In this paper, we propose the design of multilayer frequency selective surfaces (FSS) waveguide band-pass filters (WBPF). The WBPFs are designed to operate at two different frequency channels, respectively 71-76 GHz (Rx) and 81-86 GHz (Tx). The cross section surface of the FSS is imposed by the WR12 waveguide rectangular section's dimensions. The WBPFs are inserted symmetrically in a T-junction waveguide to design a compact diplexer. This is a basic component developed for an efficient integration in the future E-band millimeter-wave transceiver. The multilayer FSS structure uses only non-resonant sub-wavelength unit cell elements; metallic patch and slot. To reach high channel isolation (≈ 70 dB) a seven order filter was required. Hence, each filter is composed of 13 capacitive and inductive metallic FSS spaced by 12 ultra-thin dielectric substrate layers. The dielectric material is Rogers Ultralam 3850 (Liquid Crystalline Polymer; LCP circuit material). The filter's overall thickness is < λ/4. The numerical studies have been performed using finite element method simulator (HFSS) and CST Studio Suites Tools. The experimental validation has been also done in the X band frequency by developing a fifth order FSS WBPF. Good agreements between simulated and measured results are obtained.
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