How to make a simple 500m FM wireless transmitter?

How to make a simple 500m FM wireless transmitter?

This transmitter uses two-stage amplification, so the sensitivity is very high, and the surrounding sound can be collected clearly, so it can be used as a bug. The circuit is designed with audio input. When the audio signal is input through the jack, the microphone will automatically disconnect , It can be used as a music station. 500m FM wireless transmitter

There are many circuits similar to this form. I believe that many radio enthusiasts will not be unfamiliar. The theoretical value written in various materials is generally above 1KM, but after my experiment, this circuit is in rural areas when using a 1.2-meter whip antenna. The transmission distance is indeed close to 1KM, and it can reach more than 500 meters in the open area of ​​the city, and it can only reach 200 meters at most in the steel structure building. If you are willing to spend money to buy a special rubber antenna, the effect is estimated to be much better.

The high-frequency transmitter tube used in the circuit is D-40, but many friends say that the D-40 tube is not easy to find, and some people say that this tube is not available for sale now, but I did buy it in the Wuhan Electronic Market Of course, I can’t verify the authenticity of the tube. If you can't buy D-40, you can use high-frequency tubes such as 2SC3101 (the effect will definitely be worse) or sacrifice the transmit power to use 9018. The inductors L1 and L2 in the circuit are respectively wound with 4 turns and 10 turns on a drill with a diameter of 3mm with a 0.71mm high-strength enameled wire. After the coil L1 is wound, if possible, a magnetic core with a diameter of 3mm can be placed in L1 in the middle of the TV to enhance the circuit stability.

After the circuit is connected, start debugging. Generally, as long as the component values ​​are accurate, the transmitting frequency of the transmitter will fall within the receiving range of the ordinary FM radio. When debugging, pay attention to the distance between the radio and the transmitter, because the transmitter power is relatively large. The surrounding magnetic field is too strong and will cover all frequencies of the radio. Adjusting the value of C4 or C5 can change the transmitting frequency, and adjusting the distance between the inductance coils can also perform fine-tuning of the frequency. Note that the transmitting frequency should not conflict with the local FM radio station.

This is what I did:

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