2km FM transmitter

This FM transmitter is designed for high power. The final high-frequency amplifier utilizes three 3DG12 high-frequency medium-power transistors connected in parallel, enabling a transmission range of over 2 kilometers. Using a high-gain directional transmitting antenna and connecting the receiver to an external high-gain receiving antenna increases the transmission range to over 5 kilometers. The transmitter's transmission frequency falls within the 88-108MHz FM broadcast band, making it convenient for FM radio reception. The complete circuit diagram is shown below:

This FM transmitter has three stages: high-frequency oscillation, frequency multiplication, and power amplification. In the circuit, V1, C2-C6, R2, R3, and L1 form a capacitor-three-point oscillator. Its oscillation frequency is primarily determined by the parameters of C3, C4, and L1, resulting in a frequency range of 44-54 MHz. This signal is output from L1's center tap and coupled to V2 via C7 for amplification. C8 and L2 then select a doubled frequency signal of 44-54 MHz, or 88-108 MHz. This signal is then coupled to V3 via C9 for power amplification. V3 consists of three 3DG12 transistors connected in parallel to increase output power. During normal operation, the circuit draws approximately 80-100 mA. Appropriate heat sinks can be added to the three 3DG12 transistors that comprise V3 to prevent overheating. L1-L3 are constructed using 0.31 mm enameled wire wound in a single layer flat around a 3.5 mm diameter round rod.

The frequency stability of the FM transmitter is not very good, and there will be a certain frequency drift. After a few minutes of power on, there will be a frequency drift of 0.2-0.4MHz. This is mainly due to the large working current of V3 and the temperature rise, which causes the inter-electrode capacitance to change. This change causes the parameters of the resonant network composed of C8 and L2 to change through C9. In addition, the temperature rise of V2 also causes the parameters of the resonant network composed of C8 and L2 to change. This change is transmitted to the main oscillator composed of C3, C4, L1, C5, C6, V1, etc. through C7, and ultimately changes the oscillation frequency (generally, the oscillation frequency is reduced). During the experiment, the heat dissipation of the transistor can be strengthened, the inter-stage coupling can be reduced, the capacity of C9 and C7 can be reduced, and transistors, resistors, capacitors, etc. that are less affected by temperature can be selected. However, the frequency drift is difficult to eliminate.