A great thing about ham radio is that you can spend literally as much or as little as you can afford and still enjoy the hobby. In fact, you will likely enjoy it more by building your own simple equipment, and then making contacts with it. An inexpensive way to begin is to build your own antenna tuner. An L-network random wire tuner is probably the simplest matching network in existence, yet it does a good job as long as the antenna is at least a quarter wavelength at the lowest operating frequency.
- Skill level:
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Things you need
- Scrap wood board, about 1 foot long and 8 inches to 1 foot wide
- 4 self-stick rubber feet
- PVC pipe, 4 inches diameter and 16 inches long
- 4 L brackets, 1 inch
- 20 wood screws, 3/4 inch
- #9 drill bit
- RG-58 coaxial cable with PL-259 on one end
- 4 #10 ring terminals
- Crimp tool
- 50 feet of 14-gauge insulated solid copper wire
- 2 #10 brass machine screws, 3/4 inches, with 2 hex nuts and 4 flat washers
- Screwdriver set
- Nut driver set
- Electrician's pliers
- Clear wrapping tape, 2 inches wide
- Air variable capacitor with wide plate spacing
- F drill bit
- 2 brass machine screws, 1/4 by 2 inches, with 2 wing nuts, 2 hex nuts and 6 flat washers
- 3 ring terminals, 1/4 inch
- Soldering iron and solder
- 14-gauge clip lead
Peel and stick the rubber feet in the corners of the board, on the roughest side. Lay the board on your work surface, with the feet down.
Mount the PVC pipe upright in the middle of the wood, using the L brackets and wood screws. Space the brackets equally around the pipe, and mount to the board such that they point toward the corners.
Drill a hole in the pipe on the left side, between the brackets and three inches up from the base, using the #9 drill bit. Drill a second hole on the opposite side, one inch down from the top.
Separate the braid and centre conductor of the open end of the coax, and strip 3/8 inch of insulation from the centre conductor. Crimp a #10 ring terminal on the centre conductor.
Strip 3/8 inch of insulation from the 14-gauge wire, and crimp a ring terminal on it. Put a washer on a #10 screw, followed by the ring terminals of the 14-gauge wire and the coax centre conductor. Push this screw through the hole near the base of the pipe, and secure it on the inside with a washer and nut.
Wind the wire tightly around the pipe toward the top, leaving 1/4 inch spacing between the turns. Use the pliers to twist a small loop into the wire every second turn, so that the loops will be in a line on the right side of the coil. Strip the insulation from each of these loops, using the knife. Secure the coil to the pipe, using the tape.
Put a washer on the other #10 screw and push it through the upper hole in the pipe, from the inside. Cut the wire where it reaches the screw, leaving one inch to twist around the screw. Strip one inch of insulation from the end, twist the wire around the screw and secure it with the remaining washer and nut.
Fasten the variable capacitor to the right side of the board, using wood screws. Drill a hole through the board two inches to the right of the capacitor, using the F drill bit. Drill another hole two inches away from the first hole, toward the front of the board. Stick the 1/4-inch brass screws through the holes from the bottom, using washers on both sides and securing them with the hex nuts.
Cut two 3-inch pieces of the leftover 14-gauge wire, strip the insulation with the knife and crimp a 1/4-inch ring terminal on one end of each. Slip the terminal of one wire onto the rearmost 1/4-inch screw and solder the other end of the wire to the terminal of the capacitor that is isolated from the chassis. Slip the terminal of the second wire onto the forward screw, and solder the other end to the terminal of the capacitor that is connected to the chassis.
Cut one end from the clip lead and strip 3/8 inch of insulation. Crimp the remaining 1/4-inch ring terminal on the lead, and slip it onto the rearmost 1/4-inch screw. Connect the antenna wire to the same screw, and secure with a washer and wingnut.
Connect the counterpoise or ground wire from the antenna to the forward 1/4-inch screw, and secure with a washer and wingnut.
Strip 3/8 inch of insulation from the remaining piece of 14-gauge wire, and crimp a #10 ring terminal on it. Crimp the last #10 ring terminal on the end of the braid from the coax. Slip both of these terminals onto a wood screw to make an electrical connection, and drive the screw into the board in front of the PVC pipe. Solder the other end of the wire to the terminal of the capacitor that is connected to the ground.
Tips and warnings
- To use this tuner, connect to your radio and tune to the desired frequency. Set the capacitor to the middle of its range and select the tap point on the coil that gives the greatest received signal when connected with the clip lead. Then transmit at low power while tuning the capacitor for a dip in SWR.
- Do not use this tuner or any unshielded tuner with more than 100 watts.
- Do not touch any of the conductors while transmitting.
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