Asymmetrically-loaded interdigital coupled line for wideband microstrip bandpass filters with good out-of-band performance

An asymmetrically-loaded interdigital coupled line is proposed for design of wideband microstrip bandpass filters with good out-of- band performance. This interdigital coupled line with three fingers is formed by stretching one of two outer arms with one two-stage open-ended stub. Its characteristics are analysed in terms of admittance matrix to exhibit simultaneous emergence of extra transmission zeros in lower and upper stopbands. A wideband filter with two half-wavelength resonators is then designed and fabricated. Measured results show low-loss wide passband in the range 2.95-5.15 GHz and highly attenuated stopbands at DC to 2.2 GHz and 6.6-10.0 GHz, respectively.