On a transmission-line Butterworth lowpass filter using radial stubs

For a transmission-line lowpass filter using open-circuited microstrip stubs, the frequency at the edge of a passband or stopband tends to be higher than the frequency determined by the filter specifications. Since the length of a constituent transmission-line cannot be determined precisely because of the finite width, we introduce a radial-line stub. The equivalency of a radial-line stub to a straight-line stub will be verified from the viewpoint of their input impedances. It was found experimentally that the frequency characteristics of a trial filter using radial-line stubs agrees well with the required ones