New Coupling Scheme for Microstrip Bandpass Filters With Quarter-Wavelength Resonators

A new coupling scheme for microstrip bandpass filters with quarter-wavelength (lambda/4) stepped-impedance resonators is proposed. In the proposed coupling scheme, two adjacent lambda/4 stepped-impedance resonators are coupled by directly connecting an admittance inverter between them. By properly designing the connecting position, one may adjust the coupling coefficient between two resonators so that narrow to broad bandwidth filters may easily be fabricated without coupling gaps. Here, the admittance inverter is realized by a lambda/4 stepped-impedance line. Based on the proposed coupling scheme and the conventional magnetic coupling structure, the filters with narrow or wide bandwidth may be designed and implemented. In addition, folded lines are adopted to make the filter size compact. In this study, a two-pole filter with a 3-dB bandwidth of 5.1% and two four-pole filters with 3-dB bandwidths of 33% and 51% are successfully fabricated. Good agreement between measured and simulated results is observed.