The square spiral resonator: Investigating its electromagnetic performance for filter design

This paper investigates the electromagnetic performance of the microstrip square spiral resonator in the design of bandpass filters based on coupled resonator circuits when compared to the performance of the square open-loop resonator at the same center frequency. A full-wave EM simulator is used to calculate the resonance frequency which is plotted against the spacing between the closest lines of the spirals, against the width of the line that forms the resonator, and also against the spiral shape. The effect of those quantities on the coupling coefficient between two identical resonators and also on the external quality factor is here investigated. Therefore, plots of the coupling coefficient and external quality factor against the dimensions involved in the design of the resonator are here reported as well. The major purpose of this paper is to provide helpful information concerning square spiral resonators so that significant size reduction of the filter can be achieved.