Group delay analysis of differential-mode coupled four lines bandpass filters

In this paper, the analytic formula of the group delay of one-stage coupled four lines BPF is proposed. The analytic formula agrees well with measured group delay of one-stage BPF, except for at band-edges. In case of multi-stage BPF, the sum of analyzed group delay of each stage is quite agrees with measured result in UWB band (3.1 GHz - 10.6 GHz). A 6-stage BPF with impedance transformation from 50 Omega to 172 Omega, the input impedance of the self-complementary antenna, is shown as an example.