Introduction: How to Make a Balun or Unun
I needed a simple portable HF antenna. A long wire and some insulators is pretty easy to carry. To connect it to the radio I needed a Balun.These are such simple things but yet they cost so much from some places. I decided to make my own and did it for less than $5. A Balun is used to match a balanced line to an unbalanced line. An Unun is used to match an unbalanced line to another unbalanced line. This is a 4:1 Current Balun.That means it can match a 200ohm antenna to a 50 feed line.
If you plan on stringing up a dipole you use 2 equal lengths of wire with the Balun in the middle. You can run the wire straight across the field or better yet use a V shape, either inverted or not. In an inverted V you raise the Balun to a high point on a mast and the elements extend down to the ground. I prefer a regular V where the Balun is near the ground and the elements extend up to two masts or trees, buildings etc nearby.
Even simpler is a long wire end fed antenna. You just need a single wire and a ground rod. Here you need an Unun. The only difference between the Unun and Balun is how you connect the feed line.
For the antenna wire I have even used a spool of Mig wire in a pinch.
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Step 1: Winding the Coil
You need a good large toroid core. I was able to find some decent 40mm OD cores for $1 each online. You will make a bifilar winding. This simply means you wind 2 wires at the same time, lying flat and equally spaced. Many people use 2 different colors of wire but its easy enough to figure out with an ohmmeter.
After you are done winding the coil coat it with glue or shellac to keep it together. Now strip all 4 leads and connect your meter to one start and one end lead. if you have continuity switch to the other end lead. now that you have two leads with no continuity mark these as your antenna leads.These will go to the binding posts. Now connect the remaining 2 leads together. This is the center tap of the coil. This will complete the circuit and the meter will show continuity. This center tap will be the feed point. How it gets connected decides if its a Balun or Unun
Step 2: Pick a Connector
Pick an enclosure. I used a surplus case I got online but it was pretty slim so I had to grind down the sides of an SO-239 to fit. You can use a BNC connector if you don't plan on transmitting through it with more than 10 watts or if this is just for receiving.
Step 3: Putting It All Together
Its a tight fit in the case. the small circuit board in the bottom was to cover a slot in the back of the case. I got these from a surplus house and they have a hole in one side which I used for the feed line connector and a long slot across the back. I just epoxied over the slot to close it off.
Since I was making an unun I used one insulated binding post and one bare bolt with a wing nut for the ground connection. If I was making a balun I would have used two insulated binding posts. this makes it real easy to tell what it is from a distance.
You connect the two individual start and end leads of the toroid to the two binding posts.
If you want a balun connect the SO-239 ground to the center tap of the toroid and connect the SO-239 center pin to either binding post. it doesn't matter which one.
if you want to make an unun connect the SO-239 center pin to the center tap of the toroid and connect the SO-239 ground to the bare binding post.
I filled the center of the toroid with glue to hold it in place. You could even pot the whole enclosure to make it waterproof.This one is getting mounted under an eve so its OK the way it is. Just put the cover on and you ready to try it out.
It's been a while since I put one of these outside for my QRP rig and it still works great but I want to put another one out there for my Kenwood TS-520 and it will have to be a bit larger. I made some improvement to the case. See this instructable for the details:
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