[Elecraft] KPA500 RFI problem

[Brian Alsop]

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 <w3zv at verizon.net> 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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