Seeking dipole antenna advice

Equipment Antennas
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Greetings fellows. I would like to build a simple dipole for SOTA, which will be my first. There are a lot of kits, blog posts, videos, etc., so I don’t have the problem of too little information but rather too much.

What I’m mainly in doubt about is: should the centerpiece of the antenna include a balun?
And if so, how do I make a balun? I understand that you wind a bit of wire around a toroid but am not clear on how to decide what kind of toroid and how many windings and where the various ends get soldered.

I would also like to know what gauges of wire and what kind of feedline you all might recommend. I may transmit up to 20W SSB or 5-10W CW.

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Simple question, you are going to get a ton of advice. Very simple sota/portable dipole, 22-24 ga. wire ( i have some with 26 ga) center and end insulators, get a 2.00 cutting board from the dollar store and cut out a pattern you like for center and ends and winder Should use rg 174 or rg58 . Based on 50 yrs experience you can use a balun of your choice (this is where you are going to get a ton of answers) why not try without the balun, you could decide its not needed or can add laterr. have fun for minimal expense. add links for more bands later if y0u want
KC5BG Mike

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The answer is: anything! What do you have?

From modeling dipoles at HF, any wire >26awg bare copper will have negligible loss. I use 24awg stranded wire from ethernet cable. At this thickness, bare copper is better than tinned wire**.
After that it is a mechanical and weight issue.

Any coax will take 100W at HF: Tiny RG178, RG174.

The choice is determined by:

  • How critical is weight?
  • What is your highest frequency, and therefore the loss. (1dB is OK)
  • What do you have on hand or can you scab off a mate?

1st choice is what you have.
For HF my basic choice would be RG174, it’s light and loss is OK.

If you want longer runs, or are going to use it for 2m or 70cm, or weight is just not that critical, then some RG58 size foam coax is ideal. It’s lighter than solid, and bit lower loss.
(btw I just use 75ohm foam coax as I have it, and it works just fine)
(Oh, if anyone has a source of foam RG174, do tell)

So if you are using 80m-15m primarily, Lets say 7m of coax for a 5m pole.
RG178 has 5.6dB/30m@10M. = 1.3dB@10M, 2.45dB@50MHz This is a bit high, but RG178 will be OK for a 3m coax going to an end-fed.

RG174 3.3dB@30m@10M. = 0.77dB@10M, 1.5dB@50M
This is OK to 30MHz and serviceable at 6m

Consider a twisted pair from inside and ethernet cable. It is low loss, super light - you will put the balun at the bottom of the pole. If you are using an antenna tuner, the fact it is 100ohms will get taken care of by the tuner. If not, do trim the antenna using the actual feedline you will use in the field.

Teflon and polyethylene have the same loss. Teflon can take bigger powers as it can get hotter, but doesn’t matter at 100W HF. It is also much easier to solder tiny teflon. If you are crimping, it doesn’t matter. Foam coax has lower loss because the centre conductor has to be bigger. It is also - look it up only , the powers are huge at HF. Solid centre conductor and foil shield have lower losses, but that’s an issue for UHF.

** Perhaps. Tin is very lossy, but I didn’t find any data for how thick the plating is so perhaps it is lossy, or perhaps it is too thin to matter.

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  • Your dipole will work without a balun. You probably won’t notice any difference in the field running QRP. So don’t let the lack of one hold you back.
  • The ferrite sleeve off a power cable or VGA cable will work as a current choke balun. Cut the plastic off. Wind some thin coax or twisted wire through it 4 times (ie there are 4 through the hole.)
  • If you want to buy cores get a few Fair-rite 2643625002 . You can use them for EFHW transformers and 4:1 OCFD transformers too.

You want a current choke type balun. This is super simple: you just wind the transmission line though a ferrite sleeve. It should be located where you go from coax to balanced. So this can be at the dipole, but if you are using ethernet twisted pair to come down your pole, you could put the balun at the bottom with a short coax tail to your radio.

Now almost any ferrite sleeve will do the job, counter intuitively the size of the sleeve doesn’t change the impedance much - it just lets you fit bigger coax. Also counter-intuitive is that the RF power does not go though the core, so it does not need to be big. It is only the small current that sneaks though the choke down the outside of the coax that goes into the core. The main reason for big cores, is to wrap big coax around it.

You could use hard twisted pair of enamel wire or small coax RG178 or RG174, or that really tiny coax used in cellphone routers and so on, or ethernet twisted pair. It is only a short length - 200mm or so, the impedance doesn’t matter at HF.

How to test baluns?

  1. see how how the balun gets on CW. If it is not getting hot, it is not losing power. A 5W wire wound resistor gets really hot. Ferrite, enamel wire, and teflon coax don’t care about heat - it is just an indication of how much power loss there is.
  2. measure the current on the screen. An RF current meter is a very simple and useful thing for testing actual wire antennas in practice.

Can I just coil up coax to make a balun?
No, it is too much coax at HF, you don’t want to carry it, and it would be too lossy in RG174.

What does a balun on a dipole do?

  • It stops the outside of the coax being part of one side of the dipole. This is not really a bad or a good thing , it still works as an antenna, just a bit different, and a bit less predictably
  • It stops (reduces) rf current coming down the outside of the coax, and making you rig, microphone etc “hot”. This is really, really important - if you lugged a 1kW amplifier up the hill. At QRP, not really.
  • It stops RF noise doing the opposite - travelling from your rig (computer, smps,mains noise) back up the outside of the coax to the antenna. This important at home, but not doing SOTA, unless you are using an SMPS to power your rig.
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Hi all, great discussion about dipoles. Learning a lot. :grinning:

Geoff vk3sq

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To answer the question about bands, I am thinking of aiming for the 40M band for this first one. If it goes well I will (later) try to make myself a linked dipole to cover 40, 20, and 10.

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I wish I were more familiar with the options for coax. Sounds like you’re saying RG58 is better, but RG174 and RG178 are more lightweight? Where do you buy coax? Do you recommend getting a crimping tool, or soldering connectors?

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Hi,
I recommend you refresh your theory by reading the chapters in the ARRL AR Handbook on HF antennas and their construction. This will help you discern which is good advice and which isn’t.

The ARRL Antenna Handbook has a lot for good info and well worth acquiring

SotaBeams has an online dipole calculator that you might like to use, after reading the ARRL chapters.

73
Ron
VK3AFW

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In a word: Scab.
Contact your local hams or radio club. There will be locals with coax, crimping tools wire etc etc. They are just itching to have someone use it. Just ask. RG59 tv coax works fine too. UTP network cable is everywhere. Flat flex (zip cord) works fine for antennas too, just strip it apart.

There will also be people with SWR meters and nanovnas. Useful things to be able to use, but you don’t need to own them. It is better to borrow, because then it comes with someone who knows how to use it.

It is only when you go on a multi day hike and you become obsessive about weight, that you want thinner stuff.

Remember the only thing that is crucial to getting a SOTA contact, is being on a hill. At the moment I am taking my old TS130 (5.7kg) as it’s what I have. We carried two base station rigs up a hill last weekend. (IC7300,IC9700). It was fine.

Crimping is easiest. RG58, RG59 crimp BNC’s are common. I often have to fit RG174 or RG178 into an RG58/59 size BNC because that’s what I have. You can also just buy a patch cable (75 ohm is OK too) with BNC’s and cut it down to length.

There are BNC’s that RG58 will twist into. DON’T use them. Ever.

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Some useful information on this page, and a winding diagram; couldn’t be simpler.

        73 de OE6FEG
               Matt
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Is this Kiwi slang?

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In Australia the Irish use of scab applies. To break rank with a union strike. In NZ it means scrounge or borrow.

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That is another meaning here too.

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In the US we use “scab” as a noun for those who don’t participate in a strike; not sure I’ve ever heard it used as a verb.

I think what ZL1THH means would be expressed as “scrounge” in US English.

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For a wire-dipole for mountain top use a coax feedline is overkill. The advantage of coax is its shielding properties. On a (remote) mountain top this is not really an issue. I use twin lead for my wire dipoles and a BNC-to-4mm-Banana plug adapter at the radio. Telephone wire, in particular military field telephone wire, is a source for suitable twin lead. Loudspeaker cables as sold in building supply stores is another one. Twin lead saves weight and the hassle of attaching a coax connector.

Link: What Is a Twin-Lead?

73 Heinz

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If you use twin-lead, what does the junction between feedline and antenna look like? Do you simply split the wire so the radiating elements are a direct extension of the feedline, or do you use a balun and perhaps solder a different gauge of wire for your radiators?

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Hi Joe,

You can split the twin-lead as you suggested. I prefer to use a piece of plastic (the lid of a food container) as the center plate to support the connection of the dipole with the feeder. I have punched a few holes into the plastic and inserted cable ties to fix it to the mast. This serves as a relief of mechanical stress on the dipole wires.

I do not use any balun, neither in the middle of the antenna, nor at the end of the feeder.

I am in the office now, but I can post some fotos later today.

73 Heinz

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I assume that you have a device for one band and do not want to use an antenna tuner.
The advantage of a coaxial cable is that you can work with different lengths without worrying. Generally, this then requires a balun in the transition. (The dipole is symmetrical and the coax or the antenna socket is asymmetrical).

Of course you can keep the balun very small at qrp and combine it mechanically with the suspension point.
My recommendation is a balun for unknown impedances at the input of the dipole e.g. on a toroid Amidon XX-43.

grafik

The length of the RG 174 is then irrelevant.

Of course you can also use a parallel wire from the antenna to the TRX.

grafik

This is the easiest. You can take e.g. a speaker cable and lock it with a knot against further opening.

grafik

At the input point of the cable you will have about 100 ohms… so a 2:1 balun at the input of the TRX would be good.
But then you should adjust the complete system including the feed line to the SWR.

grafik
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I recommend in any case a current balun before the antenna input… e.g. W1JR

I must confess that I am not a big fan of classic dipoles. They are always a bit too bitchy for me with the ambient influences. :sweat_smile:

73 Armin

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Great graphics; I love that knot to stabilize the twin lead!

Are you recommending a balun at both points – the transciever input and the other end of the feedline where it feeds into the antenna?

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Of course, you only need a balun to get from the balanced antenna to the unbalanced coax cable or the unbalanced coax socket…

The current balun would not be mandatory for qrp /p applications… however, the vagabonding RF can also do stupid stuff there…
With the IC 703, for example, the paddle no longer worked for me.

73 Armin

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