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I am designing a toroidal unun transformer to connect my short helical 40m vertical, which at resonance has a resistance of 15 ohms, to 50 ohm coax. According to my calculations, based in large part on the formulas and tables in the ARRL Handbook, I figure using T106-2 I would need 10 and 18 turns, using FT114-61 I would need 4 and 7 turns.

What I am lacking is good info about how to choose between powdered iron and ferrite. This is a QRP application (5W) so the ferrite is tempting because the fewer turns will keep resistance loss to the absolute minimum. On the other hand, the unit will be outdoors and we get pretty warm here in Tucson, which leans me towards powdered iron with its generally better temperature stability.

Thanks in advance for any recommendations one way or the other,

73,

Elwood, WB0OEW