I will second Jack's observations. When we remodeled and added the room
that houses my radios, I had them put a 3" steel pipe from a 1' square
utility box in the wall straight up the wall with a weatherhead on top.
The box opens into the shack under the desk and also into the carport.
I recently bought a GAP Titan for WARC use, and cleverly mounted it on
the 3" pipe sticking up from the roof.
The bottom square 40m counterpoise is about 10 feet directly above my
radios. I can run the K2 @ 10W, and the K3 @ ~80W just fine. Anything
above that results in instantly high SWR indications, multiple flashing
LED's and strange displays, and I'm probably sterile now. :-)
I haven't done the common mode chokes yet, that *may* help some. I
don't know what effect having the coax in the steel pipe might have, but
whatever it is, it isn't enough.
Jim's [K9YC] RFI tutorial has some excellent quantitative data on
ferrite cores ... which mixes to use, how many, etc. The Titan is
essentially a center-fed half-wave dipole. The instructions warn you in
bold CAPS to have the coax exit the bottom of the mast through a hole
about 1/2" up, and NOT directly out the bottom. I don't know why, but
it's highly suggestive that the coax forms a part of the antenna
"system" beyond just feeding power.
73,
Fred K6DGW
- Northern California Contest Club
- CU in the 2014 Cal QSO Party 4-5 Oct 2014
- www.cqp.org
On 10/25/2013 12:10 PM, Jack Brindle wrote:
This is most likely an antenna problem. As my friend K9YC will
testify, vertical antennas need a good current return path, which is
usually a good radial system. Without it the return current will be
carried wherever it can, which usually winds up being on the outer
edge of the coax cable shield. This will in turn come back to the
amplifier and be indicated as a high SWR. It is real, and a problem.
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