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Say I have a wire antenna that is slightly (a few percent) too long for the desired frequency (of resonance or lowest SWR). What are the options, other than cutting the wire(s) and very likely ending up with an antenna wire that is now too short (by miss-measuring, or when re-strung, moved higher, environment changes, etc.)?

Can I fold the tips or the wires into a small flat circle or figure 8 (similar to a perpendicular capacitive hat)? Or loop the wire near the feed point? Or coil wire around a dowel somewhere in the middle? Etc.

Added: Is the answer different if the wire is insulated?

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  • $\begingroup$ Just cut the wire, and go in small steps so you don't overshoot your target. Wire is cheap. Worst case, you have to buy more wire. $\endgroup$ – Phil Frost - W8II Sep 2 '18 at 19:12
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You should not make any coil in the middle of the antenna as that will greatly change its RF characteristics. Instead, bundle up the wire at the ends in some fashion:

  • One practical solution is to fold the wire back on itself and fasten the free end onto the main line; then you have a loop you can also use for supporting the antenna, though you might want to use a “rope thimble” or other large diameter support to keep it from being sharply kinked, depending on the size and material of the wire and whether you expect to adjust it repeatedly. Electrically, this is mostly like using thicker wire, so it doesn't change the antenna characteristics much.

    There is some dispute about how equivalent this is to a shorter wire if the wire is insulated. To be sure, you could remove the insulation where the end is joined on, but of course that may introduce other problems such as excess wear on stranded wire of a portable antenna. Or you could adjust the length until the antenna is a decent match regardless of whether cutting it would produce different results.

  • If you want something more adjustable and have support solved, wind the excess wire length at the ends onto a spool; this will form an inductor, but since the far end of that inductor is open circuit it can't have significant current flowing and so has minimal effect on the antenna. There are existing antennas that work this way.

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Simply fold the excessive lengths at the ends 180° back along the dipole and tape them to the antenna. It's that simple.

See this post, where insulated wire was used.

The other methods you mention may work, but the above is an easy, time-tested and effective method of raising the resonant frequency of a dipole.

I suggest against coiling wire around a dowel somewhere in the middle.

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    $\begingroup$ Yes, coiling the wire in the middle will probably make the (electrical) length longer $\endgroup$ – Scott Earle Sep 1 '18 at 14:06
  • $\begingroup$ @ScottEarle : So if one ends up with a wire antenna a few centimeters too short, will coiling a bit of wire near the middle help with resonance or SWR? $\endgroup$ – hotpaw2 Sep 2 '18 at 17:23
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    $\begingroup$ It would "change the wire's characteristics". It would probably make it electrically longer, but be aware of losses and the like. Try modelling such an antenna using your antenna-modelling software of choice, this is the best way to learn without going out and cutting bits of wire and trying it out ourselves $\endgroup$ – Scott Earle Sep 3 '18 at 1:33
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My technique for wire antennas is to twist an eyelet about 10-20% in from the end of each leg, and attaching the support rope there. Letting the dipole ends dangle down allows a much more convenient way for trial and error fine tuning the end loops, since the attachment doesn't have to be re done each time. Also, moving the highest voltage point away from the attaching rope and trees reduces rain detuning and loss from induced antiphase currents.

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Important note: the technique of folding back to shorten the antenna's electrical length relies on a short circuit being created along the entire length of the folded-back section. Folding back the ends will have no effect on the antenna's electrical length if the wire is insulated.

Despite the obvious common sense of this note, I made this mistake - once!

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    $\begingroup$ I'm surprised by this. I would think there would be enough capacitive coupling between the two parallel wires to make it work the same. $\endgroup$ – Kevin Reid AG6YO Sep 2 '18 at 18:12
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    $\begingroup$ The insulation may have been too thick to deliver sufficient capacitance. $\endgroup$ – Brian K1LI Sep 2 '18 at 19:57
  • $\begingroup$ @KevinReidAG6YO I agree with you. I have seen too many hams write about their success, even with insulation. Here is one. But I've never personally tried it. $\endgroup$ – Mike Waters Sep 2 '18 at 22:30
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    $\begingroup$ I doubt there will be no effect with insulated. Were this so, each leg of the dipole could be folded in half, then half again, and again until the whole thing is much smaller. Would be quite convenient. $\endgroup$ – Phil Frost - W8II Sep 2 '18 at 22:47
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    $\begingroup$ I can only comment on the experimental measurements I have made. I had to physically shorten a vinyl insulated wire to raise the resonant frequency. Simply folding the excess wire back over the element changed the resonant frequency very little. Being thinner, "enamel" insulation might not give the same result. $\endgroup$ – Brian K1LI Sep 3 '18 at 2:21
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Since you know that the antenna is a few percent long, then you know exactly how long to make it (by flat fold). No cut and try if you do not add capacitance by making a loop or coil.

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