Harmonic Characterizations of Loaded Resonators for Waveguide Filters

A new method of controlling and predicting the harmonic responses, for spurious-free window improvement of millimeter-wave bandpass filters, with complete mathematical derivation, is presented in this article. This method relies on loading a transmission line resonator with inverters, permitting the increase/decrease of the harmonics-to-fundamental ratios, for predetermined inverter coupling values. It is a universal method based on circuit models and mathematical theories, independent of the fabrication implementation; only typical bandpass filter inverters are necessary. It does not affect or include the design of input/ output resonator ports, with materials and dimensions, introducing the least design complexity, cf. other earlier approaches. Resonators and bandpass filters using substrate integrated waveguide technology are designed, simulated, fabricated, measured and validated. The derived models, simulations and measured results are in good agreement. The control of the harmonic performance is demonstrated by comparing the measurements of two bandpass filters fabricated with-waveguides, in the millimeter-wave band 27.9 to 29.5 GHz. The filter, incorporating single capacitive inverter-loaded resonators, is analyzed versus a conventional iris filter operating with the TE 10 mode. The comparisons of the simulations and measurements of the two filters demonstrate experimentally the possibility of the systematic control of the harmonics-to-fundamental ratios of the proposed theoretical analysis.