Wideband Coupled-Line Microstrip Filters With High-Impedance Short-Circuited Stubs

There have been difficulties in realizing wideband coupled-line filters with microstrip technology due to tight couplings demanded at input and output ports. To solve the problem, a realization method using high-impedance short-circuited stubs is proposed. To verify it, a coupled-line microstrip filter with more than 65% bandwidth is fabricated at 3 GHz and measured. Measured results are in good agreement with prediction, showing that the measured return and insertion losses are better than 18 and 0.85 dB, respectively, within more than 65% bandwidth.