A Simple Circuit of Long Distance Wireless FM Microphone

Many enthusiasts are eager to find a wireless microphone with long transmission distance, high pickup sensitivity, long working time without frequency running, simple debugging and production, and low cost. The single tube long-distance wireless FM microphone introduced in this paper has the above characteristics.
Since the transmitting ring antenna L1 is also used as the oscillating coil, the flow in the antenna is high-frequency current synchronized with the oscillating frequency, so it is always in the transmitting state. Through practice, the transmitting distance is about 100~150m in the empty ore ground (TOLY1781 pocket radio is used as the receiver, and the receiving distance can be reached when the antenna of this machine is extended to 0.8m). In contrast, when the working voltage, working current and transmission frequency are the same, L1 is replaced by a common spiral coil, and the collector of the oscillating triode is connected to a rod antenna with a 5pF capacitance of 0.8m for transmission experiments. The transmission distance of the two transmission modes is almost the same, which proves that the transmission efficiency of the embedded ring antenna when used as the oscillating coil is quite high.
The embedded ring antenna is made of metal ring or square shaped enameled wire with a length of 160mm and a diameter of 1mm, and embedded in the housing. Adjust the capacitor C3 so that the transmission frequency falls between 88 and 108MHz for FM radio reception. When the battery voltage changes between 1.2~2V, the transmission frequency of the circuit is stable and unchanged after a long time of operation. When the battery voltage is 1.5 V, the working current of the whole machine is about 2.5 mA. During commissioning, do not put your hands near the ring antenna, and do not place them near metal objects, so as not to affect the oscillation frequency and transmission distance.
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