Tom
I think you have a corrosion problem in your antenna system.
I had a similar if not identical problem in a 2 meter repeater here----years of
good performance and then serious desense. I ran the same tests as you have
with same results.
The fix here was an overhaul of the Hustler 144-G7 if I remember the numbers
right. Corrosion was found in several places-----fix was to clean down to bare
metal to bare metal and reassemble with no-ox in the joint and more under
several layers of shrink tubing over the top for weather protection. The
antenna is still in service with no trouble.
Corrosion in the antenna results in transmitter signal rectification within the
antenna which produces noise of sufficient bandwidth to cover your receive
frequency and this of course comes back down your feedline and properly goes
right through your duplexer into your receiver. The curious thing is that under
most conditions, this wide band noise results in desense without changing the
audio noise output from your repeater receiver when your repeater transmitter
is switched off.
The fact that desense goes away when the antenna is replaced with a dummy---I
assume that you are injecting your test signal into you receiver using an
attenuating "T"----I think your problem is your antenna or something very close
by that it is exciting.
Scott
----- Original Message -----
From: Tom Elmore
To: Repeater-Builder@yahoogroups.com
Sent: Tuesday, December 23, 2008 10:03 AM
Subject: [Repeater-Builder] Six Meter Repeater Noise Issues
Several months ago I put a six meter machine on the air in my area. It is
a GE Master Pro tuned for 52.810 out and 51.110 in. One of the things still
nagging me is some sort of desense or RF phase noise, let me explain. After
tuning up the duplexers into a dummy load and running some tests I experienced
no desense all the way down to about .15uV. I moved the dummy load to the end
of the transmission line just to be sure and again the same results. When I put
the antenna in line and run the same tests this is what occurs. When I key the
transmitter and set the output of the signal generator from a starting point of
say 100 uV I hear what sounds like phase noise or just plain static just
slightly in the background. As I bring the signal generator output down the
background noise gets louder but it never wipes out or overloads the receiver
altogether as I can still hear the generator and the background noise and this
is down to the same squelch threshold I get when on the dummy load. I am
hesitant to call this desense as say when one of the duplexer cavities isn't
tuned correctly. Then it is obvious because the transmitter totally wipes out
the signal I am feeding it from the signal generator. I thought perhaps the
preamp was the culprit so I took it out of line but sill experience the same
issue. I am thinking that possibly the repeater output from the antenna is
getting back into the repeater cabinet? I took a handheld scanner and set it on
the same frequency as the receiver and connected directly to the rx port on the
duplexer and can hear the noise there as well. I do hear a slight buzzing in
the audio of the receiver almost like 60hz whenever I key the transmitter with
the squelch wide open and no input signal present using the antenna. I don't
hear it when using the dummy load though. I would like to think that the
duplexer is tuned correctly or fairly close as there isn't any desense when
terminated into a load. The last thing there is a single phase 7200 volt
primary line servicing our neighborhood probably less than 100 feet from our
house that I wonder is the culprit. I don't hear any arcing or power line noise
with just the receiver squelch open but maybe when I transmit there is some
mixing going on?
Thank You
Tom Elmore KA1NVZ
Anchorage, Alaska
