I originally designed this circuit for use on the 144MHz band to track a vehicle, but it could be used to track anything or be used as the fox for a Amateur Radio Fox Hunt.
Ranges of over 3KM have been acheived using 58cm wire antenna, receiving the signal on a REXON RL102 144MHz Handheld Tranceiver with a 3 element Yagi and the transmitter being powered of the 9Volt. This narrowband crystal controlled VHF transmitter is modulated with a 700Hz Tone outputting approximately 300mW of RF power. The circuit is crudely phase modulated hense can be detected by an AM or FM receiver, An AM receiver will receive the signal slightly better.
The original circuit was designed around a siren audio generator IC, UM3561, but as this is no longer available I have re-disigned the circuit around a LM555 flip/flop IC. The LM555 generates a 700Hz tone and modulates the output stage. If no more than 9Volts is being used, a fixed 27K resistor can be used in the O/P biasing circuit instead of a preset and the modulation fed directly to the Base. The emitter circuit of the oscillator/trippler stage is tuned to the Xtal frequency filtering the fundalmental frequency. The collector circuit is tuned to 144MHz and outputs the required modulated frequency signal to the amplifier stage. 12pf trims the O/P power into the antenna for maximum power transfer and the 22pF trims the Xtal for spot frequency. This circuit will operate between 5.6Volts and 10Volts without any circuit alterations, but the range will be a lot less at lower voltages. 12Volts can be used by inserting a 82Ohm resistor in series with the positve lead. When using above 9Volts, a heatsink shold be used on the 2N2222 as it will get very hot. It is Ideal to tune up this circuit using a wave detector meter placed 10-20cm away from the transmitter. I have a circuit on this site.
Due to the demand of a higher powered transmitter an I have added and extra circuit below. This will boost the power by 2 to 3 times. Simply remove the antenna series capacitor and connect the circuit below. The coils are 22SWG and 7mm diameter on an Air Former. I have acheived 800mW RF output from the stage into a 50ohm Dummy Load, but in practice using a 1/4 144MHz Rubber Antenna only 500 to 600mW RF Power will be radiated. With the inclusion of this circuit ranges of 7 to 8Km have been reported. Personally I managed 6-7Km.
The formulae for the LM555 being Frequency in Hz =1/((R1+(2xR2))xCx0.7
R1=47K, R2=82K, C=10nF.
This page was updated on 9th April 2001.