[Elecraft] RF feedback?

[Don Wilhelm]

James,

I did not mean to imply (or state) that the coax was any problem.  The 
real problem is that you are driving very high power into a small 
antenna, and you have not provided a good RF Ground at the base of that 
antenna.  The net result is that you have high level RF floating on most 
everything in the building - the electrical wiring, the building 
framework, or anything else that is metallic.

Get a good RF Ground - at least one 1/4 wavelength radial (two opposing 
radials is preferred) for every band that you intend to operate and 
spread them out symetrically from the base of the antenna.  That is step 
#1.  Unless you do that, please expect many 'strange happenings' from 
the high intensity RF Field that you are creating.  All that is not to 
mention the possibility of human exposure to the high RF Fields.  Be 
careful.

73,
Don W3FPR

K2QI wrote:
> Don, if that were the case wouldn't this problem manifest itself during a
> constant carrier keydown, and not just in SSB mode?
>
> Anyway, just to eliminate the feedline I took the dummy load outside and
> connected the antenna feedline directly to it. I was previously using a
> patch cable from the meter directly to the dummy load before.
>
> Testing my system in this confiiguration shows no problems eitther, so it's
> not something related to the way I have the coax run or the air-wound choke
> I'm using.
>
> Thanks for the information however.  I'm not sure if the amp is
> oscillating.  I recently replaced the parasitic suppressors along with grid
> chokes and mica caps and all seems to work fine but I'll keep my eye on it.
>
> James K2QI
>
> On Tue, Jul 14, 2009 at 6:41 PM, Don Wilhelm <w3fpr at embarqmail.com> wrote:
>
>   
>> James,
>>
>> Do not overlook the possibility that your amp is developing a parasitic
>> oscillation.
>> The behavior will be exactly like you describe - no apparent problem when
>> driving a dummy load (because it is not frequency sensitive), but the amp
>> will misbehave when driving a tuned load (like an antenna) which is
>> frequency selective.  When the parasitic starts, you will see high SWR on
>> your wattmeter.  Yes, such parasitic oscillation could be caused by RF
>> pickup in the amp itself.  Your antenna is quite close for 1100 watts power.
>>
>> I don't have an easy answer for why it only shows up in SSB mode (unless
>> the amplifier bias is changed when in SSB mode), nor why it is worse with
>> reduced mic gain settings.
>>
>> 73,
>> Don W3FPR
>>
>>
>>     
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