A simple FM radio with one transistor can be built as shown in Figure 1. The transistor T1 is working as a superregenerative stage. The gain of the positive feedback is adjusted by the trimmer C2. Two short wires twisted together may be used as this component, or use any trimmer capacitor with capacitances in the range of 4..15 pF. Potentiometer R1 is used to adjust the bias current of the transistor. Adjust it to get the maximum amplification of the signal.
Fig. 1. One transistor FM radio circuit diagram
T1 - 2N1023; R1 - a potentiometer of 2.2 Megohm;
C1 - 5..35 pF; C2 - 4..15 pF (match it); C3 - 0.1 μF; C4 - 10 μF;
L1 - 8.5 turns, L2 - 2.5 turns of 0.8 mm wire (AWG 20) wound on a 6 mm diameter former;
L3, L4, L5 - 7..9 turns of 0.5 mm (AWG 24) wire wound on a 8 mm diameter former;
BA1 - High impedance headphones ≥ 1600 Ohm;
Power supply - a 9 volt battery
The coil L1 has 8.5 turns of 0.8 mm wire (AWG 20) wound on a 6 mm diameter former, the coil L2 has 2.5 turns of the same wire, it's wound on the same former, 3..4 mm aside of the coil L1. Coils L3, L4, L5 are wound on a 8 mm diameter former, they have 7..9 turns of 0.5 mm (AWG 24) wire.
In this circuit is used high impedance headphones (≥ 1600 Ohm).
A thick copper wire of length 45 cm is used as the antenna.
"Popular Electronics", 1968, 03.