorber for absorbing waves below the cutoff frequency of a rectangular waveguide (RWG) is proposed in this study. By loading an ideal lossy capacitive metasurface (LCMS) in the cross Section of an RWG close to the radiation source, the LCMS exhibits the ability to couple with and attenuate the waves below the cutoff frequency of the RWG, that is, absorb the evanescent waves (EWs). The operating mechanism is analyzed with several parametric studies based on the momentum-matching principle, and a systematic design procedure for a practical EW absorber is concluded. Three samples made of indium tin oxide (ITO) thin films are designed, fabricated, and tested, and the experimental results agree well with the theoretical results. The maximum measured absorptance reaches up to 97%. Compared with conventional plane-wave absorbers with multilayer conductors, the proposed EW absorber only requires a single-layer conductor and has a distinct working mechanism, design principle, and application scenario. In the examples considered in this study, the samples can serve as the specific matching component for calibrating the RWG below the cutoff frequency. Notably, our EW absorption technique provides a feasible approach to suppress the electromagnetic interference (EMI) dominated by the evanescent mode in the electrically small metallic cavity packages and near field (NF) of integrated circuits (ICs).