Compact Bandpass Filter With Two Tunable Transmission Zeros Using Hybrid Resonators

This letter presents a novel type of compact, low insertion loss (IL) planar microstrip filter, which is mainly composed of two hybrid resonators with a mixed electric/magnetic (EM) coupling between them. Tunable EM coupling is constructed to realize a tunable transmission zero (TZ) very close to the passband. And two folded T-shaped feed lines are adopted to introduce another tunable TZ and obtain a desired external coupling. Advantages of the proposed filter are not only its compact size and low IL, but also its tunable TZs.