Posted on Nov 12, 2012 11170
The transmitter was designed specifically for use by radio amateurs as a radio beacon. As such, it provides a good-quality
signal that is free of unwanted harmonics. Transistor Tl, in association with crystal XI, operates as a 36-MHz oscillator. Filter L1/C3 obviates any tendency of the circuit to oscillate at 12 MHz (the fundamental frequency of the crystal). Circuit L2/C4 is tuned to the fourth harmonic of the oscillator signal (144 MHz). This signal is fed to the aerial via a buffer stage that consists of T2, a double-gated FET. The (amplitude) modulating signal is applied to the second gate of the buffer. The output power of the transmitter has been kept low, about 10 to 40 mW. The modulating signal is generated by Nl, an oscillator that switches the transmitter on and off via transistor T3. The switching rate lies between 0.1 and 0.5 Hz. When the output of Nl is low, T3 is switched off, and the transmitter is inoperative because the supply is disabled. When the output of Nl is high, T3 is on and the transmitter operates normally. The digital pattern at the gate of T2 shapes the modulating signal. Gate N2 generates a square wave at a frequency of 0.1 to 1 Hz. As long as the output ofT3 is high, N4 oscillates at a frequency of about 1 kHz. At the relevant gate of N2, there is, therefore, a periodic burst-signal at 1 kHz, and this signal is used to modulate the transmitter. The digital pattern at the relevant gate of 2 can be varied to individual requirements by altering the values of the feedback resistors in the digital chain. The transmitter is calibrated by setting trimmers C4, C7, and C8 for maximum output power. Inductors L2 and L3 are wound from 0.8-mm diameter enamelled copper wire: L2 = 5 turns with a tap of 1 turn from ground; L3A = 3 turns and L3B = 2 turns. The coupling between L3A and L3B should be arranged for maximum output power. The circuit draws a current of only 20 mA, which enables the transmitter to be operated from a 9-V battery for several hours.