On Apr 11, 2008, at 7:48 AM, Paul N1BUG wrote: > Thanks Ron, > > I will include these specs in the next revision of the Guide to > Duplexer Specifications on the RB site. > > http://www.repeater-builder.com/rbtip/duplexerspecs.html > > I'm always keeping an eye out for data that isn't in there yet. I've > added several since the current posted version so it's probably > about time to send in an update. > > 73, > Paul N1BUG
Paul, Confused here... your document at that "specs" page shows the DB-4076 as being a high UHF duplexer (485-505 MHz) while right here on RB, the duplexer's manual says it's much wider (404-512) on the top of the manual, and then that same manual says it's a completely different range (450-512) in the description text in the document. Weird. http://www.repeater-builder.com/db/db-4076-tuning.pdf I'd just say it's a typo in the title of the manual... but... A friend and I just tuned a 4075 with an added can -- making it into a 4076 (the square/brown painted variety that were often found in GE MASTR II cabinets with a GE part number on them) -- to the 446 range tonight. The "tee" section cable lengths were wrong, and had to be "messed with" to get a nice looking pass and notch, but the isolation was still very good looking prior to the change. The pattern didn't look THAT bad prior to the "tee" changes, and would have worked for a non- finicky ham, but we were shooting for "getting it right". By fiddling with the cable lengths going to the "tee" in the middle, we got it doing at least 95dB (the instrument "ran out of receiver" at that point) and a good looking "normal" BpBr trace on the test gear at near 446.95/441.95. (I say "near" because it's going on a project that hasn't had the frequency published yet... but it's tuned for that and I'm purposely being vague until that project is up and running.) By experimenting we found cable lengths for the "tee" that gave the right picture on the HP service monitor. They ended up being slightly longer than the original cables. The individual halves of the duplexer (two cans, each side, one high- pass, one low-pass) looked great individually, but the "tee" section was no longer the right length (and thus 50 ohms) at the lower frequency. The slight "double-hump"/"flat-topping" at the peaks made it so we could tell it was a bit off... on a spec-an/tracking generator in the HP service monitor... so we fiddled with the center "tee" cable length to get it having nice sharp peaks, top and bottom. Basically, just getting that 1/2 wave length (including the loops, etc...) right between the output of both sides into the tee. Once we added some length to both sides with experimental connectors and extensions, we made up quality custom cables by "eyeball" of what we figured the electrical length to be after getting to them experimentally with various L-connectors and N connector extensions to the right length. (My friend has done this a lot. I was watching and handing tools, but I understood the whole process, and was just enjoying watching him get it right the first time, without having to shorten or lengthen the cables. Not my typical evening on the workbench! When you park a computer/digital/networking guy down in front of a service monitor and a set of cans, he needs a couple more hours than the RF guy to get it right! That digital guy, would be ME... but I can usually get there if I'm persistent enough.) Anyway, once we finished there's PLENTY of tuning rod travel left over, and it is nowhere NEAR backing the tuning rods all the way out (lower in frequency) now that it's done. I'm sure this "reconfigured" 4075 to a 4076 would have gone quite a bit lower than 447 MHz. Probably not down to the low 400's though. 442 RX/447TX is way below the "spec" page and their description, but matches the title of the DB document... Hmm. Not sure which of the three is "the most wrong", but I now have seen two 4076's working just fine in the ham band, because I have another 4076 handling a link radio that's "upside down" at 447.575 RX/442.575 TX also. They're definitely set up in such a way that you want the hi-pass and low-pass used correctly -- there's a "slope" to the individual cavity response, versus say a "normal" looking cavity -- and with our high- out, low-in systems here, you typically are "reversing" how they were used prior to them coming for a ham shack visit. But the individual cans themselves are fine down at 442/447 in the two I've now worked with. Others with more experience can pipe up with what they think of them. (Special thanks to Jeff DePolo for his comments when I was first messing with that first 4076!!! That and some local Elmering got me on the right track to use the low-pass and high-pass as "advertised" on the labels, no matter what the original pair was, or if it was "upside-down" from what I was using it for. That was a key piece of information because I had tuned it ALL wrong at first, resulting in one of the strangest looking patterns I've ever seen, trying to drag the low/high-passes the "wrong" way!) -- Nate Duehr, WY0X [EMAIL PROTECTED]
