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.
73
Ron
VK3AFW
).
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