I'm working on a bandpass filter at 3.2 GHz. I built a few prototypes interdigital, pipecap cavity, and even microstrip. Microstrip filter sort of works but it can't be tuned, so I abandoned it. The other types also work, bandwidth and insertion loss is ok but they all exhibit garbage responses from 6 to 20 GHz, with some peaks having similar insertion than the fundemental. So I'm looking for a way to either build a new type of filter, (mechanically tuned) or match impedance or something. This is pretty much a homebrew design. Appreciate any info.