The circuit described is a 1.5km single-tube FM transmitter. It uses transistors such as D40, D50, or 2N3866, with a working current of around 60-80mA. However, these transistors can be hard to find, expensive, and often counterfeit. After experimenting with alternatives, the author found that more readily available transistors like C2053 and C1970 perform very well, achieving line-of-sight communication distances exceeding 1.5km.
When replacing the D40 with a common transistor like 8050, the working current remains within 60-80mA, but the transmission range drops below 1.5km. Using 9018 instead results in even shorter range due to lower current. Besides the transmitting transistor, the values of coil L1 and capacitor C3 are crucial for stable operation. If not chosen properly, the circuit may fail to oscillate or operate outside the 88–108 MHz FM band.
L1 and L2 can be wound using 0.35mm enameled wire on a 3.5mm diameter rod, with 5 turns for L1 and 10 turns for L2. C3 should be a variable ceramic or polyester capacitor ranging from 5–20pF. In practical applications, C5 can be omitted, and L2 can be replaced with a standard inductor of 10–100mH. For short-range transmission (tens of meters), a battery voltage between 1.5V and 3V is sufficient, and D40 can be replaced with an inexpensive 9018, reducing power consumption.
This single-tube transmitter has a simple design and high output power, making it easy to build. However, it's not ideal for connecting high-frequency cables to an outdoor antenna. Instead, a 0.7–0.9m rod antenna is typically connected directly to C5. Due to the Doppler effect, movement near the antenna can cause significant frequency drift, leading to distorted or lost audio on the receiver. This issue becomes even worse when used as a wireless microphone, where the antenna might be held or moved frequently.
The circuit uses a three-point capacitor oscillator, which is sensitive to changes in antenna parameters. Additionally, since it’s a single-transistor self-oscillating design, the operating current is relatively high. After a few seconds or minutes of use, the transistor heats up, causing changes in interelectrode capacitance and thus affecting the oscillation frequency. This can result in frequency shifts of up to 0.2–1 MHz, significantly reducing reliability.
Despite these limitations, the circuit is low-cost, easy to assemble, and great for beginners to experiment with. It's suitable for educational purposes or hobbyists looking to understand basic FM transmission principles.
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