Modeling a Hairpin Resonator for the Design of Bandpass Filters

Narrow banded bandpass filters can be designed in a microstrip technology using parallel coupled ?/2 line resonators 1, 2]. The main drawback is that the filters become long and bulky. To reduce their length and area, folded hairpin resonators are commonly used (Fig.1). These hairpin resonators behave differently than the ?/2 line resonators. To gain more insight in the hairpin structure and to understand how the folding of the line resonator affects the filter characteristics, we identify a model of the hairpin resonator in the frequency domain as a function of four design parameters. Such a model allows to assist the designer in the folding of the ?/2 lines to obtain a hairpin resonator design.