In message , Cecil Moore
writes
Roy Lewallen wrote:
Bear in mind that the impedance of multiple turns is proportional to
the square of the number of turns. So 5 turns, for example, through a
single core gives you the same impedance as 25 cores strung along the


Something that fooled me is the way Amidon specifies
"one-turn" impedance for beads in their brochure. Their
"one-turn" for beads is a wire running through the center
hole, wrapped around the outside, and back through the
center hole. (I would count that as two turns and
would say one-turn is just a wire running straight
through the core.)

As a result, for their FB-77-5621 bead, for instance,
they specify 270 ohms per turn. If one simply threads
these beads over RG-58, the impedance is about 1/4
of that amount, i.e. about 67 ohms per bead, requiring
about 15 of them to get to 1000 ohms.

As Roy says, ten turns of coax on an FT-240-77 core
is roughly equivalent to 100 FB-77-5621 beads strung
over coax.

Interestingly enough, Amidon specifies "one-turn" on
an FT-240-77 core to be 76 ohms, obviously a different
kind of "one-turn" than that of the 270 ohms for an
FB-77-5621.

Yes, '1 turn' on a torroid is 'once through the centre'. For any coil to
work as an inductor, there must be a return path somewhere. For a
torroid, this has to be 'around the outside'. As the permeability of the
core is generally much greater than that of air, it doesn't matter much
whether the wire is close to the surface of the ferrite, or very slack
indeed. With a single turn, the return path could be quite circuitous
(literally).
--
Ian