Two common misconceptions about toroids and rfi:
1) 'Ferrite' and powdered iron are very different materials although lots of
guys
incorrectly just say 'ferrite'. The important thing for our purposes is that
ferrite
increases the resistive loss of a conductor that passes through it as well as
the
inductance. Powdered iron has much less loss in the frequency range that a
particular
material is designed for. Therefore, ferrite is most useful for suppression of
RFI caused
by currents on the outside of coax, etc., where powdered iron is good for tuned
circuits
where you want to increase inductance with minimum loss.
2) Putting a single ferrite bead on a cable, like you see on computer monitor
cables, has
very little effect at HF. Either you need a lot of beads (choke baluns are
commonly made
with 25 or 50 beads) or you need a toroid large enough to wind multiple turns
of cable
through.
I know you technical guys know this, but I don't know how many times someone
tells me "I
put a bead on the cable but it had no effect."
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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