This short video illustrates an important tip to remember when tuning the center frequency of a cavity type bandpass filter, often used as part of a duplexer filter assembly. Using Return Loss instead of the Bandpass thru-loss characteristic will yield a more precise measurement of the filter's center frequency. Some analyzers have a directional coupler or return loss bridge "built-in", and some do not (like the one I am using in this video). In this latter case, an external directional coupler or return loss bridge can be used to measure the return loss.