Design of compact bandpass filter with improved selectivity using source-load coupling

A compact planar bandpass filter with improved selectivity using source-load coupling is proposed. The new filter consists of quarter-wavelength resonators and short-ended feedlines. An extra transmission zero is produced near the passband owing to the source-load coupling, which can improve the skirt-selectivity of the filter, and quarter-wavelength resonators are used to enhance the rejection level around the second harmonic. Both simulated and measured results with good agreement are presented.