Using 3DA87C to make FM transmitter circuit

The figure below is an introduction from an electronic magazine about a long-distance FM transmitter circuit made of ordinary transistor 3DA87C. This circuit is also an ordinary three-point oscillation circuit. The article states that the long-distance transmitter circuit uses large current transmission and can reach 1KM in an open area. The author purchased the components and assembled them according to the schematic diagram. According to the requirements of the article, the transistors with blue dot marks should have an amplification factor greater than 80 times. However, it was found in the experiment that its frequency did not fall within the normal FM band of 88-108MHz, but no matter how the capacitor and inductor were adjusted, it was lower than 88MHZ, about 70 MHZ. The author used a TESUN radio with TV audio reception function to receive the signal normally. Later, after checking the information, it was found that the fT cutoff frequency parameter value of the transistor was not enough, so its oscillation frequency could not be increased.

In order to increase the transmission distance and make the frequency fall within the receiving range of normal FM radio, we have to look for other high-frequency triodes. Many electronic newspapers and magazines often introduce the use of D40, C1971, C1972 as high-frequency oscillation or power amplifier circuits, which can increase the transmission distance with their high-power output. However, this type of high-frequency triode is difficult to buy on the market, and even if you can buy it, most of them are fake and cannot be used. Later, we found C3355, which has a cutoff frequency of several thousand MHZ and a power of 600MW, which is enough for the FM band. Then, after making some improvements to the circuit, we can easily make a long-distance FM transmission circuit.

Component selection: Capacitors C2, C3, and C4 are all high-frequency ceramic capacitors. Ct is a 5/25P high-frequency semi-adjustable capacitor. It can also be replaced with a high-frequency ceramic capacitor of the same value after debugging with a digital multimeter. L is an enameled wire with a diameter of 0.9, which is wound 6 turns on an 8mm round tube and then pulled apart by about 2CM and tapped in the middle. The transmitting antenna uses a TV antenna or a wire of the same length instead.

The actual maximum distance after debugging to the optimal transmission power is no less than 500 meters.