500m frequency modulation (FM) transmitter circuit diagram

The low-power FM transmitting circuit here uses a dedicated transmitting 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. It can work as long as the welding is correct. The circuit principle is shown in the figure below.

In the figure, a special transmitting 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 by the high-frequency signal. The carrier wave 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 circuit adjustment. 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. For L2 in the circuit, use ∮1.0mm enameled wire, wind it around 5 turns on a ∮5.1mm drill bit and stretch it 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 place it vertically. . The antenna must be set up before powering on. The operating current of the circuit is about 25±5mA. If the transmitting frequency is not within the range of 88 to 108MHz, the length of the resonant coil L2 can be appropriately adjusted.

After the circuit is installed and adjusted, an FM radio is used for reception, and the effective transmission radius can reach 500m.