Compact filters based on stub-loaded parallel-coupled resonators

Results of compact bandpass filters based on stub-loaded parallel-coupled resonators are presented in this paper. The working frequency corresponds to the low UHF band. These resonators behave like short-circuited quarter wavelength resonators, but lead to smaller lengths, allowing obtaining very compact filters in a planar technology, without adding lumped components. Such filters seem to be efficient candidates to achieve selective and compact size filters, with high flexibility and simple design rules. Measurement results carried out on a single resonator validate the background theory. Next, two-pole and three-pole filters are designed. Measurements and simulations are in good agreement. Fractional bandwidths from 3.3 % to 6.7 % are obtained, with 3.65 dB and 2.8 dB insertion loss, respectively.