COMPACT MICROSTRIP LOWPASS FILTER WITH HIGH AND WIDE REJECTION IN THE STOPBAND UTILIZING FLABELLIFORM RESONATORS

In this paper, a microstrip lowpass filter with −3 dB cutoff frequency of 3.8 GHz consisting of two cascaded resonators with flabelliform patches and two symmetric suppressing cells is proposed. To design the filter, the impact of each transmission line on the frequency response is determined by extracting the equations of the insertion loss (S21) and return loss (S11) on the basis of the equivalent LC circuit of the main resonance cell and the cascaded structure. The designed filter is constructed and tested, and a good agreement between the results of simulation and measurement is obtained. In the whole stopband region, a return loss close to zero and an acceptable suppression level of −30 dB from 4.47 to 25.17 GHz are achieved. Furthermore, a flat insertion loss in the passband and a low return loss (−23.02 dB) in this band can prove desired in-band and out-band frequency response.