Long-range high-frequency transistor FM transmitter circuit diagram

Description: The ordinary triode 3DA87C is utilized to create a long-range FM transmitter circuit, which functions as a standard three-point oscillator circuit. This remote transmitter circuit is capable of large current emissions, achieving a range of up to 1 kilometer in open areas, as indicated by schematic assembly tests. The selected transistor, marked with a blue dot, has a gain greater than 80 times. However, it does not operate within the normal FM band frequency range of 88-108 MHz. Despite adjustments to capacitance and inductance during experiments, the frequency remained approximately 25% lower than 88 MHz. A TESUN radio, equipped with TV audio reception functionality, was able to receive the signal correctly. The cutoff frequency parameter of the transistor is insufficient for the desired oscillation frequency. To enhance the distance between the transmitter and receiver, it is necessary to find a high-frequency transistor such as D40, C1971, or C1972 for use as a high-frequency oscillation power amplifier circuit. These high-frequency transistors are often difficult to obtain, and many available on the market are counterfeit and unusable. Eventually, the C3355 transistor was discovered, which has a cutoff frequency in the several thousand MHz range and a power output of 600 mW, making it adequate for the FM band. Subsequent improvements to the circuit can easily yield a long-range FM transmitter. Component selection includes high-frequency ceramic capacitors C2, C3, and C4, with Ct being a 5/25P high-frequency semi-adjustable capacitor. After debugging, this capacitor should be replaced with a high-frequency ceramic capacitor of the appropriate value, as measured with a digital multimeter. The inductor L consists of approximately 6 turns of 0.9 mm enameled wire wound around an 8 mm diameter pipe, with a 2 cm center tap. A TV or transmitting antenna of the same length as the wire is recommended for optimal performance. Actual testing indicated that the maximum transmission power achieved a distance of no less than 500 meters.

The circuit design employs the 3DA87C triode configured as a three-point oscillator, which is essential for generating the FM signal. The oscillator's operation relies on precise component values to ensure the output frequency falls within the FM band. The transistor's gain is a critical parameter, as it amplifies the oscillation signal, thus enhancing the transmitter's range. The choice of high-frequency ceramic capacitors is paramount to minimize losses at the operating frequencies and ensure stability in the circuit.

The inductor's configuration, with its specific number of turns and dimensions, plays a significant role in determining the resonant frequency of the oscillator. The center tap facilitates the connection to the antenna, which should be matched in length to optimize the radiation pattern and transmission efficiency. The use of a high-frequency transistor such as the C3355 significantly improves the circuit's performance, allowing it to operate effectively within the desired frequency range.

Finally, the circuit must be tested and tuned meticulously to achieve the best performance. This includes adjusting the variable capacitor and ensuring all connections are secure to minimize signal loss. The assembly should be housed in a suitable enclosure to protect the components and reduce interference from external sources. Following these guidelines will yield a reliable long-range FM transmitter capable of operating effectively within the FM spectrum.As shown in FIG ordinary triode 3DA87C to make long-range FM transmitter circuit, which is normal three-point oscillator circuit, the remote transmitter circuit with large current emissions, in open areas up to 1KM, according to the schematic assembly tests, the transistor magnification to choose flag with a blue dot is greater than 80 times, but found it not fall within 88-108MHZ normal frequency of the FM band, but no matter how to adjust the capacitance and inductance in the experiment, were lower than the 88MHZ about 25 percent more than a dozen MHZ frequency point, radio with TV audio reception function TESUN radio to function properly received, fT cutoff frequency parameter value of the transistor is not enough, not to mention its oscillation frequency to go up. In order to increase the distance between transmitter and receiver can change its frequency falls within the range of normal FM radio, had to look for another high-frequency transistor with D40, C1971, C1972, or as a high frequency oscillation power amplifier circuit, with its high-power output to increase the transmission distance, but such high-frequency transistor market is difficult to buy, and even buy, most of these fakes can not be used.

Later found with the C3355, this transistor cutoff frequency of several thousand MHZ, its power of 600MW, is sufficient for the FM band, then the circuit will make some improvements, can easily produce a long-range FM transmitter circuit. Select components: capacitors C2, C3, C4 are high frequency ceramic capacitors, Ct is 5 / 25P high-frequency semi-adjustable capacitor to be replaced with high-frequency ceramic value after completion of debugging test with a digital multimeter capacitance, L is a diameter of about 6 turns bodiless 8mm 0.9 enameled wire on the pipe, then opened about 2CM, center tap, using a TV antenna or a transmitting antenna in place with the same length of wire.

Actual debugging to the best of its transmission power maximum distance of not less than 500 meters.