K3/KPA500/KAT500 SWR Problem WHITE SMOKE!

[Ray Coles]

I am pleased to relate that I seem to have finally nailed this problem, and
I think many of those who gave me helpful advice and the Elecraft design
team will be interested in learning what the final diagnosis and cure turned
out to be. A quick recap: About 2 months ago, operating at about 75W into my
trusty OCFD, the KAT500, KPA500 and K3 SWR readings shot up into the red and
all DX fun ended abruptly. The problem remained, so I started looking for
the culprit: I pruned tree branches that were touching the antenna after a
recent storm. No joy. I replaced my 300W 4:1 balun at the end of my RG58
buried coax feeder with a 1KW unit. No Joy. I replaced the buried RG58
feeder with UR8 which took a new route, avoiding burial. No Joy. I dropped
the antenna and replaced the 300ohm twin feeder from the balun to the T
piece, the T piece itself and the antenna wire and dog bones. No joy. At
this point I consulted the Reflector Oracle, and the most common response
was common mode current on the feeder caused by OCFD unbalance. I did
research and decided to build the renowned GM3SEK choke balun. I chose the
mid range version which covers broadly 5-14MHz and installed it just south
of my 4:1 balun at the North end of the feeder. No Joy. Further ideas from
reflector pundits suggested that it might still be common mode, but coupled
from the antenna into the feeder where my GM3SEK choke wouldn't help.

Fortunately I had bought enough Fair Rite oval cores to build all three
(Lo/Mid/Hi) balun designs, so in a final roll of the dice I built the two
core Hi Band balun and installed it in the Shack next to my rig. The hi band
version has a fairly flat impedance curve and reaches down to 7MHz and up to
30MHz. IT WORKED! I have run 100W on all the bands from 40m to 10m and I
have seen no run away SWR readings. My 25W tune value stays rock solid. As a
final push-my luck step I turned the KPA500 to operate and ran 400W SSB. No
Sweat! All of these things were no-nos yesterday.

The final diagnosis is as follows: Some simple fault perhaps a connector or
a crimp termination caused the first occurrence. I then compounded the
problem by replacing my buried RG58 with an unburied and, I now see, a more
parallel routed feeder which locked the problem in via antenna coupled
common-mode currents. This also explains the fact that I could keep
operating provided I kept power below 40W. Some respondents felt there was a
KAT problem which compounded the problem, I always felt that the problem lay
outside the shack but that maybe, just maybe, the KAT might be a bit
sensitive to the problem. The only thing I would ask the Elecraft design
team to investigate is whether the SWR circuitry has to respond to the
common mode currents as it does. Probably not a firmware issue though!
Thanks to the many people who made so many helpful suggestions.  E Pluribus
Unum!

 

Ray Coles, C.Eng. M0XDL

10 Littlemoor Road,

Weymouth DT3 6AA

Tel: [phone]

Mob: [phone]

 

 

 

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

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html

[Stephen G4SJP]

Good news Ray, I swear by Ian's choke design.  It has worked miracles
here.  As I mentioned in an earlier post I went straight to the 3 section
GM3SEK choke, in fact two of them, one at each end of the coax section.
Works a treat.  I can't speak highly enough of the KPA500/KAT500
combination.

73 Stephen G4SJP


On 15 January 2014 15:59, Ray Coles <[hidden email]> wrote:

> I am pleased to relate that I seem to have finally nailed this problem, and
> I think many of those who gave me helpful advice and the Elecraft design
> team will be interested in learning what the final diagnosis and cure
> turned
> out to be. A quick recap: About 2 months ago, operating at about 75W into
> my
> trusty OCFD, the KAT500, KPA500 and K3 SWR readings shot up into the red
> and
> all DX fun ended abruptly. The problem remained, so I started looking for
> the culprit: I pruned tree branches that were touching the antenna after a
> recent storm. No joy. I replaced my 300W 4:1 balun at the end of my RG58
> buried coax feeder with a 1KW unit. No Joy. I replaced the buried RG58
> feeder with UR8 which took a new route, avoiding burial. No Joy. I dropped
> the antenna and replaced the 300ohm twin feeder from the balun to the T
> piece, the T piece itself and the antenna wire and dog bones. No joy. At
> this point I consulted the Reflector Oracle, and the most common response
> was common mode current on the feeder caused by OCFD unbalance. I did
> research and decided to build the renowned GM3SEK choke balun. I chose the
> mid range version which covers broadly 5-14MHz and installed it just south
> of my 4:1 balun at the North end of the feeder. No Joy. Further ideas from
> reflector pundits suggested that it might still be common mode, but coupled
> from the antenna into the feeder where my GM3SEK choke wouldn't help.
>
> Fortunately I had bought enough Fair Rite oval cores to build all three
> (Lo/Mid/Hi) balun designs, so in a final roll of the dice I built the two
> core Hi Band balun and installed it in the Shack next to my rig. The hi
> band
> version has a fairly flat impedance curve and reaches down to 7MHz and up
> to
> 30MHz. IT WORKED! I have run 100W on all the bands from 40m to 10m and I
> have seen no run away SWR readings. My 25W tune value stays rock solid. As
> a
> final push-my luck step I turned the KPA500 to operate and ran 400W SSB. No
> Sweat! All of these things were no-nos yesterday.
>
> The final diagnosis is as follows: Some simple fault perhaps a connector or
> a crimp termination caused the first occurrence. I then compounded the
> problem by replacing my buried RG58 with an unburied and, I now see, a more
> parallel routed feeder which locked the problem in via antenna coupled
> common-mode currents. This also explains the fact that I could keep
> operating provided I kept power below 40W. Some respondents felt there was
> a
> KAT problem which compounded the problem, I always felt that the problem
> lay
> outside the shack but that maybe, just maybe, the KAT might be a bit
> sensitive to the problem. The only thing I would ask the Elecraft design
> team to investigate is whether the SWR circuitry has to respond to the
> common mode currents as it does. Probably not a firmware issue though!
> Thanks to the many people who made so many helpful suggestions.  E Pluribus
> Unum!
>
>
>
> Ray Coles, C.Eng. M0XDL
>
> 10 Littlemoor Road,
>
> Weymouth DT3 6AA
>
> Tel: [phone]
>
> Mob: [phone]
>
>
>
>
>
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

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

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html

[David Cutter]

I happen to think that the K line and the off-centre-fed dipole make for an
ideal compact station, particularly for those with very restricted land on
which to put just one aerial.  As you have concluded, common mode current
must be conquered and I think the GM3SEK chokes are ideal where weight is
not a factor, eg on the ground and in the shack.  I built one in a
weather-proof box with the LF version and the HF version all in together for
a very neat solution.

Your problems were mainly with transmit, but these chokes also work on
receive keeping noise off the coax which would otherwise creep right inside
the radio.

As others have said, you need a choke at the feed point after the balun and
others down the line depending on your layout.  A feeder that does not drop
vertically from the aerial induces further cmc onto the coax.  Another good
and easy thing to do is ground the coax shield before it enters the house,
but not everyone can do that, so a choke at the radio does the job.

DJ0IP/NJ0IP is putting data on his website from a very large number of
measurements on various combinations of ocf dipoles to show which chokes and
baluns work best.  His project is not yet finished but it is the most
comprehensive set of measurements I have seen on practical aerials.

David
G3UNA



...the most common response

> was common mode current on the feeder caused by OCFD unbalance. I did
> research and decided to build the renowned GM3SEK choke balun. I chose the
> mid range version which covers broadly 5-14MHz and installed it just south
> of my 4:1 balun at the North end of the feeder. No Joy. Further ideas from
> reflector pundits suggested that it might still be common mode, but
> coupled
> from the antenna into the feeder where my GM3SEK choke wouldn't help.
>
> Fortunately I had bought enough Fair Rite oval cores to build all three
> (Lo/Mid/Hi) balun designs, so in a final roll of the dice I built the two
> core Hi Band balun and installed it in the Shack next to my rig. The hi
> band
> version has a fairly flat impedance curve and reaches down to 7MHz and up
> to
> 30MHz. IT WORKED! I have run 100W on all the bands from 40m to 10m and I
> have seen no run away SWR readings. My 25W tune value stays rock solid. As
> a
> final push-my luck step I turned the KPA500 to operate and ran 400W SSB.
> No
> Sweat! All of these things were no-nos yesterday.
>
>
>
>
> Ray Coles, C.Eng. M0XDL
>
> 10 Littlemoor Road,
>
> Weymouth DT3 6AA
>
> Tel: [phone]
>
> Mob: [phone]

---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com

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

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html