Design and application of microstrip parallelcoupled lines employing step-impedance transmission-line compensation

The application of step-impedance transmission lines (SITLs) to improving the directivity of parallel-coupled microstrip lines is presented. A simple design procedure for two configurations based on the SITL technique is given. The application of the technique is demonstrated practically with a design at 0.9 GHz on an RF60 substrate. The directivities obtained from the measured results are improved by more than 6 and 30 dB compared with the designed uncompensated coupler. The compensation technique is then applied to the design of a coupled-line resonator and bandpass filter for suppressing harmonic responses in these circuits. Experimental results show that the second harmonic in the filter response is suppressed by 30 dB.