Simplified design of microwave filters with asymmetric transfer functions

This paper presents the design of a microwave bandpass filter having an asymmetric transfer function and a symmetric geometry. The purpose is to demonstrate that an efficient synthesis can provide simpler solutions for the hardware implementation. An example of a 6th order dual mode filter in circular waveguide technology is presented. Considering the desired topology, the initial synthesis from the Chebyshev transfer function leads to two solutions with an asymmetric electrical network. A symmetric network is then synthesised increasing the number of couplings. For one of the solutions, weak couplings can be neglected so that the transfer function is only slightly affected. The modified symmetric electrical network leads then to a simpler hardware implementation of the dual mode filter. The electrical network synthesis is described, then the optimal geometrical dimensions of the filter are determined applying an electromagnetic based optimisation method. An experimental realisation is finally compared to the theoretical results.