Radio transmitter with AM in the frequency range 27-30 MHz. Scheme, description

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Radio transmitter with AM in the frequency range 27-30 MHz. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Transmitters

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The device described below operates in the 27 - 30 MHz band with carrier frequency amplitude modulation. The main advantage is that it is powered by the mains. It uses the same network to emit a high frequency signal. The receiver receives a signal using a telescopic antenna or a special network adapter. The diagram of the radio transmitter is shown in the figure.

(click to enlarge)

The master oscillator is assembled on a VT2 transistor of the KT315 type according to the traditional scheme. To power the M1 microphone, a parametric voltage regulator was used, assembled on a resistor R1 and a VD1 LED connected in the forward direction, on the anode of which a voltage of 1,2 - 1,4 V is maintained.

An ultrasonic frequency converter is assembled on a VT1 transistor of the KT315 type, the signal from which is modulated in amplitude by a master oscillator. The DC voltage at the collector of transistor VT1 is the bias voltage for transistor VT2.

The modulated RF signal from the coupling coil L2 through the capacitor C9 enters the power grid. In this case, the power wires act as an antenna. The power supply is assembled according to a transformerless circuit. Inductor Dr1 prevents the penetration of RF oscillations into the power supply. On the reactance of the capacitor C8, the excess mains voltage is extinguished. Unlike a resistor, a capacitor does not heat up and does not generate heat, which favorably affects the device's operating mode. The rectifier is assembled on diodes VD3, VD4. Capacitor C7 smooths out the ripple of the rectified voltage.

Further, the voltage through the parametric stabilizer, assembled on the resistor R5 and the zener diode VD2, is supplied to power the radio microphone. Capacitor C6 reduces the ripple of the rectified voltage. Such a power supply ensures stable operation of the radio microphone when the mains voltage changes in the range from 80 to 260 V.

M1 microphone - any small-sized condenser microphone with a built-in amplifier (MKE-3, M1-B, "Pine", etc.). Capacitors C8 and C9 must be designed for an operating voltage of at least 250 V. Inductor Dr1 type DPM-0,1 with a nominal value of 50 - 90 μH. Choke Dr1 can be made independently. It contains 100 - 150 turns of 0,1 mm PEV wire on a standard ferrite core with a diameter of 2,8 mm and a length of 14 mm (the length of the core can be reduced by 2 times). Coils L1 and L2 are wound on standard ferrite rods with a diameter of 2,8 mm and a length of 14 mm with PEV 0,23 wire. Coil L1 - 14 turns, L2 - 3 turns over and. Transistor VT2 can be replaced by KT3102 or KT368. LED VD1 - on any LED. Diodes VD3, VD4 are replaced by KD105 or others for a voltage of at least 300 V. Capacitors C6 and C7 can be larger and have a higher operating voltage, they must have minimal leakage. Zener diode VD1 can be replaced by any zener diode with a stabilization voltage of 8 - 12 V.

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