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Correctly cut dipole with few tries


#1

Hi,
I’m trying to get a 20m half wave dipole ready for the trans Atlantic event of this coming Saturday. Unfortunately the time I can dedicate to the antenna construction is few and I have to optimize things. What I find particularly time consuming is the try and error part when I find myself cutting the dipole arms little by little until I get to the desired frequency.

I remember reading somewhere some formulas to help figuring out with less approximation the correct length of wire to cut after taking note of how much the resonant frequency moved cutting a given length of wire.

Can anyone point me in the right direction?

Thanks a lot.
73s Gab IZ1KSW - EA7JUG


#2

I used Rob’s antenna designer and it just works.

https://www.sotamaps.org/extras.php

Or… measure the length of wire accurately. Setup antenna and feed with RF and adjust frequency for lowest SWR. Note frequency.

Scale factor = measured frequency / desired frequency.

Actual length = measured length * scale factor. Trim length and check again.

If the scale factor is greater than 1 then your wire is too short.


#3

Hi Gab,
You don’t say if you are going to create a dipole for use supported at both ends or an Inverted-V version with just a support in the middle. The two configurations require different lengths. The height of the antenna also affects lengths. As Andy suggested use Rob’s calculator for a linked dipole - select number of bands = 1 and it’s then no longer a design tool for a linked dipole rather just a normal Inverted-V dipole. You can also put the height of the centre posts and any end supports into the calculator.


( https://www.sotamaps.org/extras.php and select linked-dipole-calculator).

73 Ed.


#4

A useful trick for shortening dipoles by small amounts is to fold the excess length back along the wire. Ie. you’d normally pass the end of the dipole through an insulator and wrap it around or tie a knot. Instead of permanently fixing the length, just wrap it around to make a resonance test, then calculate the adjustment needed, adjust the length by unwrapping, shortening by pulling more through the insulator, then rewrapping. By wrapping I mean simply winding the free wire around the other wire several times.

You can fix the length once adjusted using any number of methods but leaving it twisted should last 10 years. My 80m dipole is an adjustable dipole using this method. I first used it about 40 years ago, it went to Brunei with me in 88 and had to be lengthened a bit due to the coconut trees at each end being a good height, but back here the ends are closer to the ground so it needed shortening again. No wire was cut during this time.

There is a neat but much heavier method used by the Rak (or Raic?) antennas I bought many years ago. They have parallel overlapped sections of uninsulated wire with two shorting bars, allowing the length of the antenna to be adjusted within the length of the overlap. Too heavy for a portable dipole.

Anyway, hopefully the combination of the calculator method (online or via the basic principles outlined by Andy) and a non-permanent termination method will make the process quicker and easier.

Andrew VK1DA/VK2UH


#5

Thanks everyone for the useful answers. I forgot indeed to say that it’s going to be an inverted V dipole supported at around 8m from the ground.
I’ll use the calculator you mentioned and this afternoon I’ll hopefully will be able to give it a go.

Andy, that’s exactly the formula I was looking for, hopefully with the help of the calculator and your formula I’ll be able to cut down the number of attempts for a resonant dipole! I’ll keep you posted

73s Gab IZ1KSW - EA7JUG


#6

Gab, if you do decide to use the linked dipole designer/calculator, you might like to know that when you export your design to PDF format, there are tips in the PDF document to help you trim the dipole (or sections of a multi-bander) to final length.


#7

Do you have an antenna analyiser available? If so make it as per the design. Measure where it is resonant and then calculate what you have to trim to get it from where it is resonant to where you want it to be resonant. It’s a 1 cut process if you do that.
Happy constructing.


#8

So, good news!
The antenna works good with a 1:1.2 SWR @ 14.170kHz
It didn’t take longer than 2 attempts, the antenna was already very close to the desired frequency, just 20cm too long so it was easy to tune using the antenna analyzer. Thanks to everyone for the help you offered me, it was really precious.

On the less good side, I realized that the balun I’m using is too heavy for my travel mast which tends to bend a lot. Today it was ok since there was no wind at all (very rare in my area) but it’s not going to work on a summit with wind. I have not enough time to work on a lighter balun so, for the trans atlantic party I’ll be on 18MHz band instead using the lighter EFHW I recently built from sotabeams.

73s Gab IZ1KSW - EA7JUG


#9

just use the dipole without the balun if you can connect it. It really does work quite ok.