[Elecraft] RF feedback?

[K2QI]

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
>
>