Varactor FM wireless transmitter circuit

Working Principle As shown in the figure, the entire transmitter consists of low-frequency amplification, varactor bias, and oscillation circuit. After the audio signal is amplified by BG1, it is sent to the varactor, whose capacity changes with the signal, and directly modulates the frequency of the oscillation circuit, eliminating the parasitic amplitude modulation phenomenon caused by triode junction capacitance frequency modulation, and has few harmonic components. The frequency-modulated radio frequency signal is radiated directly from L1 to space. Component selection: Inductor L1 is made of ∮1mm enameled wire wound 5 turns on a ∮8mm round tube, making it hollow. L2 is made of ∮0.1~0.3mm enameled wire wound 100 times on a 1/4W carbon film resistor with a resistance greater than 100kΩ. The joints are welded to both ends of the resistor. C2, C11, C4, C3, and C5 should be of high quality. Ceramic capacitor. The resistors are all 1/8W, and the decoupling capacitors of 0.01μF can be ordinary ceramic capacitors. For installation and debugging, it is recommended to use the corrosion method to make the circuit board. First weld the resistor and capacitor components, and then the semiconductor components. After the welding is completed and checked, turn on the +6V power supply and measure the voltage between the 9014 emitter and the ground. There should be 0.47~0.9V. , if not correct, R1 should be adjusted. Then measure the voltage between the emitter of the 9018 and the ground and there should be 1~2V. If not, adjust R6. Then measure the voltage on both poles of the varactor diode and there should be no voltage, or the voltage difference between the two poles should be less than 0.25V. If the pressure difference between the two poles is too large, adjust R3 or R4. After adjusting the above items, turn on the FM section of the radio, receive the signal from the transmitter, and move it to a distance of more than 5 meters. If you cannot find the signal from the transmitter, you can adjust L1, stretch L1, and the fundamental frequency will increase and compress. L1, the fundamental frequency decreases, so that the transmitter's transmitting frequency avoids local radio stations and can be received reliably. Then connect the output signal of the player and other equipment, and carefully adjust W1 so that the music is not distorted at high volume (the volume of the player and other equipment should not be turned up to the maximum, but should be opened at the commonly used volume position). After the above items are adjusted and the debugging is basically completed, put it into a strong plastic or wooden box (metal boxes are not allowed), decorate it, and use it to listen to music or as a TV sound transmitter, which is the same as an FM radio signal. Clear and stable, it can be called an "amateur professional transmitter". It does not require an antenna and is small in size, so there is no need to worry about the transmission distance when used at home. I use it as a TV sound transmitter, connect the audio input to the audio output of the TV, use a 6V battery to power it, and use an alligator clip as a switch. The circuit board is tied to the battery with a rubber band, so you don't have to worry about running out of power if you use it for dozens of days.  ●Experimental comments This transmitter is simple to make, reliable in performance and low in cost. During the specific production, the circuit board used by the author is an epoxy board with low high-frequency loss. After debugging, each coil is sealed with high-quality high-frequency wax, and the effect is very good. The 5pF capacitor can be replaced by a double-sided copper-clad plate (calculated as 1pF = 1 square millimeter).