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I'm building a balun for a fan dipole, and this chart suggests 9 turns of RG-58 on two FT240-31 cores will give good performance over the frequency range of interest.

Type #31 ferrite-cored balun frequency ranges

However, I don't feel confident winding RG-58 with it's nearly 1" bend radius, and it won't fit inside the box I intend to use. Instead, I have instead opted to use RG-174, a much thinner coax with about a third the advertised bend radius.

How will using the thinner coax with closer conformance to the surface of the toroid core affect performance? Should I increase or decrease the number of turns to compensate?

I'm planning to use it at no more than 100W on the 80m and 20m bands.

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That's a great chart you show.

You can see in the three lines for 2 toroids, that more turns help the low end, but too many turns hurt the high end of the band. This is because of the inter-winding capacitance, and especially the capacitance between the first and last turn which tends to short-circuit the toroid. (I think there's a rule of thumb about winding only about 270 degrees around the toroid).

With thinner cable you can certainly put on more turns without suffering the same problem, but I guess there's still some benefit to spacing them out slightly. So perhaps aim for 12-14 turns?

There's no issue with the cable being close to the toroid - it's not electrically conductive. Rather keep it tight for mechanical stability and to keep the centre-passing wires as far away from each other as possible.

4:1 baluns have a phasing issue with too many turns, where one path through the device is delayed compared to the other, which limits its high frequency performance. This isnt' a problem on a 1:1 balun, so the cable can be as long as it needs to be for optimal winding performance.

Be careful about the power handling if you're planning to use a mismatched antenna and an antenna tuner. RG-174 should be OK for several hundred watts at HF, but only at 1:1 SWR. If it's 2:1 the power handling will drop slightly, and for high SWR like 10:1 or more, it will be much less, maybe one quarter, I can't remember the formula.

Finally, don't worry too much about the bend radius of the cable - if you abuse it once, gently, you may affect its impedance but that won't matter much in this application.

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