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I have an unused attic crawlspace in which I can potentially install a 2-element 20 meter yagi. I know that adding the reflector will drop the feedpoint impedance of the driven element below 50 to 70 ohms, requiring some sort of matching scheme to reduce the SWR on the (unbalanced) 50 ohm RG-8U feedline. I have an MFJ antenna tuner (unbalanced), a quantity of fairly heavy-gauge solid aluminum wire, and a 1:1 balun on hand that I can use, and a radio (Yaesu FT-450) that wants to drive an unbalanced transmission line. I've also got a roll of 450 ohm window twinlead line in my deep stash somewhere... What is my most foolproof option (as an inexperienced operator) for performing the impedance match to an unbalanced line, assuming such a thing exists?

The transmission line run from the shack to the antenna feedpoint will be short, on the order of 15 meters total, and I can borrow an MFJ-259 antenna analyzer.

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  • $\begingroup$ must be one hell of a big attic $\endgroup$
    – Old_Fossil
    Oct 10, 2022 at 22:24

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There's a fairly simple approach to this, keeping the KISS rule in mind. Using a smaller type of coax (RG58, RG8) would work well since the run length is fairly short. The small amount of loss in the cable would be offset by the gain of the antenna. A common means of adjusting the feedpoint impedance for a good match is to use a gamma match. Since the driven element of a Yagi is basically a half wave dipole, the gamma match shifts the feedpoint a small amount by using a shorting bar connected to the driven element. You may lose a couple of Watts if using high power but the antenna will at least double your ERP. If properly adjusted you will not need a tuner. Gamma match detail

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