The circuit is a 1.5km single-tube FM transmitter design. It uses transistors like D40, D50, or 2N3866, with a working current of around 60-80mA. However, these transistors are hard to find, expensive, and often counterfeit. After experimenting with alternatives, the author found that the C2053 and C1970 transistors work well and are easier to obtain. These transistors can achieve line-of-sight communication distances exceeding 1.5km. When replacing D40 with a common transistor like 8050, the working current remains within the same range, but the transmission distance drops below 1.5km. Using 9018 instead results in even less range due to lower current.
In addition to the transmitter transistor, the values of coil L1 and capacitor C3 are critical 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 made using 0.35mm enameled wire wound tightly on a 3.5mm diameter rod—5 turns for L1 and 10 turns for L2. C3 should be a variable ceramic or polyester capacitor with a range of 5–20pF. In practical applications, C5 can be omitted, and L2 can be replaced with a standard inductor ranging from 10mH to 100mH.
If the transmission range is only tens of meters, the battery voltage can be reduced to 1.5–3V, and the D40 transistor can be replaced with an affordable 9018, which lowers power consumption significantly. The single-tube transmitter described here has a simple design and relatively high output power, making it easy to build. However, connecting a high-frequency cable to an outdoor antenna is inconvenient. Typically, a 0.7–0.9m rod antenna is directly connected to C5 for transmission.
One major issue is the frequency drift caused by the Doppler effect when people move near the antenna, leading to distortion or loss of signal. This problem becomes even worse if the transmitter is used as a wireless microphone, especially when the antenna is touched or moved. 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 circuit, the operating current is high. Over time, the transistor heats up, causing changes in interelectrode capacitance and shifting the oscillation frequency—often lowering it by 0.2 to 1 MHz. This makes the circuit unreliable for applications like FM broadcasting or remote alarms.
Despite these drawbacks, the circuit is ideal for beginners due to its minimal component count, low cost, and ease of tuning. It serves as a great hands-on project for hobbyists looking to explore radio frequency circuits.
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