Design of iris-coupled broadband waveguide filter using modified reflection-zero frequencies

In spite of its low loss and superior power handling capability, the waveguide filter cannot have broad bandwidth due to the strong frequency dependence of the reactance values for coupling of the cavity resonators and the guide wavelength. To achieve a broadband design of the iris-coupled waveguide bandpass filter with the Chebyshev function as the transfer function, the susceptance value of the inductive iris is optimized by specifying the ideal zero frequency of the reflection. Based on this method, a diplexer is trial fabricated and a broadband diplexing characteristic is realized. In addition, agreement with numerical results is demonstrated, confirming the effectiveness and validity of the method. The tested diplexer has a VSWR of less than 1.24 and an isolation of more than 53 dB at passband bandwidths of 32 and 7% relative to the wavelength.