Miniaturization of Triple-Mode UWB Bandpass Filters with Extended Upper-Stopband

A class of triple-mode ultra-wideband (UWB) microstrip-line bandpass filters with improved upper-stopband performance and miniaturized size is proposed. Firstly, a mode graph of the initial triple-mode resonator is studied to choose its proper dimensions, aiming at allocating its first three resonant modes located within 3.1-10.6 GHz and its transmission zeros near its 5th-7th resonant modes. As a result, a very wide upper-stopband is created. Then, the uniform resonator is bent so as to miniaturize its overall size. Moreover, two short open-circuited stubs are shunt-connected to the initial resonator such that the upper-stopband can be further extended to a range of up to 30 GHz with an insertion loss higher than 20 dB. A filter sample is fabricated and measured in final. Measured results are well matched to the predicted results in terms of insertion /return losses and group delay over the plotted frequency range of 1.0 to 30.0 GHz.