CAD of microwave junctions by polynomial curve fitting

A circuit representation of lossless reciprocal microwave junctions is presented. Exact scattering parameters of the junction are determined by solving the electromagnetic boundary value problem at a few frequencies and for a few dimensions. Curve fitting is used to characterize the parameters continuously. This model can be used in any CAD (computer-aided design) or optimization program, reducing the computation time by several orders of magnitude while preserving high accuracy. The technique is applied to the case of inductive windows in rectangular waveguides, and is used for the optimization of a difficult-to-design moderate-bandwidth 8-pole, 0.01-dB ripple bandpass filter and a broadband diplexer. The results are verified by an experimental diplexer which was fabricated and tested. The results are very close to the optimized response with absolutely no tuning.<>