Topband: Excessive noise on beverages

[David Olean]

I have been putzing around trying to improve my 160 meter setup, and 
have run into a real problem. Most of the USA is covered by two 
directions from here in Maine.  SW and W.  My problem is that I have an 
excessively high noise level on both of the beverages that run in these 
directions.  In the last six months it has gotten worse, and so bad that 
I cannot use either beverage.  The wires terminate at the side of my 
barn where I have ground rods and the RG-6 goes through PVC conduit up 
to the shack on the second floor of the barn.  During the day I see 
noise levels at -133 or -132 on the two wires, but at night the noise 
had climbed to levels around -100  or to -110 on a good night.  I 
suspected that I was getting noise coupled in from my vertical which is 
rather close and on the other side of my barn all of 55 ft away.

Yesterday, I modified the west beverage and shortened it so that it 
terminated about 150 feet away from the barn and I ran new coax back 
across the field.  I kept the SW beverage as is to compare noise levels 
at night. Lo and behold, but the noise went to -110 on the SW beverage, 
but only about -120 dBm on the modified beverage.  This is a 10 dB 
improvement.  Other directions at night run at about -130, so I am now 
seeing a 10 dB extra hit in noise to the west.

The vertical is a shunt fed Rohn 25 with a total height just over 90 ft 
with top loading from a 5 element long boom 10 Meter yagi. I am not sure 
if it is possible to simply un resonate this tower. Does anyone have 
experience with simple ways to decouple a shunt fed tower from the rx 
antennas?

The extra 10 dB of noise that I get to the west and SW  is mostly due to 
the location of a town about 7 or 8 miles away in NH. It is ripe with 
all sorts of power line noise.  I still can't rule out re radiation from 
the vertical though! I spent many years fighting with the electric 
utility to quell the noise. I involved Riley Hollingsworth and later on, 
Laura Smith, his replacement.  The effort was futile and I lost heart as 
the problem was just too big for one person to fight.  I was hoping that 
the noise would not affect 160 meters as it had messed up my VHF 
exploits, and so far I only have this ten dB swath of noise that bothers 
me in two directions.

Any help with un resonating a shunt fed tower would be appreciated.

Dave K1WHS