Single-layer air coil inductance estimate

Wheeler's short-solenoid approximation rewrites radius r, coil length l and turns N into the inductance estimate shown above. Converting your millimetre inputs into inches keeps the math consistent with the original derivation.

For example, specifying a 50 mm diameter, 80 mm long coil with 10 turns yields roughly 8 µH—ideal for a 3.58 MHz crystal filter experiment. Stretching the coil increases l, lowering inductance and raising self-resonance, while tightly spaced turns increase capacitance and limit the usable frequency.