[Wswss] Re: [VHF] Fwd:(Slightly Off Topic) Repeater Preamp For Use In RF Dirty Site

[jeff millar]

Repeaters usually live at a site with a lot other strong signals.  The only real cure for overload 
is to filter out the strong signals.  One problem comes from the fact that many duplexers have sharp 
suckout notches but wide passbands.  This works well for the repeater's own signals but not so well 
for all the other Tx on the mountain.

As others have said, you need a narrow filter in front of the Rx to get rid of as much stuff as 
possible.

Preamps cause overload in the stages after the amplifier because they make every thing stronger.  A 
good commercial radio for the public safety bands will have extremely good strong signal 
performance, and as a consequence usually doesn't have super sensitivity.  Put a preamp on it and 
the strong signal capability goes away.

So, if you want a preamp, try to get one with about 10-13 dB gain, avoid the 20 dB gain types, they 
don't help, and kill strong signal performance.  If you have a 20 dB type, put a 6-10 dB pad after it.

Try to get a cavity bandpass filter to put between the duplexer and the Preamp.  Most of these have 
a coupling adjust to trade off between loss and bandwidth.  Start low loss and wide and adjust 
narrower and narrower until you get rid of the junk.

If you can get a second bandpass filter, put it after the preamp and set it to most narrow, because 
loss after the preamp doesn't really hurt performance.  This will allow the cavity in front of the 
preamp to run less loss (but wider).

The Repeater's receiver is based on a "ham" type design, replace it with a commercial radio such 
Motorola or GE.

jeff, wa1hco



n6ze at aol.com wrote:
> My friend, Bob, KK6UE, is the curator of a 2 meter repeater in ~~DM04pg, Thousand Oaks CA.
> 
> The repeater receiver is frequently bombarded by public service repeaters near the 2m band and also has lower than desired sensitivity.
> Among the 4 preamp URLs listed below?(2 from Ramsey, 1 from ARR, & 1 from N6CA), which might be the best for his repeater operations? What other preamps not listed might?help to minimize the co-channel interference?
> 
> bt73Pete, N6ZE?

> 73's
> Bob
> KK6UE