The following schematic shows the Antenna Tuning Unit Wiring Diagram and Circuit. The coil is made from 22 swg enamel covered copper wire. The coil is tapped at 3,4,6,8,11,15,20,30,40,50 and 60 turns. Coil L2 is connected in series with the 60 turn tapping point. This is wound on a 3 inch length of 3/8 diameter ferrite and has 60 turns of 32 swg.
The circuit is a standard Pi filter, consisting of two variable capacitors C1 and C2 and a multi tapped coil L1. Switch S1 is used to select the tapping points and C1 and C2 peak up on the received signal. In this circuit an additional MW coil, L2 is used. This has a value around 200uH and can be salvaged from an old AM radio.