Wideband contiguous-band diplexer using interpolated coupling coefficient bandpass filters

As a simple method to realize a wideband contiguous-band diplexer, interpolated coupling coefficient filters whose coupling coefficients are given by a linear interpolation of the coupling coefficients in a singly terminated filter and doubly terminated filter are applied. Its effectiveness is demonstrated by theoretical studies and experiment. First, with regard to the impedance characteristics of a broadband iris-type waveguide filter with a coupling slot for the first cavity, the variations from the characteristics of a singly terminated filter are presented with an equivalent circuit. Next, it is shown that the impedance variations due to the frequency dependence of the coupling slot can be eliminated by using an interpolated coupling coefficient filter. Finally, test results in the Ka band demonstrate that a contiguous-band diplexer with a filter bandwidth of about 4% can be obtained. The test diplexer has a filter bandwidth of about 4% × 2, a guard band of about 0.8%, an insertion loss of 0.5 dB, a VSWR of 1.2, and an isolation of 20 dB.