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