[Elecraft] KPA 500 rfi problem and other curious behavior

[Jack Brindle]

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.

There are several solutions, usually involving adding a radial field (or more, beefier, radials if you already have some), and/or adding ferrites to the coax where it feeds the antenna. K9YC has an excellent tutorial discussing the use of ferrites at the feed point at his web site. Check out: http://www.audiosystemsgroup.com/RFI-Ham.pdf 
Jim has several other excellent papers at his site as well. He gave a couple of very good talks at the recent Pacificon Hamfest/Convention about verticals.

There are many problems with antenna systems that are masked with low power. This makes low power operation much simpler since we don’t have to worry about these things. When an amplifier is added, the level of the problems is suddenly increased, and show up in various ways. With the KPA, the first thing seen is usually high SWR faults. At low power the SWR looks good, mostly because the shield return currents are fairly low. But as power is increased, they become a major factor, and will have a huge effect on SWR measurements. We also see a lot of RFI in the shack with high power - if you have amplified speakers you will instantly realize the situation with your transmitted signal coming out the speaker in a very distorted fashion. There are standard techniques to resolve these problems, involving the application of ferrites in various places (power cables, speaker leads, antenna feed lines), improvement of the antenna system (coax, connectors, radials, baluns of various sorts), or some combination of these things.

The point is, check your antenna system. The problems being reported are real, they just haven’t been seen before the application of high power. This is an excellent opportunity to learn far more about the proper engineering techniques that we should use in building our stations, but didn’t realize how important they were...

Good luck with your quest.

Jack Brindle, W6FB


On Oct 24, 2013, at 4:32 PM, Robert Redmon <k5sm.bob at gmail.com> wrote:

> In addition to the display problem on 30 meters described in a previous post, I have never been able to use the KPA 500 on 80 meters. If I run the K3 at 100 watts, the swr shows very low (below 1.3:1) and is consistent. If I put the KPA500 in line, it faults with "high swr" as low as 75 watts displayed on the KPA's display. Also,the swr changes as I transmit, randomly spiking up to full scale and shutting the amp down. It does this with even more frequency at higher power. I have eliminated as many other variables as possible and have come to the conclusion that on 80 meters (only....apparently) enough rf makes its way into the KPA500 to cause this behavior (even at very low power). The antenna is a shunt fed tower, fed with buried hard line about 100 feet from the shack. I feed the same tower (same feedline) on 160 without issue.
> 
> My instincts tell me the ingress point is the AUX cable (the one thing I haven't tested), but I am otherwise at wit's end. Anyone have any suggestions?
> 
> Bob
> 
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