The radio transmitter (figure 5) is a single-stage УSW FM transmitter operating in the broadcast range 61-73 MHz. Transmitter output power when using a power source with a voltage of 9-12 V is about 20 mW. It provides a transmission range of information about 150 m when using a receiver with a sensitivity of 10 µv. Modes of transistors UAC (VT1) and the RF generator (VT2) DC voltage set by the resistors R3 and R4 respectively. Voltage 1.2 V on them and on the power to the microphone M1 is supplied with parametric stabilizer on R1, C1, VD1. Therefore, the device maintains its efficiency by reducing the voltage to 4-5 V. the observed decrease of the output power of the device, and the carrier frequency is changed slightly. The modulating amplifier is made on the VT1 transistor KT315 type. The audio frequency voltage at its input with electret microphone with amplifier M1 type FEM-3 and the like. The amplified audio frequency voltage from the collector of transistor VT1 is supplied to the varicap type VD2 SW409А through a low pass filter the resistor R5 and the capacitor C5 and the resistor R7. The varicap VD1 is connected in series with a trimming capacitor C8 in the emitter circuit of the transistor VТ2. The oscillation frequency of the master oscillator, which is made on the VT2 transistor KT315 type (KT3102, KT368), is determined by the circuit elements L1, C6, C7 and capacitance C8 and VD1. Instead of the led VD4 type AL307 you can use any other led or three series-connected in the forward direction of the diode type KD522 and the like. Coil L1 is frameless, with a diameter of 8 mm, 6 turns of wire sew of 0.8. When establishing a transmitter set up on a free site УSW FM range compression or stretching of the turns of the coil L1 or tuning capacitor C8. Frequency deviation is set by the capacitor C8 at the reception quality to the control receiver. The transmitter can also be configured to broadcast a range УSW FM (88-108 MHz), it is necessary to reduce the number of turns of L1 to 5 and the capacitance of the capacitors C6 and C7 to 10 pF. In antenna pigtail length 60 cm To reduce the influence of destabilizing factors of the antenna can be connected through capacitor 1-2 pF.
Figure 5. The radio transmitter УSW range.