Power measurement at 10 Gc/s by reversal of the angular momentum of a circularly polarised field

Theory indicates that the rate of change of angular momentum carried by the lowest-order circularly polarised field in a circular waveguide is given by P/¿, where P is the power in the field and ¿ is its angular frequency. Application of this relation to the absolute measurement of microwave power, by employing an anisotropic element suspended in the waveguide to change the degree of polarisation of an elliptically polarised H11 mode, is described. In particular, measurement of Xband power by using a dielectric halfwave plate is outlined, and it is shown that a simple light-spot torsion-head arrangement can give accuracies within ±2.5% over a 20% bandwidth. The sensitivity with a 7¿m quartz-fibre suspension is 1cm deflection per 90mW of power.