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Fwd: Re: KPA 500 rfi problem and other curious behavior

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K5SM

Fwd: Re: KPA 500 rfi problem and other curious behavior

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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Rick WA6NHC

Re: Fwd: Re: KPA 500 rfi problem and other curious behavior

What does it do into a known good dummy load?

If the same, your amp needs to visit the shop.

If it works with the dummy, it's likely RFI. Adding a common mode choke at the antenna feed along with improved bonding (grounding) would be the first steps towards resolution of your problem.

Rick wa6nhc

Tiny iPhone 5 keypad, typos are inevitable

> On Oct 24, 2013, at 4:32 PM, Robert Redmon <[hidden email]> 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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Jack Brindle-2

Re: KPA 500 rfi problem and other curious behavior

In reply to this post by K5SM

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 <[hidden email]> 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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k6dgw

Re: KPA 500 rfi problem and other curious behavior

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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Iain MacDonnell - N6ML-2

Re: KPA 500 rfi problem and other curious behavior

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 <[hidden email]> 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.
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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> Post: mailto:[hidden email]
>
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