Tunable combline bandstop filter with constant bandwidth

A new method for the design of tunable bandstop filters is presented. It is based on a dual-band combline structure consisting of a wideband bandpass filter with integrated bandstop filter. The bandstop filter exhibits the very useful property that its bandwidth remains constant as the center frequency is tuned, without the need for coupling adjustment. A prototype design has been constructed, and measured results clearly demonstrate the principle.