A Fifth-Order X-Band Frequency-Selective Surface With High Selectivity and Angular Stability Based on 3-D Coupling Slot

In this article, a 3-D frequency-selective surface (FSS) with high selectivity and large angular stability of 70° is proposed. The equivalent circuit model of the single-layer FSS is first presented and analyzed, offering physical support to the operating principle using the odd- and even-mode analysis method. By cascading two single-layer FSSs, the resulting 3-D double-layer FSS operates as a fifth-order bandpass filter. The filling design of dielectric block, metal block (MB), and waveguide wall makes the 3-D FSS produce different magnetic coupling effects in parallel and perpendicular to the electromagnetic wave propagation direction to obtain an electromagnetic filtering response with broadband and good roll-off characteristics. By introducing the outer dielectric bonding to the 3-D FSS, the angular stability is greatly improved from an angle of 0°–70°. Finally, the veracity of the design is verified by physical processing and testing.