Dual Coil / Dual Tuned Crystal Sets
Coils wound with #18 wire 3.5" dia with alternate turns pressed in to minimize capacitance and allow space for attaching taps.
Coils glued to dowels for support befor removing from form and more glue added inside along dowel after removing. Schematic is same as for set below except coupling capacitor is replaced by sliding coils and switches are alligator clips to terminals.
DUAL TUNED CRYSTAL RADIO - W4CWG
For XTAL phones add 100K across phone terminals.
For magnetic, omit 100K and use 470pF.
This set performs comparably with one using 3.5" dia. low capacity coils of #18 wire.
L1, L2 on T114A-61 Torroid, 44 turns #24 insulated wire, approx 275uH.
L1 taps at 1, 2, 4, 26 turns. L2 taps at 15, 21, 26 turns.
Range-Q Switch positions:
540-840KHz High Q (shown)
540-840KHz Low Q
840-1800KHz High Q
840-1800KHz Low Q
Note high Q relies on stray capacity coupling and may need
shielding or very small cap for optimum Q at upper freq.
Overall 3dB bandwidth is approx. 20-30 KHz at 1500KHz.
Adjust 10pF trimmer for max signal at lowest frequency.
Use separate Q switch & front panel trimmer cap.
Try moving 840-1800 tap down a couple turns.
Make headphone tap switch separate.