I did a bit of hunting for a source of #78 material
cores, and did not find any Fair-Rite cores of #78
material in sizes .82", .87", 1.14", or 2.4" (although
Fair-Rite lists them) at any of their distributors or
dealers.
Here's a list of Fair-Rite toroid products:
http://www.fair-rite.com/ds59.htm
Does anyone know someone who is selling #78 toroids
(Fair-Rite p/n 59780xxxx) for sale retail?
73,
Jeff Maass K8ND
> -----Original Message-----
> From: topband-bounces@contesting.com
> [mailto:topband-bounces@contesting.com]On Behalf Of Jim Brown
> Sent: Thursday, November 11, 2004 10:53 AM
> To: TopBand List
> Subject: Re: Topband: RFI cure kit
>
>
> On Thu, 11 Nov 2004 09:51:41 -0500, Eric Scace K3NA wrote:
>
> >What would you include in such a kit?
>
> I'll address what I've spent a lot of time studying -- common mode radiation
> from various
> computer and Ethernet sources. You might ask this question over on the RFI
> list, where
> guys like Dale, WA9ENA, and EMC engineer for Rockwell-Collins, and others
> like him,
> hang out.
>
> First, I would try to do as much of the networking wireless as possible. For
> whatever
> wired Ethernet remains, I would try to have a selection of the biggest
> Fair-Rite cores
> you can find in a selection of materials and use them to form multi-turn
> common mode
> chokes on the wired Ethernet signal and power supply wiring. The materials of
> choice
> for the lower bands are #78 and #31, while #31 and #43 are the better choices
> for the
> 20-10.
>
> Fair-Rite 5978003801 is the largest toroid using #78 material (2.4" OD).
> #5943003801
> is the same thing in a #43. These all allow lots of turns if you need them.
>
> #2643626002 is a cylinder 2" OD, 1" ID, 2" long of #43 material. 2631101902
> is a
> cylinder 1.122" OD, .543" ID, 1.125" long of #31. The larger one of these is
> big enough
> to get several turns of a CAT5 cable through it with connectors attached. The
> smaller
> one is tighter -- a couple of turns is all you can hope for.
>
> #0431177081 is a very large clamp-on (1" cable, 2.2" OD, 1.7" long);
> #0431176451 is
> a slightly smaller one (0.71" cable, 1.5" OD, 1.87" long. Because these open
> up, you
> can get more turns through them with the connector attached. The clamp-ons
> are slightly
> less effective than the solid cores of the same size, and they cost more.
>
> I would like to list #78 materials in the cylinder and clamp-on forms, but I
> couldn't find
> any in the Fair-Rite catalog.
>
> Tom is right on target when he points out that what you are doing with any
> ferrite choke
> is attempting to form an L-section filter with it, whereby the choke is the
> series
> impedance and the line has to have some low impedance to short it out. Taking
> the line
> to a common point is the usual way to do that. But be careful about that --
> the
> higher the
> frequency, the more difficult that is!
>
> But it is also true that, in general, you want that series impedance to be as
> large a
> resistance as practical, with as little X as practical, within the range of
> trash you are
> trying to suppress. That is, you can also "brute-force" the problem a bit by
> simply
> making the equivalent series Z of the line that is trying to radiate as high
> as
> possible,
> and you do that by adding the lossy ferrite choke to it. It is no trick to get
> that series R up
> to 1Kohms over an octave or more using a multi-turn choke.
>
> Some general thoughts:
>
> 1) Many of the EMC radiation standards don't kick in below 30 MHz, so that's
> the range
> where we need to concentrate.
>
> 2) None of this helps you with un-shielded boxes that radiate trash directly.
>
> 3) Some common Ethernet birdie frequencies that will help you tweak these
> chokes for
> maximum smoke are around 14,030, 21,052, and the bottom 50 kHz of 10 meters.
> I've
> also identified two in the 30 meter band, but I've suppressed them so well
> that I don't
> remember exactly where they are. And as I recall, there's also one around
> 3512 or so.
> There are probably some on other bands and on the phone bands, but I'm a CW
> guy.
>
> 73,
>
> Jim Brown K9YC
>
>
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