[Elecraft] KPA 500 rfi problem and other curious behavior

[Fred Jensen]

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.