Frequency Selective Surface Notch Filter for use in a Millimeter Wave Imaging System

A thin, low-cost, high Q 140 GHz frequency selective surface (FSS) notch filter is designed, fabricated, and measured for use in a millimeter wave imaging system for fusion plasma diagnostics. The notch filter is employed to protect the mixer arrays from spurious 140 GHz gyrotron heating power. Because of its relative insensitivity to angle of incidence, a square loop structure was selected after comparisons with Jerusalem cross and cross dipole structures. It proved to have 35 dB rejection at 140 GHz as well as insensitivity to a plane wave's angle of incidence over a range of 15 degrees. The 8" times 8" printed circuit notch filter is fabricated on 10 mil thick RO3006 using the electro fine forming technology (EF2) which permits metallization thicknesses of 25 microns together with narrow width. Good agreement between theory and experiment is obtained. Through experiment, metal thicknesses are proved to significantly affect the performance in the band-stop filter application. FSS notch filters in the millimeter wave range also have possible applications in imaging systems in other fusion machines such as the superconducting KSTAR device nearing completion in Korea