Mike Dietrich wrote: > I asked that question for most of the people on the list that have not > tuned a duplexer themselves or don't have the proper test equip to do it > right and have done it with 2 radios or signal gen and a receiver. > These work but it is so much easier with a tracking generator once you > learn how to do it.
That's why I scrimped and saved and pleaded with my wife to buy an IFR 1500 when a decent deal came along. :-) > That's the way I've always done it with the 6 or 10 db load/attenuators > on the ports being used at the time and loads on the other. > Some people tune duplexers w/out the loads on unused ports and that > throws everything off in the tuning. Definitely. Easy to see on good test gear, too. "Have a go" at it sometime as the Aussies would say! > I have never seen it done but there should be a way to "see" a > particular cable's resonance on a tracking generator, but don't know how > to do it as in shorted or open stubs. > Does it take a network analyzer to do this? Seems like it could be done just like you'd sweep an antenna with one -- a directional coupler and you'd see the return loss (lettin' the RF out of the antenna so it doesn't come back to the test gear, so to speak) but it'd be backwards for an un-shorted stub. Someone here will comment on how to do it. (GRIN) > I have not tried what you suggest to tune the cans in the duplexer > seperate and then put the cables in and see how much the tuning changes, > but will try that and see how much it changes. An RF engineer friend of mine had to explain it very simply to me this way: "What's a pass-only can doing?" "It's a 1/4 wave antenna in a can, that has a loop inside to couple only the RF at the resonant frequency of the antenna." "Right. So if you tune one you let only one frequency through. Now, assuming *everything* is a perfect 50 ohm load on your input and output and your test cables are perfect, when you hook two of them in *series* that were both tuned individually, what should happen?" "Tighter skirts and still lets RF through at mainly only one frequency." "So what's wrong if you hook them together and you get two pass frequencies and the center frequencies don't match up? (A double 'hump' or... you have to retune the tuning rods to get a single one.)" "Impedance must be wrong, most likely impedance due to the length of the interconnect cables and how they interact with the loops in the cans. The two cans aren't 50+j0 anymore, so to speak." "Bingo. How do you fix it?" "Change the cable lengths or mess with the loops until you get better coupling." "Right, and since most of these duplexers were built for frequencies very close to what you're shooting for, which one is most likely to be easier?" "The cable lengths." Whether or not this little basement conversation is 100% technically accurate, I can't tell ya... I ARE NUT UN RF INGEEENEAR. But what he was trying to stress during my "Get over here and tune this VHF duplexer, it's a test..." session was... Tune the cans individually, and GENERALLY if you then hook the duplexer back together with its original cable set and things look a whole lot worse on the test gear and you have to MESS with the tuning rods (I've left the capacitors out of this discussion altogether for BpBr type duplexers, but same concept...) much... something OTHER than the 1/4 wave stub in the can is at fault. It's resonant frequency is already "locked down" and correct. So... the lesson was... You're tweaking the wrong adjustment, so to speak, if you're messin' with the tuning rods after tuning the cans individually... in most cases. Yes... in the real world, everything's not perfect... you might have to make SMALL adjustments for absolute best-performance on both the tuning rods and the trimmer caps... but if you're really having to crank on things after tuning each can separately, something ELSE is wrong. And it's almost always the cable lengths. Now... with all that said, even my friend had a hard time explaining how DB gets those odd-ball 4066 square cans they make for UHF that have a "low-pass" and a "high-pass" clearly marked on them so cockeyed that you really do have to use the "low-pass" and "high-pass" ports appropriately, even if the TX and RX end up on the "wrong" sides. You can see it on the test gear, the slopes on each side are slanted. I guess he's saving that for an advanced lesson. Something to do with the size of the coupling loops and the choice of capacitor used inside... I didn't get it. Nevertheless, with the right test gear you can see how much isolation you can squeeze out of them and if it's enough for your transmitter power... it's enough! Nate WY0X
