A long distance FM frequency modulation transmitting circuit (500M)

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The low-power FM transmitting circuit introduced in this paper, due to the use of a special transmitting tube, has deep modulation, 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 Figure 1.

In the circuit in Figure 1, a high-frequency oscillator with a frequency in the range of 88 ～ 108MHz is composed of a special transmitting tube T2 and its outer enclosure. The audio signal picked up by the electret microphone is first amplified by T1, and then the amplified low-frequency signal modulates the high-frequency carrier wave. If the electret microphone M is disconnected, the music signal can be transmitted well by connecting the player output at the input end.
It should be noted that the special RF transmitting tube T2, whose model is FF501, is packaged in standard T0-92 (like the 9000 series triode). Its shape and pin arrangement are shown in Figure 2. Its ICM is 45mA, fT is greater than 1.3GHz, and VCEO is 13V. The advantages of the special tube are good consistency, high RF output power, easy circuit adjustment, and FF501 can work in a higher frequency band. Readers can try to use the transmitter tube for high-frequency transmission experiments of other circuits. L2 in the circuit is pulled to 0.8cm by wrapping five turns of Φ 1.0mm enamelled wire on a Φ 5.1mm drill bit, C3 ～ C8 can use high-frequency porcelain dielectric capacitor, and the antenna uses 1.2m pull rod and is vertically placed. The antenna must be installed before it is powered on. The working current of the circuit is about 25 ± 5mA. If the transmission frequency is not within the range of 88~108MHz, the length of resonance coil L2 can be properly adjusted.
After the circuit is installed and adjusted, FM radio is used for reception, and the effective transmission radius can reach 500m.