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Re: [RFI] RFI every 15 KHz on 160 meters, suspect source is a manufactur

To: rfi@contesting.com
Subject: Re: [RFI] RFI every 15 KHz on 160 meters, suspect source is a manufacturing facility.
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Fri, 2 Feb 2018 21:06:47 -0800
List-post: <mailto:rfi@contesting.com>
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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