[Elecraft] Magnetic field sensitivity

[Vic Rosenthal]

First of all, I don't see this as a problem.  But it is very interesting, and I
would like to understand it.

When I first built my K2, I put it right next to my antenna rotator control.  I
noticed that operating the rotor caused a hum modulation of received and
transmitted signals, and Wayne (I think) remarked that the VFO and BFO toroids
were quite sensitive to magnetic fields.  So I moved the rotator box a few
inches below and to the left of the K2, and there was no longer a detectable
hum.

Then I built the KPA100.  The rotator hum was back with a vengeance!  I moved
the rotator farther away, to about 10 inches to the left and two inches below
the K2.  There is still some hum when operating the rotator, but much less. 
Elecraft is now recommending that you maintain at least 18 inches of distance
between the K2 and devices that generate AC magnetic fields.

My question is: why did adding the KPA100 change this behavior so much?  Does it
have something to do with the big aluminum heat sink?  My theory (just a guess)
is that the heat sink serves as a magnetic lens -- the hum field causes eddy
currents in the heat sink, which then generate their own field which is
re-radiated near the toroids.  The original aluminum cover was too thin to
display this effect.

What do you think?

Vic K2VCO