Tutorial of making low-power FM broadcast transmitter

After the commercial power is stepped down and rectified, it is stabilized to obtain a stable 12V direct current for use by the radio frequency circuit. The radio frequency circuit is composed of a high frequency oscillator, a buffer amplifier, a final 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 frequency inductance can be changed to change the transmission frequency. The radio frequency signal is output from the emitter of VT1 and sent to the buffer amplifier composed of VT2, L2, C22, R4, etc. for power enhancement, and can reduce the impact of the final amplifier circuit on the oscillator. The final stage is high-frequency C-narrowband amplification, which further amplifies the radio frequency power, and is coupled to the transmitting antenna via C25 to radiate to the surrounding space. If the output signal amplitude of the connected audio source is too large, an attenuating resistor must be connected in series 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 marked with no unit in the capacitor are always in "pF" and must be the same as high-frequency ceramic capacitors. VT1--VT3 use ultra-high frequency NPN silicon tubes, such as 9018, B》60, Icm=50mA.fr》=600MHZ. VT3 can also use medium power transmitter tubes C2053, BF96S, etc., the transmission distance may be longer. L1-L3 are made by using 00.8mm enameled wire on a 04mm screw pen to close 4 turns and take it out. The antenna is a whip antenna, and its length is 1/4 (or 1/2) of the frequency wavelength. If the transmitting 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 first installs the oscillator, the buffer amplifier, adjusts the distance between the turns of L1 to make the frequency point fall in the place where there is no station, touches the collector of the VT2 with the black test pen of the pointer multimeter, adjusts L2 to make the pointer deflection amplitude maximum, maximum). If it is found that there is a meter marking, you can wind the test leads together until the meter is not marked. Use the same method to adjust L3 to maximize the output power of the final stage. Use the FM radio to search for the transmitted signal in a place more than 10 meters away from the transmitter, estimate the transmission frequency, and then connect the antenna, adjust the length appropriately, and then it can be put into use.

When the actual power supply voltage of this 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. When the building is blocking, use the popular machine (internal chip CXA1019M) to receive the distance up to 1000 meters.