Abstract: A variable inductance RF coil assembly including an air-core, high-voltage coil, a printed circuit board installed inside of the coil, the circuit board carrying a plurality of switch contacts corresponding in location and in number to the turns of the air coil, a contact plate attached to the circuit board and having contact fingers corresponding in location and in number to the switch contacts, and a rotary, stepped cam cylinder mounted to the circuit board, the cams corresponding in location and number to cam-follower sections of the contact arms on the contact plate. Unidirectional rotation of the stepped cam cylinder causes progressive communication between the cam steps and the cam-follower sections. Such communication causes the contact arms to connect with the switch contacts on the circuit board thereby progressively and continuously shorting out each successive turn of the coil thereby to adjust the inductance thereof.