Periodically nonuniform coupled microstrip-line filters with harmonic suppression using transmission zero reallocation

In this paper, the two-port periodically nonuniform coupled microstrip line with finite length is thoroughly studied as an equivalent J-inverter network for microstrip bandpass filters with harmonic suppression. Extracted J-inverter susceptance is exhibited to vary up and down versus frequency with the null or transmission zero when the coupled length becomes half-wavelength. This transmission zero can be adjusted by varying the periodicity and/or slit depth to suppress the first spurious harmonic passband of the filter. Firstly, two one-stage bandpass filters with wide and narrow bandwidths are designed to assure the first-harmonic passband is fully suppressed as illustrated in theory and experiment. Secondly, a three-stage bandpass filter with harmonic suppression is designed optimally. The predicted S-parameters are found in good agreement with the measured values, at least 40-dB insertion loss, at the point where the first-harmonic passband is achieved.