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Random length multi band dipole (with ATU)

hi all,

Im thinking of building a random (not cut for one band I mean) length dipole. I don’t want a linked one or a single band.

At the moment Im mainly using a random wire with 9:1unun.
35.5ft as from lamda calculators it seems like a length that is not multiple of 1/2l of most (if not all) bands.

Does the same principle work with a dipole with 1:1? my head says its ok but Im no expert :slight_smile:
If yes Ill make one with two 35.5ft legs.

I know that two 22ft legs work well for 40-10M and two 44ft legs work well for 80-20M. It’s important to use low-loss balanced feedline. I use 300-ohm windowed line and a 4:1 balun at the rig/tuner.

Barry N1EU

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Hi,
One good Rule of Thumb is to make the doublet at least 70% of a half wave at the lowest frequency you want good performance on. Shorter will work but with reduced effectiveness. A 60 m halfwave fed with balanced feeder is a good choice for 3.5 to 18 MHz. At higher frequencies the pattern has more lobes than most like.

You will need a tuner for best efficiency rather than an rf transformer at the top - the impedance will vary a lot from one band to another.

I often use a 22 ft per side doublet with 300 ohm line for 7 MHz to 52 MHz. The ATU has a 4:1 balun inbuilt. A balanced tuner like a Z-match would be better IMO. At a pinch you could use Fig 8 flex for the feeder - acceptable losses on HF IMO. Tuner may not like some feeder lengths.

Good luck. Let us know what you do and how it went.

73
Ron

70% of a half wave at 3.5MHz is 28m so not sure why you said 60m?

1/2 wave dipole for the 60 metre band?

got it, sorry - we’re on the same page :slight_smile:

Sorry, I confused the issue by posting 60 m (band) and 3.5 MHz. Memo to self. Don’t mix units, use freq for band to avoid confusion over length.

73
Ron

Have you ever read these papers:

http://xa.yimg.com/kq/groups/9536618/2035391307/name/44ft_doublet.pdf

http://ftp.unpad.ac.id/orari/library/library-sw-hw/amateur-radio/ant/docs/Introducing%20the%20All-Band%20Doublet.htm

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thank you all for the tips and links!

just to give you all a bit more info.
I got some things from our friend Sotabeams for this project. Im also going to use it with the KX3 internal ATU.
So the feed line is RG174A/U and I got a mini centre balun that can be built for 1:1 and 4:1 I believe.

maybe I have to give the 22fit legs try. this is also way shorter than I thought which is a good thing!

You absolutely should NOT use RG174 as the feedline. Much of your power is going to loss and not getting radiated. You need to use a balanced feedline. The SWR at the feedpoint and along the feedline is very high with a random length doublet, resulting in inherently high loss with a coax feedline, especially RG174. That balun helps a little but not much.

For example, that 30m doublet you suggested fed with a 4:1 balun and 10m of RG174 will have 3.6dB loss on 40m, 2.3dB of loss on 20m and 4.9dB of loss on 17m. A 1:1 balun yields worse results.

Barry N1EU

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Hint to make this configuration slightly less ineffective on the lower bands: Build the balun switchable, 1:1 or bypass for use on 40/30/(80m) and 1:4 for use on 20m and up.

With a 1:4 balun on 40/30m the “not so bad” impedances of 25/55 ohms would be reduced unnecessarily down by the factor 4.

@N1EU @HB9BCB hmm quite puzzled here. I understand the logic and implications but I was hoping for less ‘severe’ problems.
Imagine that I currently use just 10meters of wire straight on the KX3 (with 4 wires as countpoise) when Im outside.
Plus, looking at the mobile/light antennas sold online, all come with ~10meter of 174 feedline and 1:1 baluns.

Am I going to be in a worst position than just the wire?

Yes.

A coax fed doublet plus tuner is not a multi-band solution worth considering. On too many bands your rf output is going to loss in the feedline.

good to know!!

So, I should go with something like??
http://www.radioworld.co.uk/generate_product_pdf?product_id=1602461

with two legs of 22ft and then make a balun for the KX3?
–labber line–4:1–a meter of coax?or no coax??? and then KX3 with ATU on.

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Yes, that’s the way to go. The balun goes right on the KX3 antenna jack, no coax.

It makes for an excellent 40-10M antenna.

great! thanks @N1EU

to make the installation easier and not put the KX3 BNC a lot of pressure … is 0.5ft (like 10cm) of coax cable between the radio and the balun ok?

yes, it’s fine

Good luck!

My portable doublet looks like shown in the photos, antenna wire is Polys-26 or Radox-125 (0.25mm2).

BTW, Another wow effect for newbies on multiband doublets (double zepps) will be the existence of unsuitable overall wire lengths because the total wire length of the feeder and antenna leg is of relevance for the impedance occurring at the feedpoint.

To be honest I could never measure any signal difference (continental and intercontinental) when operating the doublet with or without an 1:4 Balun connected to the K2 and KX3.

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wow nice trick of linking the wire legs! you gave me ideas!

I can’t understand what you mean here :confused:

Please read the relevant sections in the different antenna books, e.g. (presented on the plate) here:
Practical Wire Antennas 2, Edited by Ian Poole, G3YWX
Chapter 4, Tuned Feeder Antennas

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