Bandpass THz Frequency Selective Surface with Flat Passband

The thickness of the substrate turns out to be approximately a quarter of a wavelength in implementing second-order passband frequency selective surfaces (FSSs) to achieve better response with faster roll-off and smooth in-band frequency response. The problem with using a thicker substrate in FSS structures is that the losses will be high, especially in millimeter and terahertz (THz) frequency applications. For that, a new approach is introduced in this paper to deal with this issue. The approach is to adopt an improvement of the coupling coefficient without increasing the thickness of the substrate. Based on this, a thin band-pass filter (FSS) with low losses and flat-top response is built. An equivalent circuit model has been provided to characterize the structure based on electric and magnetic fields distributions. Thus, it is proven in this paper that the calculated results are in good agreement with the numerical simulation results.