[Elecraft] KPA500 RFI problem

[David Gilbert]

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 <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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