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Random length multi band dipole (with ATU)

The impedance at the rig end of the feeder will depend on the length of the feeder, and some lengths of feeder will present impedances that the tuner cannot handle.


Hi Tasos

I use a doublet with the KX3 & internal ATU on the odd dry day. The antenna setup is 33ft each antenna leg and 5m of 300ohm feeder. I connect this up to the KX3 with 4m of rg174 to a 4-1 current balun going to the antenna. It was meant to be an extended double zepp antenna for 18MHz but decided to feed it using the balun for multiple bands, works well with the internal ATU on 60-10m.
I tried using the hombrew open wire feeder with spacers but ended up in a tangled mess so I invested in dipole center peice for twin feeder.

Good luck

Should be read as: Total wire lengths (length of feeder + length of one antenna leg) of one and multiple halfwaves (i.e. high impedance points) should be avoided on any band of interest. That will help making the antenna tuners happy, especially the commonly used internal/asymmetric ones. Ok?

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To calculate different lenghts I found the “Doppelzepp-Rechner” of DK1RP very helpful. You can easily find out on which bands the antenna is going to work. High impedance (marked in red in the results) should of course be avoided. Green is fine. You need to know the velocity factor (Verkürzungsfaktor) of the feederline.
Strahlerlänge = one leg of the doublet. Speiseleitung = feeder.


73, Peter

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@HB9BCB great! thanks a lot

Im already using some C code to calculate the ‘bad’ long wire length so it makes sense. I google a lot since yesterday and I found a lot of info so the sentence made more sense. Initially I was confused by the wording

@OE5AUL thanks for the link

I think ill replicate your design for starts. Im waiting for the 300ohm cable from radioworld today.

EDIT: here two examples are also possible probably. short is nice!

You’re just giving away power with that 4m section of RG174. You should extend the 300ohm feeder instead and signifcantly shorten that RG174.

Hi Barry
The 4m rg174 cable would be a bit shorter but I tend to get a bit clumsy at times operating espiaclly when looking for my many pencils that seem to get lost and rep-appear at the end of the activation. If I need to cram the kx3 into the rucksack due the weather it makes it a bit easier leaving the balun and feeder out side of the pack when operating.
I may reduce it a bit and carry some spare 300ohm feeder to extend it a bit when needed.


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@N1EU regarding power loss, goes through different settings on the german calculator it seems like the feeder to leg ratio makes it ‘difficult’ to match all bands and get the good length feeder down the fishing pole tht ill be using.
Ill try to do my best though.
Ill try to build it tonight and test it tomorrow from the same hill we had the qso a couple of weeks back! :slight_smile:

Hi Tasos,

In your calculation with the DoppelZepp-Rechner you did not use a velocity factor (Verkürzungsfaktor der Speiseleitung). In this case you get wrong results.
A standard 300 Ohm Line i.g. is 0.82.
I built a doublet with 2 x 8m and 6m twin lead feederline (velocity factor 0.61 as measured) and it performs exactly as calculated.

Good luck with your experiments
Peter, OE5AUL

let me double check: I got this feed line http://www.radioworld.co.uk/generate_product_pdf?product_id=1602461 (300ohms) and in the calculator I changed the velocity to 0.82 for the screenshot above.
is that correct?
(I’ve got a scope and 10mhz signal gen so I could probably measure it but it will a long night! :slight_smile: )

IMHO, just use whatever length of feedline you need to reach the rig. It will most probably work fine with your tuner. If there’s an issue, just add a meter or two to the feedline length and try again. I use a pair of male and female bullet connectors to extend the feedline. I typically carry a 25ft length plus two 6ft extension lengths.

73, Barry N1EU

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It would be perfect if the vf field on your plots would not look empty…

Because of some reason i barely can see the 0.82 entry in the velocity field of your screen shot but it is there, so the calculation should be ok.

Peter, OE5AUL

@OE5AUL @HB9BCB sorry for the misinformation!
ironically my excel skills are non-existent (i can code in 10 different langs but NO MS Office skills!) so the light grey almost invisible to me too is from the builder of the calculator

try one under way :slight_smile:

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You may not know it but the zs6bkw antenna is the result of finding compromise lengths for the antenna and the feed line in a mathematical analysis. It does seem to work very well. But the design is based on several important aspects that some articles overlook such as the velocity factor of the feed line.

You might like to try it.

Good luck with your antenna.

73 Andrew VK1DA/VK2UH

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Hi Barry,

That’s good advice. The feeder needs to reach the ATU. If the ATU doesn’t like some impedances add a bit more twin feeder. Usually it isn’t much extra. It can be wound on a piece of plastic down pipe - say 80 + mm in dia to make it neater. Use cable ties to secure the ends on the pipe.

I would try adding maybe 10 electrical degrees of extra feeder. eg on 3.6 MHz that would be 6 feet. Which is what you have.


So today I tested for the first time the doublet with 300ohm feed line…
worst day to try a new antenna ever! Weather was ok but the bands… forget it!

I build this:

but the weird thing is that 14.260 I was getting 1:1.2 with the KX3 ATU and on 14.275-285… I was getting 1:2.5!!!

What should be the first plan of attack? Smaller legs is the easiest for me!

Hi Tasos,

Did you push the TUNE-Button on the KX3 a second time? I found out that in some cases the KX3 tuner needs a second chance. On 14 MHz the overall length of your antenna is 0.901 lambda, which is close to a full wavelength and means high impedance. So it might be a bit difficult for the tuner in the first run anyway.

Good luck with your experiments.

Peter, OE5AUL