A compact all-frequency absorptive dual-band bandpass filter with T-shaped transmission line absorption branches

Multi-band and all-band absorptive filters can greatly promote the miniaturization of radio frequency (RF) communication systems. However, the existing absorptive multi-band filters have problems such as large size, narrow passband, high insertion loss, and low RF signal absorption capacity. Herein, a new all-frequency absorptive dual-band bandpass filter (BPF) is proposed by combining an N-shaped microstrip coupler with Tshaped transmission line absorption branches. The N-shaped microstrip coupler can flexibly implement the dualband bandpass filtering of RF radio frequency (RF) signals in a compact structure. The T-shaped transmission lines can effectively absorb the stopband signals reflected from the N-shaped coupler at full frequency. The theoretical and experimental results show that the 3-dB fractional bandwidths (FBWs) of the proposed BPF with a size of 0.41 lambda g x 0.20 lambda g are 60.5 % at 1.3 GHz and 13.3 % at 4.1 GHz, respectively, and the return losses in all frequency are greater than 10 dB, indicating that the filter has the characteristics of wide passband, small size, and all-frequency absorption of reflected bandstop signals. In short, the combination of N-shaped coupler and Tshaped transmission line provides a way to achieve an all-frequency absorptive dual-band BPF with a small size, wide passband and low return loss.