Practical Design of Waveguide Filters with Quarter-Wavelength Resonators Implementing Transmission Zeros Using Frequency-Variant Couplings

This article describes a new waveguide filter topology based on λg/4 resonators and frequency-variant couplings (FVCs) implemented with modified E-plane stubs. The combination of the new stubs with λg/4 resonators allows to increase the selectivity of the filter and to obtain, at the same time, a very compact structure. The new FVCs that we propose do not add complexity to the final structure, which remains an inline filter. A second-order prototype is designed, manufactured and measured as a validation example. The measured response of this filter matches remarkably well both the in-band and the out-of-band simulated responses, including the location of the transmission zeros (TZs).