A Compact Bandpass Filter Using Novel Microstrip Dual-mode Resonator with Hexagonal Structure

A dual-mode bandpass filter with miniaturized size is presented in this paper. A hexagonal microstrip dual-mode resonator is employed to reduce the size of the filter. A pair of tortuous parallel coupled input and output structures are used to introduce the transmission zeros, which can improve the selectivity and the stopband of the filter. Compared to other dual-mode resonator filters, a compact advantage is also achieved with the presented design. A sample circuit is fabricated with the thin film technology to validate the design. The size of the filter is 4.98 × 4.57mm², which is equivalent to $0.147 \times 0.134\lambda _g^2$ at 8.75GHz. The prototype filter has a fractional bandwidth of 5.7%, a low passband insertion loss of 0.6dB. Moreover, excellent out-of-band rejection characteristics are achieved in the hexagonal dual mode filter. Four transmission zeros are generated in the stopband to improve out-of-band rejection, and out of-band rejection greater than 20dB can be achieved in the stopband below 15GHz. In addition, a size reduction of approximately 30% is observed when compared with miniaturized rectangular ring patch resonators, for the same center frequency.