A Novel Compact Low Loss Waveguide Image Rejection Filter Based on a Backward Coupler with Band Pass Filters for 100 GHz Band

This paper describes the design and test observation result of a novel waveguide image rejection filter (IRF). The IRF is based on a quadrature hybrid coupler as a backward coupling structure, followed by Band Pass Filters (BPFs), and matched loads for the image-frequency termination. A prototype IRF shows return loss of better than 18 dB, and an image rejection ratio of more than 25 dB over 4 GHz band for stratospheric ozone spectra at 110 GHz when the LO frequency and IF frequency are 104 GHz and 6 GHz, respectively. We installed the prototype IRF into an ozone-measuring system and successfully observed an ozone spectrum at 110 GHz in single sideband (SSB) mode. This IRF has the advantages of low transmission loss, compact size, and easy scalability for sub-millimeter frequencies.