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I'm in the midst of constructing a fan dipole for 80, 40, 30, 20 and 10 meters. I have everything laid out and am about to start cutting wire. I have an ugly balun (RF choke) wound at the feed point but I'm wondering what I should do for lengths.

My sense from all of my research is that I should care about the wire length of the legs in the dipoles. For instance, each leg of the 40 meter segment will be 32.6 feet in length. I'm figuring that it's actually 32.6 feet, including the portion of the wire at the feed point that is loose and comes up to be bundled into the rest of the wires into the feed point. This would also include the piece that is folded over at the end through the egg insulator.

The alternative is that the overall linear length is more important, which feels wrong, but, admittedly, antenna calculations are not my strong suit. :)

So, should I be calculating linear length end to end from the feed point out or the actual wire length regardless of the way it comes into the feed point or the folded piece at the end for the insulator?

Update

As a followup, I've now built and hoisted the fan dipole. I have it suspended from the center feed point / RF choke and from the ends at about 40 feet. It tunes up between 1:1 and 1.6:1 on 80, 40, 30, 20, 17, 15 and 10 meters. It's a big tangle of wires until you get it into the air but well worth the time and effort if you have the space, IMHO.

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    $\begingroup$ I'm having a hard time understanding what difference there may be between "overall linear length" and "actual wire length" since the wires in dipoles are straight lines. Maybe a picture would help? $\endgroup$ – Phil Frost - W8II Feb 2 '16 at 13:45
  • $\begingroup$ What I'm saying is, is it the length of the wire or is it the total distance (width) tip to tip of the dipole elements. In the center at the feed point, the wires are loose and gather up at an angle, so the linear length to the separator is, say, 12 inches, but the wire is actually about 18" long at that point. $\endgroup$ – David Hoelzer Feb 2 '16 at 13:47
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It doesn't matter much. The tuning of a dipole is affected by many things:

  • ground conductivity
  • height above ground
  • feedline placement
  • trees, towers, gutters, etc around the antenna
  • thickness of the wires
  • (unique to fan dipoles) interaction of the multiple wires in the "fan"

As such, the extra inch or two of wire introduced by a droop in the wires isn't going to introduce any significant change.

Usual practice is to cut the wires a bit long, measure the antenna impedance (or SWR), then make them shorter until the antenna is sufficiently tuned. This "estimate, measure, adjust" approach is much more practical than trying to account for all the variable factors above.

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  • $\begingroup$ Thanks Phil. :) I had planned to cut them long but I'm trying to think through everything that might bite me in the end before I go out and string this thing up. :) $\endgroup$ – David Hoelzer Feb 2 '16 at 13:57
  • $\begingroup$ Well, today's the day. Going to go and hoist this fan up before the weather changes. $\endgroup$ – David Hoelzer Feb 3 '16 at 10:34

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