[Elecraft] KPA500 RFI problem

Wed Jan 9 13:59:31 EST 2013

 From a troubleshooting point of view jumping right to the balun 
requires making a lot of assumptions, most of them unwarranted given 
what little we know so far from his post.   More importantly, a balun 
with a ruined core is going to look bad for SWR no matter what power 
level is fed to it, and he says that things look fine at 300 watts but 
go south at 400 watts.  That's an arc over problem somewhere ...

Dave   AB7E

On 1/9/2013 10:53 AM, Brian Alsop wrote:
> Dave,
>
> Most likely a tuner is being used.
> If the balun failed one might not notice that since one generally 
> tunes to min SWR.  Unless one recognizes something changed, one 
> wouldn't know it failed.  This is especially true for auto tuners.
>
> This "not knowing it failed" is wide spread for bead baluns-- although 
> this isn't the case here.
>
> 73 de Brian/K3KO
>
>
> On 1/9/2013 17:26, David Gilbert wrote:
>>
>> 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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