There is a magic number where the output of one repeater will not bother the input of the other and vice versa.  I would want the two repeaters to be as close as possible to each other's frequency.  You can use ONE duplexer for both repeaters if they are very close to each other, say less than 75 khz or so.  Use a hybrid combiner and isolators to combine the two close spaced transmitters, and a receiver pre-amp / splitter to feed the two receivers.  I have 3 repeaters in 460 using one duplexer with a cavity combiner, and receiver preamp / splitter combination.  The repeaters are within a 150 khz window and the system works well.  The repeaters are 110 Watt Johnson VX series running narrow band FM.
Steve NU5D

At 4/5/2006 07:27, you wrote:
>How close, frequency spacing wise, can a couple of VHF (2 meter) repeaters
>in the same equipment room on the same short tower be made to work well
>without much trouble or extra expense? How does 45 Kc. with same 600 Kc.
>offset sound? What will work this close and what won't?

If the offset is the same, closer is actually better since the duplexer of
each repeater will protect each RX from both TXs, both in RX notching of TX
& TX noise suppression at the RX freq.  However, isolators on both TXs are
a MUST, otherwise you will 2A-B mix in both TXs & end up TXing 45 kHz above
& below your two outputs.  Try to keep the two antennas as far apart as
possible.  If you can't separate the antennas much, you might need dual
isolators on both TXs.  I've seen two TXs 20 kHz apart at the same site mix
strong enough to be heard 10 miles away, even though both systems had
single isolators.

When dealing with close-freq. TX spacing, equipment shielding seems to be
more important for some reason.  Stay away from converted mobiles as
repeaters, or plan on using a separate RX in an RF-tight box with EMI

Bob NO6B

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