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HF Balun/Unun Winding Insulation

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Apr 30, 2019
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How important is getting the copper wire close up to the toroid core in an HF balun or unun? I want to build an unun using an L15 core. The solid copper wire I have available has normal PTFE insulation which will separate it from the core very slightly. Is that going to degrade its performance compared to bare enamelled copper wire tightly wound on the core?

For simplicity we'll say 'thin' and 'thick'.

There's the formula for Henry value of an air coil, given coil diameter, length, number of layers, number of turns. To what degree are these parameters affected by thick insulation? The wires are separated a small distance, but what is the overall impact?

As for the core, it delivers the advantage of reducing number of turns, thus reducing copper resistance.

Theory suggests...
For greatest efficiency the wires should be as close as possible to the the core, enabling tight interaction between core and flux field. In that manner thick insulation hampers inductance.

Practically speaking however...
It's not hard to wind a few additional turns to make up for the loss.

Frankly, I don't think it matter much.
Using big toroids as FT290-43 I made baluns with different wires having various thikness on their insulation, and was no big difference in performances.
But it matter how the windings are distributed all over the circumference of the toroid.

Thanks for that. I will give it a go. I have two cores and enough wire so might build two and measure the loss through them at 50 Ohms with them both connected back-back.

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