Power measurement at 10 Mc/s by absorption of the angular momentum of a rotating field

A rotating magnetic field is set up by a 2-phase 4-pole winding analogous to the corresponding low-frequency arrangement. The angular momentum of the field is absorbed by a rotor element at standstill. Theory shows that the torque produced is given by the power absorbed divided by the angular velocity of the field. An absorption-type wattmeter based on this principle has been developed to operate at 10Mc/s. The torque on the rotor element is measured by a quartz-fibre suspension with an optical lever. Power/torque-conversion efficiencies of about 70% have been achieved, resulting in sensitivities of the order of 1 mW per centimetre deflection.