Ultrathin narrowband frequency selective surface with high angular and polarization stability at X-band frequencies

An experimental investigation of an ultrathin and narrowband frequency selective surface (FSS) with polarization-insensitive and high angular stability at X-band is reported. A roll-off and narrow-passband behavior of the FSS spatial filter is designed by coupling two layers of periodic square slot loops of resonance, which are etched on the top and bottom metal surfaces of a two-substrate-stacked printed circuit board (PCB). The sandwiched interface between these two PCB substrates is also a metal surface with periodic square slots for proper coupling. The bandpass nature is numerically simulated by high-frequency structure simulator and is further validated by experimental measurements on an FSS prototype. The prototype comprises 50 x 33 unit cells in an area of 40 cm x 26.4 cm, and the total thickness is 0.025 lambda. The experimental results show a resonant frequency at 12.23 GHz with a 3 dB relative bandwidth of 6.1%. The maximum frequency deviation of bandwidth is less than 0.4% for the incident angles from 0 degrees to 60 degrees, exhibiting good frequency stability.