Measuring S11 and S21 of the home build cavity filter

To complete the building I measured the S21 of the cavity filter
First the pass band



As the home build GHZ VNA does not have any shielding the dynamic range is rather limited but the -3dB bandwidth is about 4MHz and the loss is -3dB. From other measurements its clear the suppression of the image at 21.4MHz offset is good enough.
Certain cavity filters do have harmonic modes that will allow 2nd or higher order harmonics to pass.
I can only measure S21 till 4.3GHz so here is a wide sweep.



The peak at 4.3GHz is an artifact of my SW, its not there when you zoom in. So the choice of an interdigital filter instead of a comb filter enabled the suppression of harmonics modes

The S11 measurement shows there is still some room for improvement



but for now I'm happy.