This circuit is from a publication by Newton C. Braga in the United States. The transistor can be 2N2222 or equivalent like BF494. The antenna must be 10 to 25 cm and the L1 coil is formed by 4 turns of wire 24 in the form of 1 cm in diameter without core. C6 is a common trimmer with 20 to 40 pF of maximum capacitance. The microphone is a two-terminal electret. The dot indicates the positive side of the feed. The range can reach 200 meters in the open.
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