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Field-shortening a 40m Half-wave for 30m

What happens if I take a 68’ wire cut to be a half-wave for 40m, and wind the last 17’ or so onto a small kite-winder, in an attempt to shorten it to be a half-wave for 30m?

We’ve got all sorts of ways to shorten/lengthen a piece of wire for different bands: links with alligator clips, links with switches, chokes, etc. They work well, but they add extra “stuff” to the antenna kit.

Intuitively, it seems to me that if we can fold the end of a wire when tuning an antenna, to shorten without cutting, winding up the end is a lot like folding. Maybe winding up 17’ would create a cap hat, so I’d need to wind a little more?

I’m trying to come up with a way to use a 20/40 EFHW antenna on 30m in a pinch, without an ATU.
73 DE K4KPK/Kevin

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Hi Kevin,
I think you might need to wind that excess 17’ back up the element wire to shorten it, not onto a separate winder as that would then be a loading coil on the end of the antenna rather than electrically shortening the wire.
Of course winding back the wire on itself (as we do before cutting a wire element to get it the correct length when building an antenna), is going to be a lot of effort to do on a summit.
Perhaps simply put a link in and un-hook the last 17’ when you want the shorter (30m) antenna, connect it up when you want to use the antenna on 40 metres.
Now with an open link, you could then coil up the unneeded wire on a winder to keep it tidy, that should not cause any issues.

73 Ed.

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How close do the outbound and the inbound lengths of wire have to be in order to get the necessary coupling? Could I just bend the last 17’ back, and put twist-ties every 5 feet, or do I need to keep all 17’ of the end really close to the outbound wire when I fold it?

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I have always close wound the folded back wire around the active element. If you don’t do that, there’s the risk, I guess of the effect being different each time you do it - sometimes closer than other times.

I think some experiments with an antenna analyser attached is the best approach.
Perhaps if you do as you originally suggested and wind the excess 17’ into a winder and then tie-wrap that winder directly to the active element that “might” be enough to cause the shortening effect.

73 Ed.

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This guy did an interesting experiment on folding the end back: http://ehpes.com/n6mw/FoldWriteUp.pdf

The upshot is that a gap of an inch makes a big difference, and tightly folding has a different result than actually trimming. (He also shows that if you fold to tune and then cut the folded length, you’ll get an antenna that is too short.) Clearly, an improvised fold-back is going to have very different results each time.
73 DE K4KPK/Kevin

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Yes difficult. Variation on how the wire is folded-back, could be an issue.

Probably better to put small link piece in - they don’t have to be that big and instead of croc/aligator clips, I use 2mm gold plated RC power connectors - they have worked for years without any problems on my linked dipole.

To keep things simple and small, you could probably just tie a nylon cord from one side of the connector to the other so that the strain is taken up and the connector not pulled apart (probably less of an issue on an end-fed than when using a linked dipole element in double service as a mast guying support!

73 Ed.

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I am maybe off topic but I would not fold back wire. I would make 30m dipol with trap for 40m. No folding back wire and automatic change of bands

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@9A3IV - Thank you for the suggestion. After botching a number of projects, I have decided that I’m happier as an “appliance operator.” I try to understand what’s happening, but construction beyond tying, cutting, or crimping does not improve the quality of my life! =:-o

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I think it is rather a loading coil. But as loading coils at the end tend to have no effect, I think this should work. If it is only for switching between 40m and 30m, this should work well and introduces no fuss in the middle (which can break, get tangled up, …). The only disadvantage is that you cannot easily switch back and forth between bands (as eg with a linked wire).
Determining the correct winding might be a bit tricky, though. My G90 can also measure SWR across a 100 kHz range. I think after a few activations you know how much to wind.
Folding it back is trickier still, I think, you’d have to make sure that the folded part is on the first part. Too complicated.

This is also a viable solution. There are only two things to consider:

  • The trap might get trapped :slight_smile: in a branch or so and increases the weight
  • the bandwidth of the trapped bands is lower.

Summing up, I’d try with the winding solution: You have a good antenna for 40m. You loose nothing by winding it up for 30m and testing it.

Just my 2cents. Disclaimer: I’m just a beginner, so not everything above might be correct.

73 de Martin / HB9GVW

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Fold it back or roll it up. Yes, it is an inductor, but the end of a 1/4 wave element has zero current, so the inductance doesn’t do anything. It does add a bit of capacitance to ground, which matters because the end is a high voltage point.

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