Machine 48Build a crystal radio← → step through the guideboard · 4 posts
Turns–
Coil–
Tuned to–
Station–
Signal–
What this step adds; in part B, the received signalParts already in placeA crystal set needs no battery: the only power it uses is the radio signal itself. The tuner taps 20 to 120 turns of a 120-turn coil on a 7.5 cm tube; Wheeler's formula, L = r²n² / (9r + 10ℓ) in inches, gives 42 to 531 µH. With the antenna's own capacitance, about 178 pF for 30 m of wire 6 m up (C = 2πε₀ℓ / ln(2h/a)), the coil resonates at f = 1 / (2π√(LC)): 518 to 1,843 kHz, the AM broadcast band. The model assumes a loaded Q of 20 and that the crystal detects as the square of the signal; the three stations and their strengths are examples, and the mast, lead-in post and earth stake are drawn shortened. This is a guide to how the parts go together, not engineering drawings. Keep the antenna well away from power lines, and disconnect it and earth it before a thunderstorm; galena is lead sulfide, so wash your hands after handling it. A crystal set only receives: a transmitter is another matter, and illegal to run unlicensed in most countries.
