Thanks for the great info Eric! I obviously cannot afford new so in order to weed out the stuff that cannot be used that is often found on E-Bay I thought I post the question here to find out what I should be looking for. When I mentioned cheap, I was thinking in the range of $250-300 or so... not a $1500 TXRX set ;-)
On top of which, this is by far, not for building a wide coverage repeater. I only have a 60ft tower; this is more or less for me to learn with. How to build effetive systems for a knowlege base so when I do wind up with some commercial tower space, I have the knowlege to do it right quickly the first time. I have a decent UHF system using low cost stuff that required me to work at it to get it so it has decent coverage with the surounding terrain around me. VHF would help fill in the spots where the UHF does not make it - hence my want to build up a VHF machine. If it requires me to work on something to get it to work right, heck, that's what ham radio is in my eyes.... not some order it from the internet catalog then plug and play. By building it, working it into a working system, if something goes wrong, I can very quickly identify the problem and fix it. Thats the way I look at it. Again, thanks for the info and for keeping it within the subject I started ... not varing off explaining about using seperate antennas or saying I should go into another aspect of the hobby if I cannot afford top of the line stuff. Dave Schmidt / N9NLU --- Eric Lemmon <[EMAIL PROTECTED]> wrote: > Dave, > > It is important to understand that ANY duplexer that will handle a > 600 > kHz split at 2m is neither cheap nor abundant. The only 2m duplexers > that I usually see on the hamfest market are those that just don't > make > the grade in a good system. You won't normally see really good 2m > duplexers being sold for any price on the used market for one simple > reason: The owners know they have a good duplexer, and they keep it > in > service! > > Okay, here's some guidelines. If you have a crystal-controlled > repeater > that puts out 50 watts or so from a tube PA in the transmitter, and > the > receiver has a string of helical resonators in the front end, you > MIGHT > get by with a four-cavity duplexer of the bandpass/notch variety. > However, if you have a synthesized transmitter, you now have to deal > with a lot of "synthesizer noise" that is not present in a > single-frequency transmitter. Your synthesized transmitter probably > has > a broadband solid-state PA that amplifies the broadband synthesizer > noise along with the fundamental carrier. Not only is such a "low-Q" > amp noisy, but its untuned output circuit has an impedance that > varies > with power level- not conducive to a good match to the duplexer. > Finally, your synthesized receiver probably has a very broadband > front > end, a virtual "barn door" for all the PA noise and adjacent channel > noise to enter and cause major desensing. > > Not all duplexers are created equal. I have had several WACOM, > TX-RX, > Sinclair, and EMR duplexers pass through my hands in the past few > months, and I have learned some interesting things. First of all, > size > does not indicate just the power rating. A larger cavity has a > higher > "Q" meaning that it can be tuned more sharply and with a greater > notch > depth. They are also more costly. In many cases, a pass/notch > duplexer > with six, four-inch cavities will outperform most duplexers having > four, > seven- or eight-inch cavities. But, the bandpass tuning of such > duplexers is often rather vague. An ideal pass/notch duplexer SHOULD > have a deep sharp notch for the reject side AND a narrow sharp peak > for > the pass side. Reality check: Most pass/notch duplexers do well on > the > notch task, but fail miserably on the pass task- they have a small > "hump" at the pass frequency, but there is practically no useful > selectivity. If you have a noisy PA with lots of synthesizer spurs, > you > must filter out all of that chaff and keep just the wheat. You guys > with the GE tube PAs are gloating, because such tuned PAs are > legendary > in their freedom from noise. > > The solution is to follow your pass/notch duplexer with at least two > bandpass-only cavities at the RX input. This will filter out your > PA's > noise, and will also protect your RX front end. I am quite certain > that > many other repeater builders will agree that it's possible to have a > good repeater when the duplexer, antenna, and feedline are the best > you > can afford, when mated with a cheap transceiver. The converse, > however, > is not true. Please don't spend all your dough on the transceiver, > and > then skimp on the rest. > > 73, Eric Lemmon WB6FLY > > cworksmidwest wrote: > > > > I'm building a VHF high band repeater... on a tight budget. I have > > everything except a duplexer. I need some recomendations on cheap > > abundant duplexers that will do the 600Khz split. Max power I'll be > > using is 50 - 75W so recomendations for some Monster wacom or TXRX > > duplexers that can handle a bazilion watts do not need to be > posted, > > besides, I probably wouldn't be able to afford ANY used wacom or > txrx > > duplexer sets. > > > > If you have something low cost that can work or if you know of > > something cheap that can work, let me know. > > > > Thanks > > > > Dave / N9NLU > > > > > > > > Your use of Yahoo! Groups is subject to > http://docs.yahoo.com/info/terms/ > > > > > > > Your use of Yahoo! Groups is subject to > http://docs.yahoo.com/info/terms/ > > __________________________________ Do you Yahoo!? Protect your identity with Yahoo! Mail AddressGuard http://antispam.yahoo.com/whatsnewfree Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
