Single- and dual-passband microwave planar filters with coupled double-transmission-line resonators

A type of planar double-transmission-line resonator, consisting of two open-ended half-wavelength transmission-line segments edge inter-coupled along their entire length, is proposed. This microwave resonator, because of the generation of double resonances, allows coupled-line bandpass filters with reduced longitudinal length to be implemented. This is the main contribution of this work. Furthermore, as an added feature of the aforementioned resonator, its two-line geometry makes it suitable for dual-band bandpass filter design. To prove the concept experimentally, two different examples of single- and one dual-passband microstrip filter prototypes within the 1-1.5 GHz range are synthesised, constructed and tested. The major advantages of these filter solutions are emphasised. A complementary section dealing with the analysis of the split-type double-transmission-line resonator is also reported. Here, as a result of this study, formulas for its exact design are provided for the first time.