The noise floor is really decreasing the utility of the new
repeater. The noise source seems to come and go as a quiet signal
on the repeater input can become suddenly noisey, and vice versa - a
noisey signal can become suddenly quiet.
It looks like you have a 10dB degradation. Many times this is from
phase noise from other transmitters, but intermod can give you the
same problems. I would look for a VHF paging transmitter if phase
noise is suspected. This type of troubleshooting requires a good
spectrum analyzer in most cases. I typically use the RX part of a
duplexer on the front end of an ANRITSU 2721B to keep the first mixer
in the spectrum analyzer happy. I hook up a 10dBd Yagi to the
duplexer and start looking for the noise floor to rise. Once I have
found the site that is causing the increase in noise floor, then the
hard part comes, getting the other site to cooperate in further
testing. What we ultimately want is for the other site to completely
shut down for the time it takes to test the sensitivity of the
receiver that is experiencing the degradation.
Since you mention that the degradation is intermittent, you may be
able to monitor other signals and see a correlation between when the
suspect transmitter keys, and an increase in the noise floor in your
bandpass. If the problem is because of intermod, then it becomes a
little more difficult as you have multiple culprits.
The spectrum analyzer you use should have a noise floor of around
-120 dBm @10 KHz bandwidth.
73,
Mark N5RFX
