[RFI] shopping for toroids
jim at audiosystemsgroup.com
Sun Dec 1 14:41:50 EST 2013
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
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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