Jim Brown wrote:
>
>I'm showing that bifilar windings are a very effective low cost alternative
>to winding big coax through multiple cores or the biggest clamp-ons. A
>major reason for doing so is that one of my friends in the UK says that
>they are experiencing exhorbitantly high mark-ups on Fair-Rite cores.
>
That would be me...
The technical background is the same all over the world, but the cost of
"named ferrite" [1] is much higher outside of the USA so
cost-effectiveness is a much bigger part of the design equation.
That led me to investigate the chokes designed by W2VJN that were
published in the Transmission Lines chapter of the 2010 ARRL Handbook.
These are based on a large format #43 bead that happens to be available
in the UK at a more reasonable price than 2.40in #31 toroid or the big
#31 snap-on [2].
The full story is in an expanded Radcom article that can be downloaded
from:
http://www.ifwtech.co.uk/g3sek/in-prac/index.htm#1005
Performance is not so good as Jim's biggest and best, but adequate for
the large majority of applications - including the low-stress
applications like TV antenna downleads and all kinds of non-RF cables.
The performance of these low-cost ferrite chokes is far better than
air-wound chokes (which are narrowband and very easily detuned by series
resonance effects) and at least equal to most under-designed strings of
beads.
If even higher performance is needed, then by all means go for the "big
ferrite" designs like Jim's. There is no technical conflict between this
approach and Jim's - it is simply influenced by the high price of
ferrite.
For the record, I strongly agree with K9YC, W8JI, W1HIS, G3TXQ and
others that 500 ohms is *not* an adequate design basis. 500 ohm chokes
will often work, but only on "soft target" problems. Tougher common mode
problems need a much higher impedance that is predominantly resistive.
Sorry, I won't be able to respond further until next Tuesday, as I am
leaving for a DXpedition-type contest. As there will be no time for
experiment we are playing safe with 2.4in toroid chokes at the
feedpoints, and the RFI kit will include big #31 snap-ons and a box of
the #43 beads... and a clamp-on RF current meter.
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
[1] It is essential to use "named ferrite" [3] from a reliable supplier
(if you don't know what grade of ferrite you're using, you are wasting
your time) but customers in Europe have to pay for international
shipping, assorted handling charges, markups and ransom demands, and
then about 20% tax on the whole bill.
[2] With Jim's encouragement we are also working on driving down the
markups on the #31 products.
[3] Further reading
CURRY GARDENS MENU:
Curry with Vegetable 8p
Curry with Meat 10p
Curry with NAMED Meat 15p
(Terry Pratchett)
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