[Photo] Self-made low-power FM radio transmitter

Li Shifeng

The author uses the existing components at hand and comprehensively refers to the production articles related to "Beijing Electronic News" and other newspapers to make a long-distance FM broadcast transmitter, working in the 88-108MHZ frequency band, and playing it in spare time music.

The circuit principle is now shown in the attached drawings. Figure (1) is the power supply part. After the mains is stepped down and rectified, the voltage is stabilized to obtain a stable 12V DC power for the RF circuit. The radio frequency circuit is composed of a high-frequency oscillator, a buffer amplifier, a final-stage power amplifier and an antenna. The high-frequency oscillator is used to generate the carrier frequency signal, the frequency point falls within 88-108MHZ, and the transmission frequency can be changed after completing the frequency sensing. The radio frequency signal is output by the emitter of VT1 and sent to a buffer amplifier composed of VT2, L2, C22, R4, etc. for power boosting, and can reduce the influence of the final amplifier circuit on the oscillator. The final stage is high-frequency C narrowband amplification, which further amplifies the RF power and radiates to the surrounding space through the C25 coupled to the transmitting antenna. If the amplitude of the output signal of the connected audio source is too large, it is necessary to string in a weakening resistor to avoid sound distortion.

The circuit board can be made of copper-clad board, pay attention to the influence of distributed capacitance when wiring. The numbers of the capacitors without unit labels in the figure are all in "pF" as the unit, which must be combined with the high-frequency ceramic capacitors. VT1--VT3 uses ultra-high frequency NPN type silicon tube, such as 9018, B> 60, Icm = 50mA.fr> = 600MHZ. VT3 can also use medium power transmitter C2053, BF96S, etc., the transmission distance may be farther. L1-L3 is made of 00.8mm enameled wire tightly wound around 4 turns on a 04mm screw pen. The antenna is a whip antenna, and its length is 1/4 (or 1/2) of the frequency wavelength. If the transmission frequency is 100MHZ, the antenna length is 0.7m (or 1.5m)

It should be installed step by step during production. The radio frequency part is first equipped with an oscillator, a buffer amplifier, and adjusting the turn-to-turn distance of L1 so that the frequency point falls where there is no platform. Use the black test pen of a pointer multimeter to contact the collector of VT2. Adjust L2 to make the pointer deflect the maximum amplitude maximum). If you find that there is a phenomenon of hitting the watch, you can twist the test leads together until you don't hit the watch. Then adjust L3 in the same way to maximize the output power of the final stage. Use the FM radio to search for the transmitted signal at a distance of more than 10 meters from the transmitter. Approximately estimate the transmission frequency, then connect the antenna, adjust the length appropriately, and then put it into use.

When the power supply voltage of the machine is 12V (in fact, it can work normally within 6-15V, the higher the voltage, the farther the distance), the working current is only about 45mA, the emission frequency is about 104MHZ, and it is placed on the third floor balcony without excessive When the building is blocked, it is received by a universal machine (internal chip CXA1019M), and the distance is up to 1000 meters.

Li Shifei

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