Norm, Thanks for the update on your "quest." The loops have two adjustments, and they are not complementary. The degree of coupling, and the amount of insertion loss, is adjusted by loosening the three screws that secure the loop mounting plate, and turning the plate slightly to achieve the desired insertion loss- which is generally around 0.5 to 0.8 dB per can. The variable capacitor mounted in the loop plate is used to move the notch closer to, or away from, the bandpass peak. Are you following the Q202G tuning instructions shown on the RBTIP? Those instructions are here: <www.repeater-builder.com/sinclair/ci-1069-Q-Series-Tuning.pdf>
Because the bandpass peak is so vague, the best way to tune it is to use a network analyzer or a spectrum analyzer with a return-loss bridge. When tuning for return loss, the image on the display is a very sharp notch which is easy to get right on the money. Then, the analyzer is switched to transmission loss to set the notch capacitor. Finally, the loop plate is rotated to achieve an insertion loss of between 0.5 and 0.8 dB. If the jumper cables between each pair of cans are the correct length, the insertion losses should add exactly. I normally go through this routine at least three times to get the tuning as good as I can. I looked at some Q202G loops from a 2m duplexer, and they are plain copper- not silver-plated. The standard VHF loop assembly has a 1/4" wide copper strap bent into a rectangle that measures about 1-1/8" by 3-1/8", so if your loops are much different in size, they may be unsuitable for 2m operation. The notch tuning capacitor is a Johanson 5602, which is rated 1 to 30 pF, and has a Q of greater than 800. I have to wonder if the added capacitors you found were high-Q silver micas or ordinary ceramic capacitors. If the latter, the cavity cannot be tuned properly. The Johanson capacitors are on page 4 of this brochure: <www.johansonmfg.com/pdf/Air-Capacitor.pdf> One thing to keep in mind about BpBr duplexers- and not just those made by Sinclair- is that the presence of the notch tuning capacitor means that there is no DC ground anywhere on the feedline between the antenna and the receiver. A high-voltage spike caused by a nearby lightning strike, or an electrical system fault, can sail right into the receiver front end. That's one good reason for having a true bandpass cavity- which has DC grounded loops- somewhere in the RF chain. I have heard a few reports of notch tuning capacitors that were destroyed by high-voltage arcing. This damage would not be visible. 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of NORM KNAPP Sent: Sunday, July 26, 2009 3:58 AM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Sinclair Q202 duplexers (frankenstein series) Ok, it is official. I have a set of sinclair Q202 duplexers that didn't come as a set. At least two of the cans have different serial numbers and different factory tuned frequncies. DRAT! Ok, so here is the problem. With my trusty Aeroflex 2945 (I think that is the model) I can easily get a -35dB notch on each of the hi pass cans at 147.825 pass and 147.225 notch. No problem with that side. Ok, the problem is on the low pass pair (mix-mached I am sure). I can barely get a -30dB notch on each can even if I spin the loops. So, just out of (morbid) curiocity, I pulled the loop out of one just to show a fellow ham what was in there. Lo and behold there was a 12pF cap soldered accoss the notch tune cap. Hmmmm... What would happen if I took this cap off there since to get the -30dB notch the tuning cap is almost screwed all the way out (min capacitance). So I got out the soldering iron and removed it. It worked, sort of. I was able to get a -35dB notch on my lo pass (147.225 pass 147.825 notch) side, but at the expense of some pass loss higher than the high pass side. What happend is it seemed to make the low pass can into a high pass can. The notch went to the other side of the pass. I spun the loop to get the best result and tuned the notch cap near its max capacitance to get the notch where it should be. It worked barely and with a bit more loss in the pass. I am thinkin I may need to go back and put a 6pF cap accross the notch tune to get it where I want it, but I am not sure. On the second can of the lo pass high notch side I tried to remove the cap, but it didn't turn out the same. The loop inside it was not silver plated like the previous can, it was just plain copper and appeared to be a bit longer. When I removed the cap on this can, the notch went to the low side of the pass and I couldn't get it to come back around. I will definitly have to try a 6pF cap accross the notch tune cap on this one. Now, I just replaced the harness with RG-214/U jumpers the length needed to get 14" between centers of the "Tee's". The old ones were RG-142/U and were also apparently the correct lenght as well. Changing the jumpers didn't seem to affect the tuning ability of the cans. It was a waisted effort really, but oh well, they have a really nice new harness on them, makes em look 10 years newer. Any suggestions guys. It lookes like I may have a modified pair of cans that are actually high pass instead of low pass. Should I play with the length of the loop? What about changing the 12pF caps accross the notch tune caps (parallel) to 6pF or so? Right now, they are working, but the high pass pair look really good and the low pass pair look so-so to kinda oK. The repeater is working very well, but I feel if I get the cans right it will be even better. Anu suggestions? Call me if you like 251-234-0295. 73 de N5NPO Norm
