Measurement of power by absorption of the angular momentum of a circularly polarized wave

The absorption of angular electromagnetic momentum produces a torque on the dissipative element directly proportional to the power and inversely proportional to the velocity of rotation of the field. This technique is well known at low frequencies in its application to the induction motor. Thus at 50/cs the starting torque on the rotor of such a machine, operated from a polyphase supply, is quite considerable even when the power input is small. In fact, at all frequencies up to the v.h.f. part of the spectrum, sufficient torque can be obtained in this way, at medium power levels, to give a direct reading of the power absorbed. At 200 Mc/s. for example, 50 mW will give an observable torque. With microwaves the conditions are very different, and the forces tending to rotate the absorbing termination are several orders of magnitude smaller, so that some means must be found to amplify the effect. In this paper it is proposed to develop a wattmeter for 35 Gc/s using a local oscillator and a mixer to give an output at the difference frequency, thus setting up a rotating field to operate on the absorbing termination, at a correspondingly reduced angular velocity. Some calculations are given to show what can be achieved by this method and also by direct-reading instruments in the v.h.f. band.