I was looking through this old box of parts and I came across an old set of TV rabbit ears my father modified to use with a SW radio. He replaced the twin lead with a PL-259 connector ignoring the mismatch since it was only for receiving. Even a receiver will benefit greatly from a good match and I wanted a portable antenna for my NOGAnaut and Tuna Tin 2 QRP transmitters. I decided to add a balun to improve the match so I can transmit with it.
Step 1: Disassembly
First thing I did was to disassemble the antenna to see if it needed cleaning. The copper side washers were a little dirty but some carburetor cleaner fixed that. I left the center conductor of the PL-259 attached to one side of the rabbit ears. If you're building one of these you will have to remove the original twinlead and drill a hole down the center of the mounting post for the wires. It helps if you cut off the ball at the end first. Starting the drill will be easier on a flat surface. You have to solder the wires to the PL-259 before you screw it on the antenna base. Once its screwed on the collar wont slide up enough to get to the side soldering ring. If you screw in on while its still hot from soldering the ground lead it will go on a bit easier. Don't worry about the slot the twinlead used to go through. We will use it for the balun wires.
Step 2: Winding the Balun
I was able to find some decent toroid cores for $1 a pair 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 if you don't have that option. Here I use a red and green pair of enameled magnet wire.
After you are done winding the coil coat it with glue or shellac to keep it together. Now strip all 4 leads and connect the red wire from the start of the winding to the green wire and the end of the winding. If you used the same color wire you just 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 just 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.
Feed the wires through the slot vacated by the twinlead and solder each of the single leads to a rabbit ear. Then solder the center tap of the core to the ground of the PL-259. Connecting the center pin of the PL-259 to either rabbit ear completes the wiring.
Step 3: Closing It Up
I glued the core to the side of the housing and cut a small square of wood to cap the wiring. I could have just used some RTV to fill the top and attache the coil. Encapsulating it in Black RTV would have been a nicer look. I still may do that when I get some RTV. Right now I only have Blue and Red RTV on hand.
Step 4: Trying It Out
Just flatten out the ears and give it a try. Fully extended its a bit over 7 feet long. I took a piece of 1-1/2 PVC pipe and a PVC cap and made a small mast. I put a SO-239 feed through in the pipe cap so I can connect a short rg58 cable to the bottom. You can use any connector or adapter to match your radio and feedline. Bulkhead connectors work great here. I always keep a bag of adapters handy. A couple of zip ties or bungee cords can be used to attach the mast to a fence or post. If you bring a 1-1/2 tee and extra piece of pipe you can just drive the extra pipe into the ground and top it with the tee and mast adapter.