Microstrip dual-band bandpass filters using parallel-connected open-loop ring resonators

This article proposes a microstrip dual-band bandpass filter that uses parallel-connected open-loop ring resonators. Compared to many microstrip dual-band filters, the advantages of using microstrip open-loop ring resonators are easy calculation (half-guided-wavelength), easy fabrication (equal width for all 50-Ω lines and without grounding holes), and direct connection to external feed lines (reducing insertion loss caused by gap couplings). Another advantage of the filter is an asymmetrical feed on the ring resonator that provides sharp rejections at its adjacent bands. The input and output matches of resonators to the external feed lines are derived using a simple transmission-line theory. The results of the derivation provide a simple design rule for filter designers. Simulated and measured results are presented with good agreement. The filter has minimum insertion loss of 1.25 dB at 1.85 GHz and 1.6 dB at 2.33 GHz. The 3-dB fractional bandwidths are 5.9% for the 1.9-GHz bandpass filter and 4.7% for the 2.4-GHz bandpass filter, respectively. © 2008 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2008.