Yesterday, with a buddy, I attempted to tune my first set of hybrid ring duplexers. I have successfully tuned many sets of duplexers and didn't expect to have any trouble with a minor freq move. I have read and understand the theory behind the design.
http://www.repeater-builder.com/antenna/hybridring.html In practice I saw some unexpected results. My buddy has a new to him HP 8935 (SA/tracking) so we started off using it. I have a HP8921A that we also used. We used good shielded cables, 3db pads x3 and good dummy loads, etc. We started off with a working good on 145.23/144.63 set of duplexers. They were taken off the air and brought to me to be moved up to 146.76/16. I am sure these were many megs up originally and were modified to get down to 145.23. It appears only the termination stub coax lengths were changed and they are longer than the 144-146 instructions suggested as starting points. So I understand the rings/stubs and antenna port for each side may not be *perfect* but they were performing and we are only moving them up a meg or so (towards the original harness freq).. First thing off the bat we looked at the current high pass side and it looked like it only had 45 db separation, the low pass looked better at 65, other wised shaped pretty good. How could they be working good all these years like that? Reading the HP correctly? Yes. Hmmmm. We used the Sinclair instructions to retune them substituting our tracking gen/SA test gear for the instructions sig gen/receiver. We followed the laborious instructions carefully (course tune/fine tune) and ended up with similar separation numbers on the new freqs. Hmmm, must be something quirky with the 8935, fired up the 8921 and it showed we had even less separation. Half what the 8935 showed. When placed into station service into a dummy load, they didn't work, I'll spare you the numbers. Time for a beer. We fiddled with it some more using both service monitors, a talkie and a bird and didn't move anything much, everything was still peaking/nulling. It has been my experience that with 3db pads on the 3 ports, patch cable lengths to test gear are not critical, I'm beginning to think this style ring system is finicky. After sleeping on it I am thinking we are really close. I'm guessing we are unable to see the ring cancelation/null/pass with the gear we are using even though we are able to see the pass and reject peaks during our steps? I'm looking for suggestions on what I'm missing and why we can't we use our expensive test gear to fine tune these freaks? 73 Don W5DK
