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Hidden transmitter hunting, aka fox hunting, is a fun activity with serious overtones. Ideally it’s a game, but the same equipment and skills can help find those in distress or track down a jammer. This post provides a schematic and construction details for a circuit developed by Dave Sharpe, KIØHG. His circuit below is by far the simplest direction finder I’ve run across. It works with two dipoles on a pole and provides a tone when the antenna array isn’t quite pointed right, and nulls the tone when it is. I’ve used mine in many a two-meter fox hunt and it takes me right to the fox. Let’s start with the schematic.
Let’s walk through the schematic. (At the very bottom of the post I’ve provided part numbers for jameco.com.)
- The single IC is a 74HCT04, a CMOS hex-NOR-gate that takes very little current. Wire it as shown
- The audio tone is generated by two of the NOR-gates back to back with the 22K fixed resistor and the 10K potentiometer along with the .05μF capacitor.
- The diodes are all 1N4148 or 1N914 RF switching diodes. These are really inexpensive. The two in the middle go on the perf board along with the 680pF capacitors. The other two are at the very ends of the equal-length coax sections, as shown in the figure below
- The inductors can be store-bought, or can be made by winding magnet wire using a 1/2-watt high-value resistor—the old-fashioned cylindrical kind. The resistor should be greater than 100KΩ. The resistor is not important to the circuit.
- The pieces of coax from the circuit board to the antennas must be of the same length! Get this right, down to the millimeter!
- A Radio Shack perf board makes a great circuit board, as shown in the figure below.
Here’s what it looks like with the antennas inserted:
Components are available at Radio Shack or at www.jameco.com.
- 12μH inductors, Jameco 372171, 15¢ each
- 1N4148 diodes, Jameco 151247 (also available from Radio Shack), 2¢ each.
- 470Ω resistors, Jameco 690785 (also available from Radio Shack), 2¢ each.
- 22KΩ resistors, Jameco 691180 (also available from Radio Shack), 2¢ each.
- 10KΩ potentiometer, Radio Shack 271-343, $2.99 each, also available from Jameco as part number 241294.
- 0.05μF capacitor, Jameco 332507, $1 each (seems to me very pricey)
- 680pF capacitors, Jameco 332427, 10¢ each
- Circuit board, I used Radio Shack 276-150, $1.99 each.
- 74HCT04 hex inverter, Jameco 44898, 25¢ each
Be careful wiring and have fun! Fox Finder operating instructions in another post.