Novel quasi-elliptic microstrip filter configuration using hexagonal open-loop resonators

In this paper, a novel filter configuration in enhancing the selectivity of the quasi-elliptic microstrip bandpass filters is presented. The filter topology is based on hexagonal open-loop resonators and this offers additional electrical coupling. This extra electrical coupling together with the cross coupling between several pairs of adjacent hexagonal open-loop resonators relocates the attenuation poles of the quasi-elliptic filter so as to provide higher selectivity. Compared to a traditional six-pole square open-loop resonator microstrip filter with a fractional bandwidth of 7.8% and center frequency of 1GHz, this new filter configuration offers additional 13dB rejection improvement at the attenuation pole in the lower rejection band whilst the fractional bandwidth reduces about 5% of that traditional filter and the filter dimension diminishes by 27%