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I want to build a 1:1 unun for a 6M dipole. Power level 100W. What core material should I use?

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  • $\begingroup$ Good question, but did you mean balun, as in feeding a balanced dipole with unbalanced coax? And the power level has nothing to do with the choice of the ferrite material.I'm confident that someone will answer your question soon. $\endgroup$ – Mike Waters Mar 3 '18 at 2:10
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As Mike Waters points out, your application requires a balun since a 1/2 wave dipole is balanced and the coax is unbalanced. The purpose of the balun is to enforce equal current in both dipole legs. It performs this function by preventing current from flowing on the outside of the coax shield by presenting a high impedance path to exterior shield currents.

You can use a toroid form comprised of mix 31 or mix 43 material to construct a 6 meter balun. A 240 size toroid with three or four turns of coax wound uniformly through the toroid will prove effective. Wide, evenly spaced turns are important to minimize the interwinding capacitance.

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  • $\begingroup$ You are correct, I need a balun not an unun. I mispoke in my original post. Most of the literature is for HF baluns and not for VHF. I will see if I can find a toroid of the correct mix. Thanks! $\endgroup$ – Bruce A Friend Mar 4 '18 at 4:51
  • $\begingroup$ to be honest, 6m (50 MHz) is technologically pretty close to the upper end of HF; the fact that the "HF band" ends at 30 MHz is pretty arbitrary. $\endgroup$ – Marcus Müller Mar 4 '18 at 12:52
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In addition to what Glen W9IQ said, also consider using a Fair Rite #46 material mix. This is electrically similar to #43 and in some regions is much less expensive.

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