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/
