WR-3 Band Quasi-Elliptical Waveguide Filters Using Higher Order Mode Resonances

Two types of WR-3 band quasi-elliptical waveguide bandpass filters (BPFs) using higher order mode resonators are presented based on physical cross coupling and modal bypass coupling, respectively. Under the situation of physical folded structure, a TE102-mode in an oversized waveguide resonator is utilized to reverse the magnetic field direction of main path to implement a negative cross coupling (Filter-I). Limited to the structure of Filter-I itself, the simulated 3 dB fractional bandwidth (FBW) of 8.5% is slightly narrower than desired 10% FBW. For the other case, two TE101/TE201 overmode resonant cavities with suitable input and output coupling locations are employed to generate two transmission zeros in the vicinity of the passband through modal bypass couplings (Filter-II). The benefit from the realization of the wideband source and load couplings in the fringe first and fourth oversized cavities is that the FBW of 10% is achieved in the Filter-II. The two BPFs fabricated by computer numerical control milling technology exhibit an insertion loss (IL) of about 0.7 dB and a 3 dB FBW of 8.77% centered at 257.7 GHz (Filter-I), IL of around 0.5 dB, and FBW of 9.83% with center frequency of 256.3 GHz (Filter-II), which are all in good agreement with the simulations. The performance of the two BPFs based on one or two oversized waveguide resonators is highlighted comparing with the reported similar terahertz waveguide filters.