A Modified Parallel-Plate Waveguide for Wide-Angle Characterization of Frequency Selective Surfaces

This letter presents an innovative parallel plate waveguide (PPW) system for wide-angle characterization of frequency selective surfaces (FSSs). The structure is made of two parallel metallic plates, with increased height at the center to accommodate a 1-D FSS array, and reduced height at the ends to achieve wideband impedance matching. The plates are further tapered toward the edges to suppress the edge radiation and leakage. For validation under normal incidence, the setup is tested using three distinct FSS prototypes: a bandstop filter, a two-layer wideband absorber, and a switchable absorber/transmitter structure. For oblique incidence characterization, two identical PPW sections are joined at an angle 2θ, forming a controlled angular excitation path with the FSS inclined at θ. The measured results of FSS samples for both normal and oblique incidences closely match unit cell and full-wave simulations, thereby confirming the accuracy of the proposed system for wide-angle characterization up to 8 GHz.