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