A coaxial trap antenna is a popular antenna design for amateur radio enthusiasts with limited space who are interested in obtaining increased bandwidth. Multiple coaxial traps can be installed on the same antenna line, which affords the radio operator access to radio frequencies otherwise unavailable -- without building a second or third antenna. Build this coaxial trap antenna developed for the 20-meter band.
- Skill level:
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Things you need
- Coaxial cable, RG-58, 50-ohm, 12 feet long
- 2 PVC tubes, 1 1/4-inch inner diameter, 3 inches long
- Plastic sheeting, 3 inches square
- Electrical wire, 16-gauge, 34 feet long
- 4 wire nuts, 16-gauge
- Coaxial connector
- Fishing line, 50 feet
- Power drill, 1/4-inch and 1/8-inch bits
- Utility knife
- Soldering iron and solder
- Coaxial cable crimping tool
Cut two 3-foot lengths from the coaxial cable, setting aside the additional 6-foot segment. Strip one inch of outer plastic insulation from both ends of each 3-foot coaxial cable. Slice through one side of the braided insulation of each cable end and twist this insulation until it forms a wire. Strip 1/4 inch of inner plastic insulation from each end, exposing the inner copper wire.
Drill a 1/4-inch hole in a PVC tube, 1 inch from the end. Thread 2 inches of cable through the hole, and then wrap the cable six times around the PVC tube. Drill another 1/4-inch hole after the sixth turn, and thread the end of the cable through this hole. Do the same procedure with the second PVC tube and 3-foot cable.
Solder the twisted braid wire of one end of the coaxial cable to the copper wire of the other end of the same cable. Do this with the other PVC tube assembly as well. This procedure may take a few attempts, since the space inside the PVC tube will be tight.
Cut the electrical wire into two 17-foot segments, stripping 1/2-inch of insulation from the ends. Drill two 1/8-inch holes side-by-side, 1/2 inch from the end of a PVC tube. Loop the end of one 17-foot segment through both holes to lock the wire in place, and connect the wire to the coaxial copper wire with a wire nut. Terminate the unused twisted braid with a second wire cap. Perform this procedure with the other 17-foot wire and PVC tube assembly.
Strip one inch of outer plastic insulation from the end of the 6-foot coaxial cable. Slice the braided insulation, and twist it to form a wire. Strip 1/2 inch of inner plastic insulation, exposing the copper wire.
Drill two 1/8-inch holes in two corners of the square plastic sheeting. Loop the free end of each 17-foot wire through a set of corner holes, locking the wires in place.
Solder the twisted braid of the 6-foot coaxial cable to the free end of a 17-foot wire, and solder the coaxial copper wire to the free end of the other wire.
Spread the antenna into a T-shape. The electrical wire should form the 34-foot horizontal segment, with the PVC tubes located at opposite ends. The 6-foot coaxial cable should form the middle vertical leg of the T.
Crimp a connector to the free end of the 6-foot coaxial cable that is appropriate to your radio receiver's external antenna jack.
Drill a 1/4-inch hole in the outer end of each PVC tube, and string fishing line through each hole. Hang the coaxial trap antenna in your attic or outside between poles, keeping the antenna in a T shape.
Tips and warnings
- Use an antenna analyzer to measure the resonance of the antenna. Trim the 17-foot wire segments 1/4-inch at a time to find the correct resonant length. Wrap the centre cables and square plastic insulator with electrical tape to weatherproof the antenna.
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