Some thoughts from past experience. Sinclair Q202 cans are not high pass OR low pass, they are both. They have 2 notches, one on either side of the pass, separated by typically 10 MHz more or less. The 2 notches move as a pair by varying the series capacitance on the loop. The more capacity the lower they are. A low capacity places the lower of the two notches close to the low side of the pass and gives you a high-pass can. Higher capacity lowers the notches until the higher notch is close to the high side of the pass and yields a low-pass can.
The value of the capacitance you need for a VHF Hi-band is 1-30 pF, adjusted typically to less than 10 pF for a high-pass config and 20-30 pF for a low-pass config. One more very important consideration. The Q of the loop series capacitance must be very high (1000+) to get a good deep notch. Most capacitors do not qualify. The Johanson variable caps (5600 series) used in the VHF Q202 cans have a Q of 800+ (at 250MHz), higher at 150MHz. The UHF cans use Johanson 1-10pf variable caps (5200 series) with a Q or 5000+ (at 250 MHz). If you think about parallelling the variable with a fixed cap, the only fixed caps I know of with sufficient Q are silver mica units. The bare uninsulated UNELCO brand transmitting caps work well. Other brands may also work but ceramic capacitors definitely do not have enough Q for this application. The other alternative I have used at times on VHF cans is to parallel 3 1-10 pF Johanson variables. One advantage of this is that 1-10pf Johanson trimmers are common in surplus for 50c-1$ whereas 1-30pf caps are very rare in surplus and new cost is upwards of $15 (my Scottish ancestry is showing:-) Good luck in your experiments. Burt VE2BMQ NORM KNAPP wrote: > 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 pas s 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 wher 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 lenght of > the loop? What about changing the 12pf caps accross the notch tune caps > (parralell) 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 > >
