Hi Mat, thanks for your kind reply. Sorry for pictures not being uploaded, I'll try again right now.?
I do not have VNA, I'm thinking to buy NanoVNA because I read very positive reviews, but now I use a Siglent SA that is enough for my present needs. Also, a HF RF Bridge prototype is waiting for me for many months?in order to be finalized, let us see next future...
About the modification: the cold side of each bandpass is RF grounded via capacitors, so the bandswitching PIN diodes can act being diven by direct or reverse polarization. But so doing the filter cold side is not really a solid RF ground because the capacitors are not ideal, an a low value resistance is inherently present inside. So the ultimate attenuation values far from the bandpass region is dependant on this residual resistance. You can also imagine the filter as a frequenct selective attenuator, where the ideal ground path has a low value resistor in series to the real ground, so that the desired attenuation out of band can't be achieved. Probably 40 years ago the electrolytic caps within the equipment were good enough for the purpose, but after this long aging the ESR tend to raise. I simulated the original circuit inserting a low value resistor in series to ideal bypass caps obtaining responses thhat are very similar to the real behaviour.?
Yesterday I built a prototype of a 28-30 MHz filter that could replace the original one. Measuring it as a stand alone unit results are very good, next days I''ll try to put it into the rig. With a very little arrangement the new filter can be referred to the real solid gnd and in theory no adverse spur variables will worse the ultimate out of band signals rejection.
Kind regards
Enrico IK2XEI
