A Miniaturized Interdigital Bandpass Filter with Capacitors and Inductors Loading

In this paper, a microstrip interdigital bandpass filter based on capacitors and inductors loading is proposed. The filter is miniaturized with capacitors and inductors loading at each end of the resonator. The principle of resonator length reduction after loading is interpreted by the ABCD matrix. Simulation results show that the 3 dB fractional bandwidth of the filter is 35.8% from 6.43 GHz to 9.24 GHz. Besides, the insertion loss of the proposed filter is less than 2.67 dB and the return loss is over 14 dB. The size of the proposed filter is $0.2755 \times 0.2027 \lambda_{\mathrm{g} 0}{ }^2$, which is reduced by 21.2% compared to the interdigital filter without capacitors and inductors loading.