Using 3DA87C to make FM transmission circuit Introduce a long-range FM transmission circuit made with ordinary triode 3DA87C. This circuit is also an ordinary three-point oscillation circuit. The article shows that the long-distance transmission circuit uses large current emission, which can reach 1KM in open area. The author purchased the components and started the assembly test according to the schematic. According to the requirements in the article, the magnification of the triode with the blue dot mark should be greater than 80 times, but in the experiment it was found that the frequency is not falling at 88-108MHZ. Within the FM band, no matter how the capacitance and inductance are adjusted, it is lower than the frequency of 88MHZ, which is about 70 MHZ. The author is the TESUN radio with the TV sound receiving function to receive the radio. Later, the data was found. It is the triode's fT cutoff frequency parameter value is not enough, so that its oscillation frequency can not be raised.
In order to increase the transmission distance and make its frequency fall within the receiving range of the normal FM radio, we have to find other high-frequency triodes. Many electronic newspapers often use D40, C1971, C1972 as high-frequency oscillation or power amplifier circuits. The transmission distance is increased by its high-power output, but such high-frequency triodes are difficult to buy on the market, and even if they are bought, most of them are fakes and cannot be used. Later found C3355, the cut-off frequency of this triode is several thousand MHZ, its power is 600MW, it is enough for the FM band, and then the circuit is improved, it can be convenient to make the long-range FM transmission circuit.
Component selection: Capacitors C2, C3, C4 are high-frequency ceramic capacitors, Ct is 5/25P high-frequency semi-tunable capacitors, can also be converted to the same value of high-frequency ceramics after testing with digital multimeter test Capacitance, L is an enameled wire with a diameter of 0.9 mm
The round tube is wound around 6 turns, then pulled about 2CM, the middle tap is used, and the transmitting antenna is replaced by a TV antenna or a wire of the same length.
Actually debug to the best transmit power with a maximum distance of no less than 500 meters.
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