Dual-Mode Dual-Band Microstrip Bandpass Filter Based on Fourth Iteration T-Square Fractal and Shorting Pin

A new class of a dual mode microstrip fractal resonator is proposed and developed for miniaturization of the dual band bandpass filter. The perimeter of the proposed resonator is increased by employing fourth iteration T-square fractal shape. Consequently the lower resonant frequency of the filter is decreased without increasing the usable space. The self similarity of the usable structure enables it to produce the two degenerate modes which are coupled using the proper perturbation technique. The shorting pin is placed at the null in the surface current distribution at the center of the resonator. This shorting pin is coactively coupled to the resonant circuit of the resonator, effectively coupled to the lower degenerate mode and reduces the lower edge band resonant frequency. By adjusting the resonator dimensions and the size of the shorting pin, the resonant frequency and the out-ofband rejection around the transmission bands can be controlled to meet the design requirements. The simulated response of the designed filter has two transmission bands; the first band is from 2.34-3.65 GHz with resonant frequencies at 2.47 GHz and 3.55 GHz, the second band is from 4.37-5.324 GHz with resonant frequencies at 4.5 GHz and 5.13 GHz. In the pass bands, the group delay is less than 0.65 ns. The proposed filter can be applied to WLAN (2.4 GHz and 5.2 GHz) and WiMAX (3.5 GHz) and Bluetooth and ZigBee (4.9 GHz).