Design of compact wideband multi-band and ultrawideband band pass filters based on coupled half wave resonators with reduced coupling gap

In this paper we propose a technique to design compact multi-band and UWB bandpass filters based on coupled half wave resonators. The proposed design consists of the modification of a conventional parallel coupled Chebyshev bandpass filter structure by setting a very small or null coupling gap between the resonators of the center sections jointly with a very small spacing between resonators of the extremity sections. This spacing determines the performances of selected frequency bands. An ultrawideband response is accomplished by applying null spacing between all the adjacent resonators. We analysed the effect of the separation distance between the coupled lines on both the fractional bandwidth and group velocity of the filter response. The effect of the order assumed for the initial Chebyshev filter was also discussed. As an illustration of the proposed technique, we designed and measured a dual band and a tri-band filter for the frequencies covering the WiMAX/WLAN/X system bands demonstrating an excellent performance, with a fractional bandwidth covering the 40% and 100% of the FCC bandwidth respectively. The proposed technique alleviates the fabrication accuracy requirements. The designs show an optimal improvement in terms of group velocity flatness.