VHF radio transmitter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transmitters
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The radio transmitter is a single-stage VHF FM transmitter operating in the broadcast band 61-73 MHz.
The output power of the transmitter when using a 9-12 V power supply is approximately 20 mW. It provides an information transmission range of about 150 m when using a receiver with a sensitivity of 10 μV.
The modes of the UZCH transistors (VT1) and the RF generator (VT2) for direct current are set by resistors R3 and R4, respectively. A voltage of 1,2 V is supplied to them and to the power supply of the M1 microphone from a parametric stabilizer to K1, C1, VD1. Therefore, the device remains operational when the supply voltage drops to 4-5 V. In this case, a decrease in the output power of the device is observed, and the carrier frequency changes slightly.
The modulating amplifier is made on a transistor VT1 type KT315. The audio frequency voltage is supplied to its input from an electret microphone with an M1 amplifier of the MKE-3 type and the like. The amplified audio frequency voltage from the collector of the transistor VT1 is supplied to the KV2A type VD109 varicap through a low-pass filter to resistor R5 and capacitor C5, and resistor R7. The VD1 varicap is connected in series with the trimmer capacitor C8 in the emitter circuit of the transistor VT2. The oscillation frequency of the master oscillator, made on a transistor VT2 type KT315 (KT3102, KT368), is determined by the circuit elements L1, C6, C7 and the capacitance C8 and VD1.
Instead of the VD1 LED of the AL307 type, you can use any other LED or three diodes of the KD522 type and the like connected in series in the forward direction. The L1 coil is frameless, 8 mm in diameter, has 6 turns of PEV 0,8 wire.
When setting up, the transmitter is tuned to a free section of the VHF FM range by compressing or stretching the turns of the coil L1 or adjusting the capacitor C8. The frequency deviation is set by the capacitor C8 according to the highest quality reception on the control receiver.
The transmitter can also be tuned to the VHF FM broadcast range (88-108 MHz), for this it is necessary to reduce the number of turns L1 to 5 and the capacitance of capacitors C6 and C7 to 10 pF. A piece of wire 60 cm long is used as an antenna. To reduce the influence of destabilizing factors, the antenna can be connected through a capacitor with a capacity of 1-2 pF.
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