High power radio transmitter at 27-28 MHz. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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High power radio transmitter at 27-28 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 radio transmitter operates at a frequency of 27-28 MHz with amplitude modulation. The carrier frequency is stabilized by quartz, which allows you to increase the communication range when using a receiver with quartz frequency stabilization. The device is powered by a 3-4,5V power supply. The audio frequency amplifier is made on a transistor VT1 type KT315. To power the microphone and set the DC modes of transistors VT1, VT2, VT3, a parametric voltage regulator is used on resistor R2, LED VD1 and capacitor C1. A voltage of 1,2 V is supplied to an electret microphone with an Ml amplifier of the MKE-3 type, "Pine", etc.

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The audio frequency voltage from the microphone M1 through the capacitor C2 is supplied to the base of the transistor VT1. The DC operation mode of this transistor is set by resistor R1. The amplified audio frequency signal, taken from the collector load of the transistor VT1 - resistor R3, is fed through the capacitor C3 to the master oscillator, thereby carrying out the amplitude modulation of the transmitter. The master oscillator of the transmitter is assembled on two transistors VT2 and VT3 of the KT315 type and is a push-pull self-oscillator with quartz stabilization in the feedback circuit. The circuit consisting of coil L1 and capacitor C5 is tuned to the frequency of the quartz resonator ZQ1. The circuit, consisting of coil L2 and capacitor C7, is designed to match the antenna and transmitter.

The device uses MLT-0,125 resistors. Capacitors are used for a voltage of more than 6,3 V. The VT1 transistor can be replaced with any npn transistor, for example, KT3102, KT312. Transistors VT2, VT3 can be replaced by KT3102, KT368 with the same current transfer ratio. A good result can be obtained using the KR159NT1 microcircuit, which is a pair of identical transistors.

The contour coils are wound on a frame with a diameter of 5 mm, having a tuning core made of carbonyl iron with a diameter of 3,5 mm. Coils are wound in increments of 1 mm. Coil L1 has 4+4 turns, coil L2 has 4 turns. Both coils are wound with PEV 0,5 wire. The Dr1 choke has an inductance of 20-50 μH. A wire about 1 m long is used as an antenna. As a power source, you can use one flat battery KBS-4,5 V or four elements of the A316, A336, A343 types. The VD1 LED of the AL307 type can be replaced with any other or you can use an analogue of a low-voltage zener diode with low current stabilization.

Setting up the transmitter begins with setting the modes of transistors VT2 and VT3 for direct current. To do this, connect a milliammeter to the break in the power circuit at point A and select the resistance value of the resistor R4 so that the current is 40 mA.

The tuning of the circuits L1, L2, C5, C7 is carried out according to the maximum RF radiation. Moreover, they are roughly tuned to the operating frequency with capacitors, or rather, with the core of the coil. The trimmer of coils L1, L2 should be located at a distance of no more than 3 mm from the center of the coils, since in its extreme positions, generation can be disrupted due to a violation of the symmetry of the shoulders of transistors VT2, VT3.

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