OK guys... I thought I could figure this out on  my own, but I was 
wrong. I could use some wisdom from the group! I have been chasing a 
stubborn case of "duplex noise" for a long time without success.

The 2 meter repeater will run clean all day at 100 watts (or any 
lower power level) into a quality RF load placed on the antenna port 
of the duplexer. It will run clean all day with the same load placed 
at the antenna end of the feedline.

With the antenna connected it will SOMETIMES run clean. At other 
times we get the crackling and popping of "duplex noise". And at 
times we get increased noise floor (I can see it on the receiver 
limiter current but it has no specific "sound", it's just like the 
normal receiver noise floor came up 20 to 30 dB!) None of this 
happens when it is run into the RF load. It is only when running 
into the antenna, and then only sometimes. The steady high noise 
floor, when present, happens only when my transmitter is up. It 
happens even if no other transmitters on the hill are up. It will 
occasionally go for days or even weeks without a glitch, then be 
essentially unusable for hours or days.

The crackling does seem to be worse in windy weather. The steady 
noise does not seem to be better or worse in any kind of weather, 
but occurs completely at random as far as I can tell.

Three different antennas (all DB or Sinclair, two of them NEW) have 
been tried with no significant change. The feedline (Andrew 
LDF5-50A) has been swapped out once with no change.

I think that leaves the tower or other nearby metal structures as 
the prime suspects.

The tower is 100 feet of Rohn 25G guyed with 3/16" EHS. I have tried 
more than once to "bond" the sections of tower with straps across 
the leg joints, and similarly where guys attach to the tower and/or 
turnbuckles etc. at ground level. These efforts did not help and 
seemed to actually make matters worse. By the way, the guy ends use 
Big Grips, not clamps. Was that a mistake for a repeater tower?

I am looking for advice. If anyone has solved noise problems in a 
similar tower, I would very much like to know specifically what 
materials you used and how you installed them that worked for you!

I do have another newly erected 100 foot tower close by. It could be 
part of the problem. However, I was having this problem long before 
that tower went up so I'm still pointing fingers at my own stuff.

A 440 repeater on the same tower does not have any problems. Neither 
repeater seems to be affected by the other's transmitter. It is only 
the 2 meter repeater killing itself.

Both my tower structure and the new adjacent tower structure are 
"hot" with RF from my 2 meter transmitter, as evidenced by horrible 
noise when something like a screwdriver shaft is lightly rubbed 
against the towers or guys. I do not find any other metal structures 
in the vicinity that react that way.

There is NO loose hardware in my system. I've been over it time and 
time again. There is NO visible rust anywhere.

Any suggestions before I pull the rest of my rapidly thinning hair out?

Here's one for ya... this problem first reared it ugly head right 
after I put up the tower. For years prior to that I had run the 
antenna on a rusty metal mast with loose fitting joints without ever 
a hint of trouble!!!

Paul N1BUG
147.105 and 444.950

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