On 2/2/2018 1:00 PM, Cortland Richmond wrote:
quite small loop will do very well if one is in close proximity to the
magnetic field of an emitter. That is, there is a good deal of
utility in entering the near field very closely.
Yes, indeed! Most electronic sources (and this is almost certainly and
electronic source) are current sources -- they radiate by simple antenna
action, AND also couple via their magnetic field. In the near field of a
current source, the magnetic field is strongly dominant.
That said, there are other flies in the ointment. First, the shielding
(or lack thereof) of the receiver. If it's poor, internal wiring will be
the antenna (or the magnetic loop). Second, in systems that are
physically large, wiring carrying the noise current generates both an EM
and magnetic field. Variable speed motor controller are a common, and
very strong, noise source, and the noise Don is chasing is typical of
them. It's all too common for controllers and motors to be widely
separated, and the wiring between them radiates. And if that's not
enough, bypassing the noise to "ground" can create a loop for the higher
order harmonics that both radiates and establishes the magnetic field.
Something like 15 years ago, Neil Muncy (the Pin One Problem guy) was
called in to troubleshoot massive audio frequency buzz from the power
system at the newly built New Jersey Performing Arts Center. The cause
turned out to be magnetic coupling of neutral current from controllers
for the motors that run the elevators, which coupled into microphone
wiring for the performance spaces. And the coupling mechanism within the
audio system was another important concept that Neil discovered, a
defect in how mic miniature, multipair mic cables are manufactured. He
called it "shield current induced noise," and the coupling mechanism is
magnetic. There's a detailed description of the mechanism on my website.
BTW -- it also couples well into the MHz range.
73, Jim K9YC
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