When a tower is loaded as a vertical, by pass with a .1 ufd each rotator
lead and ground the coax shield to the grounded tower where it leaves it.
Just running them inside the tower often is not enough. Cables laying on the
ground can still pick up RF also. By pass the rotator leads and ground the
coax shield prior to coming in the shack also.
I had a typical beam location with a 20' tower on top of a house and a
tribander. The coax ran horizontal on the roof under the beam in the field
of the beam then down to the ground and in the shack. There were power lines
50' in front of the house. The beams balanced feed point was fed direct with
the coax. As I rotated the beam, the SWR changed a fair amount. I thought
it was due to the power lines. I added RF ammeters in series with each lead.
The currents were unbalanced (3 and 4 amps) and varied as the beam was
rotated. When a balun was added, the currents balanced up and varied very
little when the beam was rotated. The balun prevented the RF induced on the
shield from changing the feedpoint currents on all three bands. The balun
works both ways stopping RF Spill-Over & Spill-Up. The power lines were not
the culprits I thought they were. I have a chart of all the readings on 3
bands that appeared in a book I wrote. I can send it out as an attachment
when I get it scanned into the computer.
Palomar Engineers has a RF Current Meter Model PCM-l that will measure
relative levels with 3 sensitivity levels of RF Spill Over or stray RF on any
line or coax. It will show the effectiveness of radials of taking RF off the
coax shield of a center fed vertical. I use it all the time. I've even
detected bad and/or loose RF connectors. You can test the effectiveness of
applying toroids on a line or coax shield.
K7GCO Ken Glanzer
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