A 9:1 unun is a great way to set up a non-resonant “random” wire antenna. With the right (non)random length radiator and counterpoise, these antennas will tune up easily on many bands and make for a very versatile antenna when you’re using a radio with a tuner. For 40 m and up operation, consider using a 41 ft radiator and 17 ft counterpoise, and to cover 80 m as well, use a 84 ft radiator.
NOTE: the 9:1 unun build does NOT use the capacitor in the kit.
Step 1: Solder the Counterpoise and Antenna Wire Leads
Cut the length of Poly Stealth wire roughly in half. Strip about ⅛” of insulation from the end of each piece of wire.
Note the direction each wire is installed. Although this is not critical, I prefer this orientation for convenience of use.
Step 2: Install the Jack and the Jumper from Center Pin to Board
Use the nut and washer to secure the SO-239 jack to the PCB. Use some loc-tite if you have it.
Solder one end of the magnet wire to the center pin.
NOTE: The magnet wire in the kit is 155 C enamel which is relatively low-temp and burns off with a hot soldering iron. Most builders can be successful burning off the enamel by dwelling on magnet wire solder joints for 5-10 seconds. If you want to ensure a good connection and are not confident with this method, use sandpaper or a razor blade to strip away the enamel before soldering magnet wire.
Trim the wire to the length shown. Position the wire so it does not rub on the SO-239 jack. Solder the other end to the “CENTER PIN” pad on the PCB.
Step 3: Wind the toroid
Start winding the toroid as shown. Wind 6 or 7 turns evenly around the toroid. Each pass through the center is one turn.
CAUTION: Do not exceed 7 turns or you may run out of magnet wire before finishing the toroid.
Create a twisted tap about 1 inch long as shown.
Continue winding 7 more turns.
Then, 7 more turns after that for a 3rd lap around the entire toroid.
Congratulations, you’re done with the toroid!
Step 4: Install the Spacer and Solder the Toroid to the Board
Place the spacer onto the PCB/SO-239. Position the magnet wire in the notch on the spacer. This prevents chafing and shorting of the center pin jumper wire.
Insert the wire leads into the PCB holes as marked. Solder into place and trim flush.
The final assembly should look like this.
Step 5 (Optional): Install Strain Relief Cord.
Cord is not included in the kit, but if you’d like an attach point for strain relief during operating, install cord as shown.
Step 6: Test.
Inspect your solder joints. Ensure no solder bridges exist.
Test for cold solder joints with a DC multi-meter. You should find continuity (close to 0 ohms resistance) between the BNC center pin solder pad, BNC ground solder pad slots, and the antenna wire. If you find an open circuit or high resistance, reheat each magnet wire solder pad for 5-10 seconds to allow solder to burn off the enamel and flow into the joint.
You can also test the transformer by installing a 450 ohm resistor across the ANT and C-POISE wires, then test with an antenna analyzer. The SWR should be relatively low across the HF bands.
Step 7: Install the Heat Shrink
Position the 2″ heat shrink as shown. Carefully begin heating the heat shrink with a heat gun. Go slowly and adjust the heat shrink to position it properly.
The finished assembly should look like this.
Step 8: Add Wire and Operate!
Connect 41 ft or 84 ft of wire (or another appropriate length) to the ANT wire, connect 17 ft to the counterpoise, hoist it up in the air and have some fun.