DIY RDF

Amateur radio operators can modify portable receivers for radio direction finding.

The Federal Communications Commission considers the amateur radio community to be self-policing. Among other things, that means you are expected to do your own detective work to locate sources of radio interference within your band allocations. This responsibility has spawned a ham radio sport known as RDF -- or radio direction finding -- "foxhunts." The designated "fox" hides a small low-powered transmitter, and then the other participants hone their skills in locating the transmitter. Typically, foxhunters attach a DIY directional loop antenna to an existing portable receiver, to create an RDF receiver.

Things You'll Need

• PVC pipe, 1/2-inch diameter by 2 feet long
• 1/2-inch PVC cross
• Measuring tape
• Hacksaw
• 20 feet of insulated electrical wire
• Electrical tape
• Wire stripper
• Variable capacitor

Instructions

1. Cut the PVC pipe into four equal-length pieces. Insert the pieces into the cross.

2. Use the hacksaw to notch the exposed ends of the pipes, to hold the wire. Wrap the wire around the perimeter of the cross for five turns. Leave 2 feet of free wire on each end. Tape the wire to the cross.

3. Bring the free ends of the wire along one of the legs to the center of the cross. Strip a half-inch of insulation from each wire at the center of the cross.

4. Connect the variable capacitor across the two wires where you stripped the insulation. Tape the capacitor to the cross.

5. Connect one of the wire ends to the receiver's antenna input. Then connect the other wire to the receiver's ground connection.

Tips & Warnings

• In use, hold the antenna in front of your body while you slowly rotate your body to obtain the strongest signal. Then tune the capacitor to peak the signal, and turn your body clockwise until the signal nulls -- indicated when it disappears or becomes much weaker. When you find the null, the signal will be arriving 90 degrees counterclockwise from where you are facing.

References

• The ARRL Antenna Book; American Radio Relay League