Home >> Hobbies >> Ham Radio

How to Build an Antenna Tuner Using a Jennings Cap

If your ham radio system is confined to a narrow band of frequencies, an antenna tuner may expand your frequency range, and boost transmission power. A tuner matches the antenna's impedance, or resistance to electrical current, to that of the radio, providing optimum transmitting power. A Jennings cap is a heavy-duty capacitor that, when combined with a wound coil, can create a resonant circuit that will balance the antenna's current. This resonant tuning circuit can improve your long-wire antenna, and save you the cost of a commercial alternative.

Things You'll Need

Jennings variable capacitor, 300 to 500 pf, vacuum type
PVC tube, 1 inch diameter, 3 inches long
Magnet wire, 24 gauge, 6 feet long
3 pieces of electrical wire, 16 gauge, 6 inches long
Plywood board, 6 inches square
2 machine screws, 1/4 inch-by-1 inch
4 machine screw nuts
Alligator clip, solderless
Plastic zip tie, 6 inches long
Power drill, 1/4- and 1/8-inch bits
Wire stripping tool
Utility knife
Soldering iron and solder
Epoxy
Pliers
Sandpaper
Pencil

Instructions

Preparation
- 1
  Drill two 1/4-inch holes in a corner of the plywood board. Insert two screws through the bottom of the board, with the threads sticking up. Fasten nuts to the screws, securing them in place.
- 2
  Drill two 1/8-inch holes in the PVC tube, 1/2 inch from the end. Thread the magnet wire through these holes. Wrap the wire tightly around the tube 120 times, twisting the wire into a loop every 10 turns. The loops should stick out from the tube.
- 3
  Drill two 1/8-inch holes after the 120th wire turn on the PVC tube, and thread the remaining magnet wire through the holes, locking the wire in place. Place a dab of epoxy over both sets of drill holes to secure the wire permanently. Cut one end of the magnet wire short, and trim the other end to 3 feet. Remove 1/2 inch of insulation from the 3-foot end.
- 4
  Place the capacitor on the board in the corner adjacent to the screws, with the tuning knob facing out. Drill an 1/8-inch hole through the board on either side of the capacitor. Thread the zip tie through the holes, and secure the capacitor to the board.
Assembly
- 5
  Epoxy the PVC tube vertically to a corner adjacent to the screws.
- 6
  Strip 1/2 inch of insulation from both ends of the three lengths of electrical wire. Solder one end of a 6-inch wire to the rear frame of the capacitor. Twist the free end of this wire around a screw. This will be the ground screw.
- 7
  Solder a 6-inch wire to the terminal on the front of the capacitor near the knob. Twist the free end of this wire around the empty screw. This will be the antenna screw.
- 8
  Crimp an alligator clip to one end of the remaining 6-inch wire. Twist the free end around the antenna screw, and clip the other end to a loop on the PVC tube.
- 9
  Connect the antenna wire to the antenna screw, and the ground wire to the ground screw. Fasten a second nut to each screw, securing the wires in place.
- 10
  Connect the 3-foot magnet wire end to the external antenna jack of your radio. Tune the antenna by turning the capacitor knob, and clipping different loops on the tube.

Ham Radio