I needed a smallish 1:1 Bal-un for an HF multi-band dipole which I’ll be using from a portable location.
Looking at what was available ready made didn’t impress me too much so I decided in my attempt to avoid commercially made stuff as much as practical to have a go at building one myself.
One of the reasons I didn’t like the small sized ready made ones available from a couple of places was the fact that a lot of people had them go into meltdown even at QRP levels. My guess would be a combination of wire insulation type and core material choice with a bit of user over-expectation thrown in the mix too was to blame for the “melt-downs”.
After a bit of reading I decided on what I needed and sourced the parts.
Amidon 240-61 type core
14 AWG enamel coated wire, about 12 feet.
I already had some banana plugs and sockets which came from Mouser.
Small plastic box to put it all in from Amazon.
Amphenol BNC socket.
Total cost was about $25, not bad for a Bal-Un that will handle 1KW depending on how you put it together. I chose to use a BNC connector since thats my preference for lower powered portable ops. It will still be perfectly fine for 100 watts though.
I started by cutting the enameled copper wire into 32″ lengths, four of them.
Then wound two of them doubled up round one half of the core, then done the same thing on the other side with the other two lengths of wire.
The next step was to identify each end and then connect the two pairs in parallel and solder them together. The end result is two connection points at each end.
Next I drilled holes in the plastic box one at one end for the BNC socket, and two at the other end for the banana sockets.
Then I placed everything in the box and marked the wires where they needed cut so they were close to the connector.
The copper wire is quite stiff to work with, so I also bent it into place where it needed to be for soldering to the connectors.
Stripping the enamel coating from the wire was done with a file and some sandpaper, I tried my usual method of doing this by burning it off, but the coating is really good and won’t melt. (no need to worry about it melting while in use!)
After soldering everything together I tested the Bal-Un with the analyzer and it looks good. There’s a little anomaly at 16mHz but since thats not in the Amateur bands its not an issue, If it had been in one of the bands adjusting the winding spacing on the core would probably have fixed it.
I don’t have a VNA so I cant test the return loss or the common mode attenuation of this bal-un so I’ll have to trust it works to some degree.
The pictures below show the bal-un and also the impedance on the Amateur bands using two 100 ohm resistors in parallel to give a nice 50 ohm load.
Inside view and showing the test connections with the carbon resistors.
Finished item ready for use.
Anomaly at 16mHz, it started at about 15.500 and ended at 16.900 so far enough away not to be an issue.
I didn’t take pictures of the higher bands, but 17m 15m & 10 meters were flat at 50 ohms too. It would also cover 6m if needed but it was about 1.4:1 there so not as good as lower down the spectrum. The impedance curve was steep right after 51mHz. I think if I needed it to go higher one or two less turns would have fixed that at a cost of maybe 160 meters.
These notes were for my own use, however if you read this far then I hope you found it useful.
thanks and 73,