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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.
Date of Publication: April 2009