COILS LIóL2 óL3
THREE COILS HAVE TO BE HAND MADE FOR THE 904 AND IT IS A SIMPLE TASK TO BUILD THEM.
ALL YOU NEED IS A 3/16" DRILL BIT AND THE SPOOL OF # 22 MAGNET WIRE.
LI AND L2 ARE IDENTICAL:
USING THE SMOOTH END OF A 3/16" DRILL BIT, WIND 5 TIGHT TURNS AGAINST EACH OTHER LEAVING ABOUT 1/2" OF LEAD BENEATH EACH END OF THE COIL WHERE YOU STARTED AND WHERE YOU FINISH.
L3. L3 IS MADE IDENTICAL TO LI AND L2 WITH THE EXCEPTION IT HAS 6 TURNS INSTEAD OF 5.
LI AND L2: 5 TURNS 3/16" BIT # 22 MAGNET WIRE
L3: 6 TURNS 3/16" BIT # 22 MAGNET WIRE
This Transmitter Repeater circuit was originally engineered for Law Enforcement Agencies. They would pick up a low powered transmitter from a nearby concealed receiver and this unit would retransmit the signal to a couple of miles away on an entirely different frequency.
It can be made tunable into the FM 88 to 108 MHz Band or into the 144 to 174 MHz band by simply adding a 4 to 22 pF adjustable capacitor across L3 replacing the 12 pF. The 22 uH chokes shown were originally Mouser Electronics Part Number: 43LQ225 that had a Q of around 30 at 2.52 MHz.
T1 ( The Transformer ) was a small Radio Shack 8 Ohm to 1K Ohm. Antenna can be any small stranded wire of between 11 to 18 Inches. Samtronics Import and Electronics was one of several companies that I operated back in the days. Enjoy, John.