Tracking Transmitter

Posted on Feb 6, 2014

This is a small battery operated Tracking transmitter which broadcasts a repeating radio signal which can be detected by a directional antenna. The transmitter broadcasts an isotropic (radiates in all directions) signal on the FM band. An ordinary FM radio can be used to locate the transmitter, the reception of the signal will improve when the rad

Tracking Transmitter
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io antenna is pointed in the direction of the transmitter. The transmitter is a standard hartley oscillator, designed to transmit across the FM band approximately, 87 to 108MHz. The transmit frequency is adjustable by means of trimmer C8. The combined capacitance of C4 plus C8 and L1 set the resonant frequency. Modulation for the transmitter is an interrupted square wave signal using all 4 gates of a CD 4011. Gates U1 and U2 interrupt the signal at about 2Hz while gates U3 and U4 create a higher tone in the audio range. The output of U4 applies this modulation to the base of Q1 via R3. Because the modulation is a squarewave, the radiated signal becomes a little bit "dirty" and harmonics will be heard across the FM band. This is not a big problem as it will aid in locating the transmitter. The transmitter is tuned by the tank circuit L1 and parallel capacitance of C4 plus C8. L1 is approximately 0. 15 uH. The value of "C" in the equation below is the sum of C4 plus C8. L1 is 7 turns of 20swg wire wound on a 6 mm drill bit. This forms an inductor of approximately 150 nH. The value of the inductance can be altered by compressing or expanding the turns. Hold one end of the wire against the drill bit and start to wind the turns clockwise. Continue until all 7 turns have been completed, and the coil looks like the image of the right hand side. Trim any excess wire so both ends are same length and scrape about 3mm of insulation...

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