A novel double loop dual-mode bandpass filter is proposed which is composed of a quadrate resonator and a small quadrate loop for the purpose of perturbation. The small quadrate loop has two tunable parameters which increase the degree of freedom of dual modes and make the miniaturization easier. Tapped coupling can be used to change the transmission zeros so that the performance of filter is improved. A prototype filter is designed and fabricated with a measured center frequency of 2.41GHz
17.77% fractional bandwidth
a low passband insertion loss of 0.67dB
a high passband return loss of 17dB and a wide stop band with rejection better than 20dB.