Design of an elliptic‐function bandpass filter using microstrip spiral resonators

In this paper, a new equivalent circuit representation for the electromagnetic field coupling between distributed‐line resonators is discussed. Based on this circuit, the coupling coefficient is defined. In this definition, the coupling coefficient between resonators takes a positive, negative, or zero value. Hence, this equivalent circuit representation is useful in the design of multipole filters with poles in the stopband, in which positive and negative coupling are needed. Based on this definition, the design of an elliptic function four‐pole bandpass filter is carried out with the Sonnet em simulator and the effectiveness of the approach is discussed.