A Novel Tap Input Coupling Structure for a Narrow Bandpass Filter Using TM010 Mode of a Microstrip Circular-Disk Resonator

This paper discusses a new method to couple into the TM010 mode of a microstrip circular-disk resonator. This method can achieve reasonably strong input coupling, which is useful for narrow-band filters with fractional bandwidths of approximately 0.5% and above. A comparison between this newly proposed input coupling structure and the comentional gap input coupling structure will be addressed. A decision threshold for using either the top input or the conventional gap-coupled input is also explained. Experimental results of a filter fabricated using this novel input coupling structure is also presented.