[RFI] Help with Bad RFI.

[Hare, Ed W1RFI]

Hi, Jim,
 
We had such a page for a couple of years on the old ARRL site, and I don't recall more than a single outside contribution to the sounds page.  As my earlier post said, I hope to see that page placed on the site again, soon.
 
The new ARRL site has a lot more content-management capabilities, so I am hoping that we can see more input from hams worldwide to help make the site better.  As I figured out long ago, all good things in Amateur Radio don't come from Newington, CT!  With RFI, we are in pretty good shape, because our staff have had a LOT of experience with both the social and technical aspects of RFI issues, with material that starts with the social issues on purpose, but almost all of the smarts we have came from people that help to make us smart. :-)
 
I have especially appreciated your work documenting the characteristics of ferrites vs numbers of turns.  We had submissions of some of that in the past from others, but alas, they had used hard-to-find and obscure ferrite materials.  I hope to get links to your ferrite graphs added to several places in the ARRL RFI material. 
 
Although a wide range of ferrite materials can be used, in general, ARRL had recommended -75 material for low HG and -43 material for upper HF and VHF.  As noted by you and others, the -31 material is slightly better across a wider frequency range, so it is now our recommended "all around" material. It is available in the FT-240 size that is very useful for making common-mode chokes for larger cables.
 
Unfortunately, we still see problems with hams equating ferrite with all common-mode chokes (CMC).  Our material outlines why it is necessary to wind a number of turns onto a toroidal core, but again and again, hams tell me put a couple of "ferrite beads" onto a cable and expected it to serve as a CMC on HF.  Unfortunately, beads are just so much easier that hams talk themselves into using them.  They may work, in a few cases, where they are located at current node in the complex distribution of current and impedances in a large wiring system and only a few dB are needed,  but as you have so aptly noted, toroids and inductance are needed.
 
Ferrites work by three mechanisms -- resistive loss, inductance serving to reflect energy and inductance coupled with self-and stray-capacitance to do an even better job of sending unwanted energy back from whence it came.  I think that the dominant mechanism in most applied common-mode chokes is reactive, as each wire passing through the core adds the resistive losses linearly, but adds inductance by the number of turns squared. The ideal CMC effectiveness does involve resonance, but I think that with the complexities and variations in the systems, the stray capacitance and self-capacitance is hard to predict, so most of the time, the Z of the choke is probably predominantly inductive.
 
I'll take it either way. Energy that is lost as heat in the core is considered ideal, but if there is a strong common-mode signal on a cable, as long as I can prevent it from getting inside the equipment I am trying to protect, the exact mechanism is pretty much moot.   For the most part, this is also true of internal grounding issues in equipment, as most of the time, hams are not going to open up equipment to correct the internal grounding, so it may not even be possible to know whether it is an internal grounding issue is the dominant cause, of some other stray coupling mechanism inside a piece of equipment.  No matter the cause, if common-mode signals are kept out of equipment, its potential for being susceptible to RFI is significantly reduced. 
 
 
73,
Ed Hare, W1RFI
ARRL Laboratory Manager
225 Main St
Newington, CT 06111
Tel: 860-594-0318
Email: W1RFI at arrl.org

________________________________

From: Jim Brown [mailto:jim at audiosystemsgroup.com]
Sent: Sat 1/15/2011 12:33 PM
To: rfi at contesting.com
Subject: Re: [RFI] Help with Bad RFI.



On 1/15/2011 2:55 AM, Hare, Ed W1RFI wrote:
> I hate to bring up the thread gain, but the noise sounds like it could be coming from a high-powered indoor grow light. I say that because the one model we looked at had a peak of 9 MHz, with strong noise also above and below it.

Ed,

Something that could be a quite useful resource is a library of sound
files and spectral samples of noise produced by sources that have been
solidly identified. This sounds like something that the League
could/should logically do.  I suspect that once it was set up that could
be contributed to by various EMC engineers in the field.

73, Jim Brown K9YC
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