This circuit is so small and one can easily fit it in a match box. This circuit generates a small RF power on 27 MHz band. But the out put power is strong enough for hearing on any 27 MHz band receiver in a building. If you want to use the circuit on only one frequency then remove
the socket of crystal and directly solder the crystal of desired frequency on the circuit. Transistor Q1 is hep 50, use the crystals from any junked unit. Use walkie talkie type antenna and telephone receiver mic or any other carbon mic. To tune circuit to its maximum out put power adjust C3 trimmer capacitor. If starting is not consistent slowly adjust capacitor 3 until it become consistent. The power can be taken from 9V battery. L1 = 10 turns of # 16 enameled wire wound on 1/3 inch form. Spaced 1 inch end to end.