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These are usually called a Magnetic Longwire Balun. Its really an
impedance transformer (9:1) to feed a high impedance long wire (~450 ohm),
down to a 50 ohm unbalanced coaxial input. I have heard them called an UnUn
which seems more appropriate. I almost bought one of these for around £30,
but then decided to make one. The toroid is a T130-2 Iron Powder core, with 3
x 9 turns of 18SWG enammeled copper wire, and the connections can be seen
below.
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