A long-distance FM transmitter circuit (500M)

The low-power FM transmitter circuit introduced in this article uses a dedicated transmitter tube, which has a deep modulation degree, no amplitude modulation, small distortion, long transmission distance, and stable operation. The circuit is simple and easy to make, and it can work as long as the welding is correct. The circuit principle is shown in Figure 1. 　　In the circuit of Figure 1, a special transmitter tube T2 and its peripheral components form a high-frequency oscillator with a frequency in the range of 88 to 108 MHz. The audio signal picked up by the electret microphone is first amplified by T1, and the amplified low-frequency signal is then amplified. High frequency carrier is modulated. If you disconnect the electret microphone M and connect the output of the player to the input end, the music signal can be transmitted well. It should be noted that the RF transmitter special tube T2, its model is FF501, adopts the standard T0-92 package (like the 9000 series triode), the appearance and pin arrangement are shown in Figure 2, its ICM is 45mA, and fT is greater than 1. 3GHz, VCEO is 13V. The advantages of the special tube are good consistency, large RF output power, and easy adjustment of the circuit. FF501 can completely work in a higher frequency band. Readers can try to use the transmitter tube for high-frequency emission experiments in other circuits. L2 in the circuit is made of ∮1.0mm enameled wire, wound around 5 turns on a ∮5.1mm drill bit and stretched to 0.8cm. High-frequency ceramic capacitors can be used for C3~C8. It is best to use a 1.2-meter pull rod for the antenna. , and stand vertically. The antenna must be set up before powering on. The operating current of the circuit is about 25±5mA. If the transmission frequency is not within the range of 88~108MHz, the length of the resonant coil L2 can be appropriately adjusted. After the circuit is installed and adjusted, use an FM radio for reception. The effective transmission radius can reach 500m.