[Elecraft] RF feedback?

[Don Wilhelm]

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

K2QI wrote:
> Hi Monty,
>
> I too have assumed that RF from the antenna was causing the issue, but why
> can't I recreate the problem when transmitting a constant carrier?
>
> The behavior of this phenomenon is strange to say the least.  What usually
> happens is the SWR alarm will activate upon initial keydown in SSB mode.
> However, if kept keyed long enough, the SWR reading drops back to 1.1 or 1.2
> and operation resumes normally.  This drop in SWR usually only takes a
> second or two.  It's the initial alarm blast that concerns me.  When the
> alarm trips, I've seen SWR readings higher than 100 to 1, and sometimes as
> low as 3.5:1.  I can also hear the amplifier relay clicking in rapid
> succession.
>
> There is no real pattern as well.  The only thing I've noticed is that I can
> increase the frequency of tripping the alarm the lower the microphone gain
> is set (which to me seems like it should happen the opposite way).
>
> The Elecraft hand mic is connected directly to the front panel.  Changing
> the bias from high to low has no impact.  The SB-610 is an old Heath scope.
> Its only connections are 110v in, and RF in and out.
>
> I also put ferrite chokes on the amplifier relay cables that go into and out
> of the SWR meter (MFJ-828) and that did not solve the issue.  I do not
> experience any hotspots in the shack either.
>
> I'll try bonding the components together as well to see if that helps.
>
> Many thanks,
> James K2QI
>
> P.S. Apologies to the group for my off-topic problem.  There are a lot of
> smart guys here though, and I can't seem to get much help from QRZ...
>
> On Tue, Jul 14, 2009 at 4:32 PM, Monty Shultes <montys at mindspring.com>wrote:
>
>   
>>  James,
>> Not having the problem with a dummy load is revealing.  RF from the antenna
>> is getting back into the shack and causing mayhem.  As I understand it, the
>> problem is the SWR meter tripping from high SWR.  On the surface it sounds
>> like the SWR meter needs shielding and/or grounding.  I thinking connecting
>> all your chassis together may help even without grounding.
>>
>> Rectification can occur wherever dissimilar metals meet.  I would not
>> expect it to increase with reduced audio drive, just the reverse.  SWR
>> meters have diodes for sensing voltages, and diodes are rectifiers.  They
>> may be overloaded by the modulated carrier - the modulation has something to
>> do with it, as you have no problem with a constant carrier.  Is your mic
>> connected directly to the radio or through the SB610?  I don't remember all
>> the functions of the SB610.
>>
>> Do you notice any current fluctuation in the SB220 when the problem occurs,
>> or in forward power?  Can you modulate the rig with a computer sound card to
>> eliminate RF feedback in the mic as a source?
>>
>> Monty
>>
>>
>>     
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