A Third-Order Bandpass Frequency Selective Surface With a Tunable Transmission Null

We present the design and analysis of a frequency selective surface (FSS) capable of providing a third-order bandpass response. The transfer function of the proposed FSS has a transmission null, the frequency of which can be tuned to suppress strong interference signals with frequencies close to or within the main transmission band. The principles of operation of the device are presented along with an approximate analytical procedure for synthesizing the desired FSS response. Two prototypes are fabricated and characterized using a free-space measurement setup and the FSS's different modes of operation are experimentally demonstrated. Finally, the tuning performance of the FSS's transmission null is experimentally demonstrated in a WR-90 waveguide environment.