At 9/25/2007 07:59, you wrote:
> >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. The spectrum analyzer method is of course an excellent way to look for interference sources. However, a noise floor of -120 dBm @ 10 kHz RBW is a bit on the deaf side when you consider that your repeater RX will be almost 10 dB better than that. You might consider adding a preamp & an additional pass cavity in front of the analyzer to maximize sensitivity if at first you don't find anything. Bob NO6B
