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Re: [RFI] Inverse Square Law and RFI

To: rfi@contesting.com
Subject: Re: [RFI] Inverse Square Law and RFI
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Mon, 11 Jun 2018 22:35:51 -0700
List-post: <mailto:rfi@contesting.com>
While this is all solid, there's a LOT more to it than that.  First, we must understand the difference between impulse noise generated by arcing on the power system, which is extremely broadband, and is radiated by the wires connected to the source (the arc). Because it's an arc, it's spectrum is theoretically infinite. But because the highest frequency components are most strongly radiated by the wires close to the source, the most effective way to find the source is at the highest frequency where it can be heard.  Lower frequencies are radiated more strongly by longer sections of the power line.

Noise is often attenuated by transformers, which helps localize the source a bit more.

Unfortunately, noise sources inside a house often DO go much further than the house. They are mostly radiated as common mode signals on wiring connected to the source.  These are NOT usually impulse sources, but rather electronic sources. They mostly fall into three categories: 1) clocks in microprocessor systems, which tend to be quite stable in frequency and appear as a steady or pulsed carrier; and 2) switch-mode power supplies, variable speed motor controllers that rectify the power line, do almost no filtering of the DC, then switch that DC at high audio frequencies (10-20 kHz is common) to produce a square wave, again do only enough filtering to take the ripple out of the DC (and often regulate it). While it is POSSIBLE (even easy using smart circuit design and layout) to prevent the strong and rich harmonics of that square wave, it is rarely done (or done very well) for most consumer products.

In general, these power-handling devices are free-running and rarely frequency stable, and the poor filtering the DC produces an amplitude-modulated "hump" of noise at harmonics of the  switching frequency that drift (because they're not frequency controlled by a clock). This mush of  noise is conducted as a common mode signal to wiring to which it is connected, again as a result of poor design and/or construction, and is radiated by the wiring (just like any other RF current would do on any other antenna). THESE signals can often travel a significant distance (thousands of feet) or even miles for strong sources that produce large enough currents.

Now, noise CAN be radiated by wiring internal to products due again to poor design, but THIS component of noise generally does NOT travel very far because the wires are too short. Plasma TV sets are an example of this -- noise is produced by the switched currents driving the display.

I've written a lot more about noise, and it's on my website. k9yc.com/publish.htm

BTW -- what the EMC rules call common mode is the voltage between neutral and ground, which is NOT what hams call common mode, and it's not what radiates. Rather, common mode current can best be described as the algebraic sum of all currents on a cable, and it's that common mode current that radiates. This often happens when the "green wire" is not properly connected to the shielding enclosure. EMC line filters external to equipment won't do anything to kill this current -- it's on the green wire, and the green wire goes right past the filter, which is between phase and neutral and neutral and ground (green). The only way to kill this current is a ferrite choke. For the HF bands, we need multiple turns of the cable through a suitable core. Lots of details at the link posted above.

73, Jim K9YC

First, think On 6/11/2018 3:20 PM, David Eckhardt wrote:
from a device located in a neighbors home over long distances before being
attenuated?

This is why its so difficult to isolate arcing or corona using RF
emissioins and walking the length of the power line.  Generally sources
inside a neighbors home don't go much further than the house.  However, if
they are strong sources, they will.  The reason we do conducted emissions
in regulatory testing is that lengths of conductors in housing are good
radiators of RF energy below 30 MHz.  So, we measure conducted emissions
that become radiated emissions once they couple onto the house wiring.
Conducted emissions are measured and regulated from 150 kHz to 30 MHz for
most home electronic equipment.  The generator is usually not a good
radiator due to small size wrt wavelength.


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