Design of stubs loaded SIR filters by reflection coefficient analysis

In this paper, a new method for designing stubs loaded SIR filter is presented. We can calculate the reflection coefficient of stubs loaded resonator directly by converting the admittance to input terminal. This method replaces the transmission matrix and even-mode and odd-mode analysis. Then, genetic algorithm (GA) is applied to search the corresponding electrical parameters, which provides a more effective and intuitive way to design filters. Then, an ultra-wideband bandpass filter using short-stub loaded SIR is realized, which exhibits a good frequency selectivity from 3.1 GHz to 10.4 GHz. In addition, by applying this method, a dual-passband filter with short and open stubs is achieved. It contains two wide bands at 2 GHz and 5.8 GHz, which concerns the frequencies of GPS and WLAN. Meanwhile, the sizes of these filters are further minimized by bend structure. Finally, the filters are manufactured. As a result, the measured results are in good agreement with the calculated ones. These filters exhibit great frequency-selection characteristics: flat passbands and extremely sharp rejections around the passband. So, based on the verifications of the manufactured filters, this method can be widely used in microwave filter design field.