Alexander,
That is a good start. Now you have some more work to do
(sorry I didn't have a chance to comment before your trip
to the site!)....
With the preamp OUT and repeater transmitter DISABLED, get
someone to give you a weak signal (something that is far
from full quieting). Now put the preamp back in line.
Does the signal quieting improve substantially? If not,
then you have a bad preamp, or the noise floor at your site
is too high for a preamp to help you, or the preamp is
being overloaded by other RF at (near) the site, or
possibly the preamp is overloading your receiver. Try a
bandpass cavity before the preamp and see if there is any
improvement.
If it passes the above test (signal is better with the
preamp), then do the desense test again. Do you have more
desense with the preamp in? If so, some attenuation after
the preamp might help. Or it might not, depending on
exactly what the problem is. As Jeff explained, some
attenuation before the preamp may be helpful too, if you
have a high noise floor at the site. If you have
substantially more desense with the preamp than without it
and adding attenuation doesn't help, your duplexer probably
isn't providing adequate isolation to run with a preamp.
In this case there are a number of possible configurations
involving added cavities that may help.
Try these tests and see what you come up with....
To answer your question below, the bandpass cavity will
probably do more good if placed before the preamp: duplexer
> bandpass cavity > preamp > receiver.
Paul N1BUG
On Monday 23 May 2005 04:34 pm, Alexander Tubonjic wrote:
> Hey Guys,
> Well few of us played with the repeater today. We took
> the preamp out of line and then did the "desense test".
> We shut the transmitter of, opened the squelch and
> listened to a weak station on input of the repeater, when
> the transmitter was enabled I was the only one to notice
> any desense. It was a very minute amount. I think I am
> going to try the preamp route with an attenuator in line.
> Just thinking, what if we ran the setup like this:
> repeater, then band pass filter on the rx side, then
> preamp, then duplexer. Would that help alleviate some of
> the desense from the preamp? Thanks for the help.
> Alexander
> KG4OGN
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