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Re: [RFI] shopping for toroids

To: rfi@contesting.com
Subject: Re: [RFI] shopping for toroids
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Sun, 01 Dec 2013 11:41:50 -0800
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
Hello Pf,

As it turns out, I am currently organizing a group purchase of #31 cores for several local (Northern California) ham clubs. The parts we are buying this time around are 0431177081, which is a 0.75-in i.d. "clamp-on" intended for multi-turn chokes on small cables (home entertainment systems, computer cables, battery chargers) and 2631181381, which is a 1.4-in i.d. clamp-on that is 2.5-in long. It is equivalent to five of the 1.4-in i.d., 2.4-in o.d. toroids, but sells for about half the cost of 5 toroids, making it a real bargain for high power transmitting chokes. The other part we order is that #31 toroid. We are not buying any this time because we did a large purchase (1,100 pieces) two years ago.

Several years ago, GM3SEK made me aware of the very high cost of ferrite cores in EU, so I started working on chokes that could be effective with fewer cores. This resulted in the "bifilar" designs that you'll find in the latest version of my RFI tutorial (about 2010). The principle is simple -- take two insulated AWG #12 copper conductors about 1.3 m long, tape them together, wind 12-16 turns around a single #31 toroid, and connect them as a short section of parallel wire transmission line. Insulated "house wire" will give Zo of about 85-95 ohms, enameled wire gives Zo about 50 ohms. Both work very well with antennas in the range of 50-100 ohms. 16 turns is good for 160M to about 17M, 12 turns is better for 80M to 10M. AWG #12 copper is 0.08-in diameter.

These bifilar chokes can handle transmit power of 1.5 kW if the antenna is reasonably well balanced and operated near resonance. In other words, a center-fed half-wave dipole, a typical beam, and a vertical with a good radial system. They will NOT handle high power of the antenna is badly unbalanced (off-center fed), or if the VSWR is VERY high (like an 80M dipole fed on 40M).

I've always bought these parts as a group purchase, and I'll bet that most vendors will be happy to take your order and give you a good price if you're ordering in full box quantities. There are 100 toroids to a box, 30 of the big 2631181381 clamp-ons to a box, and 110 of the smaller clamp-ons in a box. Last week, I was quoted $620.40 for two boxes of the big ones, and $267.30 for one box of the smaller ones. About two years ago, we paid about $4 each for 1,100 toroids, including the cost of shipping.

As to the #43 parts -- the discounts for buying full boxes of #31 parts make them cheaper than #43 parts, and the #31 are more universally useful. That is, #31 is greatly superior below 5 MHz, roughly equivalent from 5-15 MHz, and still quite effective to 100 MHz. #43 is a dB or so better between 20 MHz and 50 MHz.

I'm guessing that the major reason #31 parts are so expensive in EU is that industry pays attention only to EMC compliance above 30 MHz, where the #43 parts are quite sufficient. But a study of the Fair-Rite catalog shows that #31 is the right material for EMC below 30 MHz -- indeed, no #43 parts are listed. As hams, we need to awaken distribution to our needs, and get these few #31 parts in their catalog. A large group purchase would be the way to do that.

BTW -- Newark Electronics is Farnell in the US, and they did not even respond to my request for a quotation on a nice order.

73, Jim K9YC

On 12/1/2013 10:33 AM, Pierfrancesco Caci wrote:
I'm trying to come up with a shopping list for assorted toroids for RFI
suppression. At the moment, I can identify these needs:

- a choke for a 80-40 m dipole. I'd like to make this by passing
   rg213-like cable through the biggest 31 material cores. Fair-rite
   2631814002 should be the one, and I think I'll need 5. I'm pretty sure
   this is overkill (I have 100W, and can go up to 500 maybe one day), so
   maybe 2631803802 used with rg58-like cables is a better choice?

- an assortment of tubular (for cables I can solder myself) and clamp-on
   cores (for when I can't cut, think USB cables and such) for reducing
   RFI from assorted equipment. In choosing the internal diameter, should
   I go for the "easy" fit, or for the tight one? Think e.g. rg58 cable
   that is 5 mm diameter, there are cores at 4.95 mm and other that are
   6.35 mm. Same for rg213, which is 10 mm, and cores are available as
   9.50, 10.15, 12.80 mm.

For applications above 20 MHz but still under 200, is there any reason
to prefer material 31 over the slightly cheaper 43? The graphs on Jim's
RFI-Ham and the data on Fair-rite site seem to say 43 is as good, am I
missing something?

Where do my fellow Europeans get their supplies from? I'm looking at
Mouser, as Farnell redirects me to the .it site that only sells to
businesses with VAT number.



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