foxhunt fm transmitter circuit schematic

Foxhunt FM Transmitter Circuit

This 2 meter 144 MHz fox hunt transmitter is used in amateur competitions where a hidden transmitter is to be “hunted” using mostly homebrewed receivers and antennas. The foxhunt electronic circuit is the transmitter. It radiates a high quality signal without unwanted harmonics. Transistor T1 and the crystal togheter make the oscillator that generates a 36MHz signal.

The unwanted 12MHz basic frequency of the oscillator is suppressed by the filter circuit made of L1, C3, C2. The L2/C4 circuit is set to the fourth harmonic or 144 MHz. The signal goes to the dual-gate-FET driver stage before finally radiating through the transmitting antenna. The output power is from 10…40mW.

The radiated signal is also modulated by the gate circuit made of U1, U2, U3, U4. Gate U1 is a low frequency oscillator which generates a signal from 0.1 to 0.5Hz. This signal modulates the transmitter through the transistor T3.

If the U1 output is “0”, transistor T3 is off and the transmitter is also off. On the other hand, if the U1 output is “1”, transistor T3 is on and the transmitter is on. During the “1” period, gate U2 generates a squarewave signal with a frequency form 0.1 to 1Hz. Gate U3 works as an inverter only. It determines whether gate U4 generates a 1KHz signal or not. A periodic burst signal is now present at the gate FET T2 to modulate the transmitter used at foxhunt.

Calibration of the foxhunt transmitter: Adjust the three trimmer capacitors to produse a maximum signal amplitude at the output.
Coil data:
L1 = 470 nH
L2 is made of 5 windings of 0.8mm copper wire, 8 mm winding diameter. It is tapped at the first winding from the ground.
L3 is made of 0.8mm copper wire, 8mm winding diameter, 3 windings at the FET side and 2 windings at the antenna side. Adjust the coupling between the two windings sides to get a maximum signal output amplitude.

The circuit can be powered with a 9 volt battery. It consumes around 20mA only.

Fox hunt transmitter circuit schematic

foxhunt fm transmitter circuit schematic

Fox hunt transmitter active components

  • T1 = 2SA256
  • T2 = 3N205
  • U1, U2, U3, U4 = IC1 = 4093


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