Synthesis and realization of novel ultra-wideband bandpass filters using 3/4 wavelength parallel-coupled line resonators

In this paper, a novel synthesis theory is proposed to design ultra-wideband (UWB) bandpass filters using 3lambda₀/4 (lambda₀ is the wavelength at central frequency of the filter) parallel-coupled line resonators. By using the first two resonant modes of the 3lambda₀/4 parallel-coupled line resonators, this kind of UWB BPF can achieve a wide bandwidth larger than 100%. In order to alleviate the requirement on fabrication precision, aperture-enhanced coupled lines are introduced in the filter. A three-degree UWB filter with a midband frequency of 6.85 GHz is synthesized, simulated and fabricated. Measured results indicate that the filter is low loss in the passband, and the attenuations in the lower and upper stopbands satisfy the indoor limit issued by the Federal Communications Commission (FCC).