[Elecraft] KPA 500 rfi problem and other curious behavior

[iain macdonnell - N6ML]

If I understand the problem description properly, the VSWR was NOT
observed with the K3 running 100w, but WAS observed with the KPA500
making only 75w. That seems to suggest it's not the antenna. An
external SWR meter between the KPA500 and the antenna would be useful.

It may be meter calibration issue in the KPA500 - support should be
able to help with that.

73,

    ~iain / N6ML



On Fri, Oct 25, 2013 at 1:56 PM, Fred Jensen <k6dgw at foothill.net> wrote:
> 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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