The use of a loop antenna of large size (12 18`) provides a large signal pickup at AM broadcast frequencies. It has
about 18 turns. Tl is a toroidal transformer of about 3:1 turns ratio (not critical). The primary winding should have about 1.5 mH inductance. By using the circuit at (B), a varactor diode can be used in place of the 10- to 365-pF variable capacitor.