Circular Waveguide Filter Design based on Multi-mode Equivalent Circuit Model

Mode matching method and equivalent circuit method are the two main waveguide filter design methods. The mode matching method have well calculation accuracy, especially when the waveguide size is relatively large compared with operating wavelength, but this method does not provide physical insights, therefore traditional filter design theories and methods cannot be applied. However, in most of the engineering applications, the size of the waveguide filter is not large to prevent higher-order modes from affecting the performance of the entire system. Under this condition, the equivalent circuit, providing physical insight, and efficient with acceptable accuracy, becomes the preferred method when designing waveguide filters. Unfortunately, the equivalent circuit of the waveguide filter that has been established so far is realized by connecting multiple Y-parameter elements with transmission lines. This method does not show the internal structure of the waveguide filter cavity, so in fact it cannot provide physical insight. Moreover, compared with the mode matching method, the reduction of the calculation amount is also very limited. This paper gives the complete equivalent circuit model of the waveguide filter with analytical expressions for the first time. The establishing process of equivalent circuit represents by capacitors and inductors are given in detail by taking circular waveguide filter excited by TM
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mode as an example. The effectiveness of the proposed method is verified by finite element simulations and experiments.