Improved microstrip hairpin-line bandpass filters for spurious response suppression

A new fractal technique is applied to hair-pin microstrip bandpass filters for the second spurious suppression. The coupled edges of hair-pin microstrip resonators are changed by a new fractal to equalise the odd-mode and even-mode phase velocities so that the nearest spurious harmonic can be suppressed. Simulated and measured results show that the second spurious level is reduced below 30 dB for a 1 GHz and 25% bandwidth bandpass filter. An analysis is presented to show the effects of tap point height on fundamental parameters of the filters. In addition, the circuit size is reduced by 11% in comparison to the conventional filter.