Topband: 160m noise
w8ji at contesting.com
Wed Nov 10 11:23:51 EST 2004
> I'll defer to your experience. Given the availability
> of #43, #61, #73, or #77 toroids (all of which are
> commonly available), which would be your first
> choice? (Fair-Rite also recommends #31, but it
> does not appear to be available in toroid form).
First remember the number order is sometimes random, a
higher number does not always reflect higher permeability.
When soft iron materials are made larger than a certain
physical thickness and diameter, they exhibit internal
effects that render them useless as suppression devices.
It's similar to the velocity factor in dielectrics. make
them big enough and impedance drops.
That's why you only find higher permeability cores in
31 43 and 44 material are about the limit for larger cores
like common snap on cores.
The optimum core to use "randomly" would be one that appears
as a pure resistance and the highest possible resistance for
the size you need. That's why you have to, as Jim often
suggests, look at the manufacturer's catalog.
Permeability and resistance also change with frequency. It
is possible to have too much permeability (case in point,
the cores suggested by Misek for Beverages) and actually
wind up requiring more turns than a lower ui core.
BTW, Amidon is a retailer and distributor. NEVER rely on
second hand information. Go right to Fair-Rite's data.
I've found multi-turn cores to peak at the mid HF range with
43 material, but again it depends on how many turns.
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