I've tried loops in a variety of configurations, shielded, unshielded, balanced
and unbalanced, but when I could get close to the H field source, shielding and
balance really weren't a problem.
I've also used semi-rigid plastic tubing [HV insulation] with a short exposed
center conductor of coax inside to probe for the E-field on PWB's and EUT's
while investigating conducted emissions. In the photograph below, one trace
shows the near-E-field high-frequency ringing of an on-board SMPS while the
other trace shows trigger current from a LISN.
One adapts to the requirement of the moment,eh?
>From: Jim Brown <firstname.lastname@example.org>
>Sent: Feb 3, 2018 12:06 AM
>Subject: Re: [RFI] RFI every 15 KHz on 160 meters, suspect source is a
>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
>RFI mailing list
RFI mailing list