Radio transmitter with compact loop antenna. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transmitters
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The device operates in the range of 65-73 MHz with frequency modulation. The operating range when using a compact frame antenna is about 150 m. The operating time of the device when using Krona batteries is 30 hours. The schematic diagram of the radio transmitter is shown in the figure.
(click to enlarge)
The low-frequency signal of the M1 microphone of the type MKE-3, "Sosna" and others is amplified by a two-stage low-frequency amplifier with direct connections. The amplifier is made on transistors VT1 and VT2 type KT315. The operating mode of the amplifier is set by resistor R2. The master oscillator of the device is made on a transistor VT3 type KT315. The frequency-setting circuit is connected to the base of the transistor VT3 through a small capacitor C6. Capacitors C8, C9 form a feedback circuit.
The generator circuit consists of an inductance L1, a capacitor C5 and two back-to-back diodes of the KD102 type. Under the action of the modulating voltage, the capacitances of the diodes VD1, VK2 change. Thus, the frequency modulation of the transmitter is carried out. From the output of the generator, the modulated signal is fed to the power amplifier. The output amplifier is made on a transistor VT4 type KT315. It works with high efficiency in class "C" mode.
The amplified signal enters the loop antenna, made in the form of a spiral. The spiral can be of any shape, it is only important that the total length of the wire is 85-100 cm, the diameter of the wire is 1 mm.
Inductors Dr1, Dr2 - any, with an inductance of about 30 μH. Coils L1, L2, L3, L4, L5 - frameless, 10 mm in diameter. Coil L1 has 7 turns, L2 and L4 - 4 turns each, L3 and L5 - 9 turns each. All coils are wound with 0,8 mm PEV wire. Setting up the transmitter has no special features.
See other articles Section Transmitters.
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