The modified Hartley oscillator, seen in figure 2, can generate signals ranging from 3 MHz to 10 MHz, adjusted on the variable capacitor of 120 pF or larger. For the indicated frequency range, the coil can have between 20 and 30 turns of 28 wire on a 1 cm diameter ferrite rod with a central socket. With fewer turns, the circuit can oscillate at frequencies of up to 50 MHz. The 10 nF capacitor must be ceramic, and the supply voltage can be between 6 and 9 V. Transistors equivalent to the BC548 can also be used, and for an output of higher power with 12 V power supply, the BD135 can be used.
