Dave, You might be able to take the TX tower shunt wire to ground with a
relay and change its resonant frequency. Another solution might be a cage
wire feed which when taking directly to the ground might act as a
decoupling sleeve. Broadcast engineer have experience in this since
electrical power towers can drastically change
their multi-tower directional patterns. I would Google this since
consulting engineers have written papers on how to do this.I would strongly
advise you to put a 1:1 balun and only ground the shack side coax. DX
Engineering makes one of these and the cost is under $50. Long coax runs
seem to carry the noise back to the antenna feed point and these will
provide good isolation and may reduce the noise pickup significantly.
Before you do anything it is important to identify the noise source and
type.
Herb Schoenbohm, KV4FZ
On Tue, Mar 6, 2018 at 1:30 PM, David Olean <k1whs@metrocast.net> wrote:
> 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
>
>
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