Novel wideband bandpass filter using open-ended stub loaded parallel coupled short-circuited three-line unit

Open-ended stub loaded parallel coupled short-circuited three-line structure is proposed to realize a new wideband bandpass filter. The characteristics of the new coupling unit were firstly investigated based on simulation, followed by proposing a two-step design scheme: 1. A Chebyshev filter is synthesized on the desired passband, using symmetric three-line coupling units. 2. The transmission zeros are then introduced to improve the skirt and stopband characteristics by properly designing the stubs. Then, a UWB bandpass filter covering the lower UWB band (BW: 3.1-4.8 GHz, FBW: 43%) is designed to explain the proposed design procedure. The filter was then simulated, fabricated and measured to compare with the theoretical predictions. A good agreement validates the effectiveness of the proposed new coupling structure and corresponding filter design technique.