|Crystal Radio with Air Core Coil - Schematic|
The 35 turn primary series-tunes the capacitive 60' random wire antenna to resonance at 612kHz (frequency of the lone 200kW local AM broadcast station). The secondary is parallel-tuned using a 500+500 pF PVC gang condenser.
|Crystal Radio with Air Core Coil|
Reception, using sensitive balanced-armature headphones, is quite good. Headphone current, measured using a 1mA FSD 60 Ω meter, is 350 μA.