Description
This is a kit to build a 1:1 current balun (a common-mode choke) on an FT140-43 toroid, inside a 3D-printed case I designed for it. It is made to sit just behind your radio: coax from the rig goes in one SO-239, your feedline comes out the other, and the choke keeps RF off the outside of the coax and out of your shack.
It is designed as a 1:1, but nothing about the case locks you into that. There is extra room inside for thicker coax or wire, so if your antenna needs a different ratio balun or an unun, wind it however you need.
What is in the kit
- 3D-printed case and lid
- FT140-43 ferrite toroid
- RG-316 coax for the windings
- 2 SO-239 chassis connectors
- M3 screws and nuts for the connectors and the lid
- Zip ties to hold the windings in place
- Optional: 2 SO-239 to BNC adapters (+$7.50) if your radio or feedline uses BNC
Power
Rated for 100 W. RG-316 is PTFE coax that handles a lot more than that, and the parts have headroom past 100 W, but 100 W is what I rate it for. The thing that limits it is heat inside a closed plastic case, so if you run more power, long key-down transmissions, or a badly matched antenna, check that the case stays cool.
How to wind it
Count turns at the center hole: each time the coax goes down through it is one turn. Wind 5 turns on one half of the core, bring the coax over the top to the other side (that crossover is not a turn), then wind 5 more on the other half, going through the hole the same direction. The two ends finish pointing away from each other, one to each SO-239.
What you will need
- Soldering iron and solder
- Wire cutters and strippers
- A small screwdriver
The case
- Interior: 55 x 70 x 32 mm
- Sized for a 140-43 toroid with a little extra room for thick coax or wire
- M3 screws all around mount the end connectors and hold the lid on
- The photos show a green case. Yours is printed in the color you pick.
- Not made for a permanent outdoor install, but great for temporary outdoor setups like POTA.
This is a kit, so it shows up as parts, not a finished balun. If you want to print a spare case, the design is on Printables: Balun / Unun Antenna Case for Toroid.
Free shipping, US only.
