Compact Wideband Microstrip Bandpass Filter Using Quasi-Spiral Loaded Multiple-Mode Resonator

In this letter, a new model for further reducing the size and increasing the bandwidth (BW) of a class of bandpass filter (BPF) utilizing multi-mode resonator is presented. A new technique based on spiral transmission line loading to achieve higher BW and further size reduction is analyzed for this class of BPF. A compact ultra wideband microstrip BPF is proposed by using this technique. The capability of tuning the transmission zeroes in the proposed model is also studied. Estimates for the frequencies of these transmission zeroes are directly presented. The overall BW of the proposed BPF is shown to be increased more than 12.6% and its size to be reduced more than 70% compared with the conventional ones. The proposed BPF provides a relatively wide 3 dB fractional bandwidth of 72.6% frequency response with two transmission zeros at the lower and upper stop-bands to provide a very sharp cutoff and low passband insertion-loss and good return-loss characteristics.