Desert Sage replied the topic: waveguide cut-off and the attenuation below that

Yes, small length of waveguide should work. WG below cutoff is surprisingly effective as HPF. Surprised how little rejection you are getting. If I wasn't lazy I could calculate how much guide you need but you forgot to say how much rejection you were looking for. As a guess, a couple inches should give you plenty.

sm4rzw replied the topic: waveguide cut-off and the attenuation below that

Thanks for quick reply :) Not that used to waveguides, but I'll learn. Those wg-to-coax adapters are quite short, so the radiators inside them get's quite close, thats why I had that thought. Will try to track down a 3" or 4" WG on ebay and try it out.

sm4rzw replied the topic: waveguide cut-off and the attenuation below that

With 3" WR137 the attenuation improved from -17 to -28 dB. Still too much. Tried the trick with a steel ball and a magnet, but every position only made it worse... Desperately, I ac-hoc'ed a stub notch out of .141 semirigid and tuned to 4100 MHz. In cascade with the waveguide adapter filter it became perfect. The combination of a file and a network analyzer is great sometimes ;)

Desert Sage replied the topic: waveguide cut-off and the attenuation below that

Thank you for closing the loop and letting us know results. Couple of thoughts. I should have checked dimensions. You are operating only about 7.5% below cutoff. The next smaller guide would have worked better. The steel ball sounds too large. BB to pea sized I estimate. But your solution works. Thanks again.

sm4rzw replied the topic: waveguide cut-off and the attenuation below that

Yes, I 've learned that cut-off frequency doesn't mean totally stop, but rather like -20 loss. My first mistake. The steel ball, yes it was a small bearing ball, like 4mm.
I just took a SMA-T-coupling, (which I normally hate because people connect things without having a clue of the impedance), put a peice of .141 in one female port, soldered the shield to the threads, hooked it to the VNA and started with the file ;) It's great fun!
As for the smaller waveguide, that would have been WR112. But I needed the passband down to 5.7 GHz. But yes, it would probably worked better, it's easier to handle 10 dB loss in passband and -80 to kill the carrier power than 2dB loss in passband, and -20 to kill the carrier.. I guess it all cooks down to the meaning of Fco, repeating to myself " It doesn't mean totally silent, but -20 dB loss" Once more, thanks for helping