1:1 balun turns based on toroid type? (where to get ferrite toroids in the UK?)

Hi all,

I’ve been googling around and reading up 1:1 buy baluns the last couple of days but I think I’m missing some basic info.

I want to build a 1:1 balun for a dipole but I haven’t found any clear info about the way we calculate the number of turns based on the toroid someone has (I might be missing the right terminology to put to google :slight_smile: ).

Is there some sort of calculation I can follow based on the toroid, the type of wire / enamelled copper and type of balun?
I so fr have found a 9:1 design (for a long wire) that uses the toroid I have, but in general I would like t understand how to calculate what I need based of the items I have in hand.



Also read K9YC’s detailed paper on what baluns do for you and how to build current baluns and chokes.



thanks a lot both.

I checked some posts with the formulas based on type and AL but probably was the posts that weren’t very clear for my head :blush:

Ill read the pdf that seem to be very details.


Roy Lewellan wrote the best article I have ever seen about understanding baluns. It is copied here:

It’s a bit shorter than K9YC’s paper - less details.


I;ve done some more research so I need to also find a couple of ferrite toroids to test.

Any pointers to places in the UK I can get (in reasonable prices if possible) some FT240 and FT140 toroids? type 43, 52 and 61.


Depends on what bands you want to use Tasos,

61 material is better for frequencies > 18 MHz. You will find this material and 43 used everywhere for broadband inductors and transformers in various hmbrew projects.

43 material tends to work better at lower frequencies.

You will see plenty of people using a “green” core in the US, which is 31 material (I think). They can be sourced from surplus PC power supplies in the EMC filter.

52 is a half way compromise between the two.

Personally I think your better with 61 as general rule. Jabdog (Web) and AMTools (Ebay) do these cores at a reasonable cost.

You will see large cores being used, I wouldn’t agree with a large core being necessary as with different loads they made little difference here w/rpsct to testing. Think I used 8-9 turns, on a FT240-61 here with RG-316.

Do not use micro-metal mix two this will worsen the broadband performance of your balun. E.g T200-(2) etc.

Hope that helps.


Hi Tasos,
Lots of good info there.
A 1:1 current balun is easy to make because it is less critical of the core than most. Almost anything will work. You can go to a lot of trouble or just use what is on hand.

Here are some suggestions.

You can make an air cored 1:1 current balun by winding a length of coax (a tenth of a wavelength at the highest frequency of use) or twisted pair around a short length of plastic pipe. This requires about 700 mm of coax for use up to 10 m when the velocity factor of the coax is taken into account. No ferrite required and only 5 turns on a 50 mm diameter former!

You can use a ferrite or iron powder core toroid to reduce the balun size and improve the low frequency coverage. About 6 to 12 turns of RG174 (thin 50 ohm coax) on a toroid about 20 mm in outer diameter works pretty well from 7 MHz up and is useable down to 3.5 MHz. You can use twin lead - light weight “figure eight” speaker cable for example - instead of the coax. Or if you have 600 mm of enameled wire about 0.05 mm in diameter cut it in half, twist the two wires at about 2 twists per centimetre and wind on the balun as before. The lesser number of turns is for a core with medium mu and the higher number for a lower mu core.

Now to check if you have enough turns just measure the inductance on one winding, say the shield of the coax. The inductive reactance should be very roughly 500 ohms on your favourite band. It’s not at all critical and the balun will still be useful at half that frequency. 10 microhenry is good for 3.5 MHz up. The idea is to present an impedance that is greater than the alternative 50 ohm current path.

The toroid material should have a low to medium permeability and be intended for use across the HF region to get the best response and lowest losses. It is less critical IMO for 1:1 current baluns than for other types. For example i’s not critical as the whether you use powdered iron or other ferrite material.

What if you can’t measure the inductance and have a small core but don’t know what it’s characteristics are?

You will get plenty of advice to overcome these difficulties BUT the good news is that even if you use a core designed for switch mode power supplies down on 200 kHz you can still make a useful HF current balun with it.

I know of one VK who used a steel bolt for his current balun and still uses it in spite of the fact I gave him a custom made toroidal one.

The current choke has intrinsic resistance to unbalanced current flow. Even if the core is lossy (ie not designed for HF) and the unbalance current was high before the balun was inserted it will be much less unbalanced with the lossy core. Some of the errant unbalance current will probably turn to heat in the core but better that than giving you rf feedback!

I can source toroids from Jaycar and Rockby here in VK and there is a local agent for Amidon. I also have a junk box with unknown cores bought at an irresistibly low price.

To test my baluns I make two. I have a small box wit coax connectors one each end. I connect the two baluns back to back in the box so I have 50 ohms in and out. I feed a tx in one port and have a dummy load and power meter on the other port. I then go to FM on the TX press PTTand start winding the power up from 5 W to 100W at 7 MHz. So far I’ve not seen any smoke. I expect to see no more than 2 W total loss at 100 W - generally I can’t measure it. ie 99% efficiency for each balun. After 60 seconds of key down I release the PTT and lightly touch the winding and core. It should not have warmed much. If after 3 minutes key down at 100 W - as much as dare with my present rig - it is no more than just detectably warmer then I am satisfied it will be OK for normal duty cycles at up to 100 W.

For QRP the two holed TV balun cores can be used with an enameled wire pair to make a useful 1:1 current balun. 4 to 8 loops seems to be OK for 7 MHz up.

The worst that can happen with your home brew 1:1 current balun is a small power loss and incomplete suppression of the common mode current.

Good luck.


Thank you both for the info. Ill invastigate your pointers.

I will pop you one in the post today.

73 Richard G3CWI

Thank you Richard! Must appreciated!

(Offtopic about your 174 cable: amazing termination! Super strong!)

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talking about diy-ing baluns, how do you clean the enamel from think wire?

I did the usual hot solder even 400oC on my Hakko and using a lighter to burn it off… but nothing…

should I get some sandpaper? i don’t have one at the moment…


Rub the iron tip over it a few times - It does remove. Failing that gas stove works well OR as you suggested sand paper. The first suggestion usually works. Make sure those turns are tight.


Means ‘solder iron’ or the ironing… iron? the first one I did, with much persistence…
the second… my lady will probably hung me from the tree and try to find my resonant frequency :smiley: heheheheheh

“solder iron” – soldering iron. HA ! the second wouldn’t get anywhere near hot enough. Although that “iron” is particularly useful in making PCB’s at home, that will upset your other half for sure.

Its all technique. Don’t think I have ever needed to reach for the sandpaper in that instance. Either that or your solder has the worse flux imaginable.


If you are using old enamelled wire, maybe recovered from a transformer, the enamel was not designed to melt with a soldering iron as it is nowadays.
In that case sandpaper is the safest way to remove it, but I usually scrape it with a very sharp blade. Take care not to nick the wire, though, or it is likely to break at that point…

73, Adrian

My cable is new i got it last week but its thick. I tried with a lot of solder (multicore good quility)
Will try again!

Ooooh I remember building hardware with this kind wire. Wiring up hundreds of HCTTL chips with 28SWG enameled wire. Heat it up and the varnish/enameling would evaporate. I used to wonder why an afternoon in the lab would mean I was coughing all evening. Then we had fans fitted and no more coughing. Then we had CPLDs and FPGAs and there was much less red wiring. Then everything went SMD and we had to have PCBs produced. Now everything is virtual… I never see anything I work on in physical form until it appears in customer devices 3-4 years after we prototyped it.

Anyway, the big stuff was hard to get started. The wire was thick enough to conduct a lot of the heat away so you couldn’t start the stripping. However, scratching/cracking the enamel was often enough to get the process going. Once it started to evaporate it was quite easy to clean up. So ensure you have a hot enough iron (Weller TCP #7 bits are hot enough) and if you are using 16SWG or heavier make sure you crack the surface finish or it may take a long time to strip.

Thanks! I got some sandpapers so ill propably get them tomorrow in the mail.ill try again.

I just got Richards toroid too!!

What size is the toroid?
Thanks again!

EDIT: FT-114-43? size seems ok. toroids.info say shiny black… its a bit shiny if you ask me! :smile: