If that balun had overheated and become ruined on 12m it almost
certainly would have affected other bands as well. A ruined balun is a
changed balun.
Dave AB7E
On 1/9/2013 7:26 AM, Brian Alsop wrote:
Sounds like the 4:1 balun may be overheating on 12M. Chances are if
you've been doing this for a bit, it's ruined. Heat anneals the core
and changes it's electrical properties for the worse. Heat may have
cracked it.
Now whether a 4:1 balun is the thing to use is another issue.
You don't say how long your open wire line is so it is impossible to
estimate the impedance seen by the balun.
Here are the approximate antenna feedpoint impedances for a 120'
double 50' up.
80M 47 -j188 SWR=17:1 4:1 balun at antenna feedpoint makes
situation worse
40M 1724 +j1869 SWR =75:1 4:1 balun at antenna helps
20M 415 + j880 SWR =45:1 4:1 balun at antenna helps
15M 186+281j SWR =13:1 4:1 balun at antenna helps
12M 1293-155j SWR =63:1 4:1 balun at antenna helps
10M 122.4-j100 SWR =4:1 4:1 balun at antenna helps
It is clear that 12M may be close to the worst of the lot
If one knew the electrical properties and length of the ladder line,
one can do the impedance transformations and determine just what
impedances the balun sees. One may end up with entirely different
conclusions from above. The G5RV is an example of a "doublet" antenna
where the ladder line length is pruned to find an acceptable match on
several bands.
73 de Brian/K3KO
On 1/9/2013 13:41, Jim Miller wrote:
I strongly suspect you have a common mode problem. These can be very
frequency dependent.
Read: http://audiosystemsgroup.com/RFI-Ham.pdf especially the section on
common mode chokes. There is a cookbook near the end for band specific
chokes.
For more details on the math read:
http://fermi.la.asu.edu/w9cf/articles/balun.pdf
I'd also recommend you take out the 4:1 transformer.
Good luck
jim ab3cv
On Tue, Jan 8, 2013 at 8:06 PM, Ron W3ZV <[email protected]> wrote:
At least I'm pretty sure its an RFI problem. I have a K3, KAT500 and
the
KPA500 connected as recommended with Elecraft DB15HD cables. The
antenna is
a 120ft. doublet fed with ladder line to a 4:1 balun and 10ft of
coax to
the KAT500. I have a comment point ground to a ground rod six foot
from the
shack. I have operated the amp at 400+ watts from 80m to 6m. The
only band
that gives me a problem is 12m. When I am operating at 400 watts
ssb, the
amp will fault with a high swr indication within 5 to 20 seconds.
Immediately prior to the fault the swr on the amp display was 1.2:1.
Reducing power to below 300 watts seems to avoid the problem.
Assuming the consensus is that this is RFI, what would be the most
likely
suspect? Coax cables in/out? DB15HD cable? Power cable? What's the
easiest
way to diagnose and correct the problem? Thanks
Ron W3ZV
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