Power and energy: National standards

The development of RF and microwave power standards over the last 20 years is reviewed in the light of the evolution of the commercial power meters which they are used to calibrate. Although many different types of power standard have been investigated, the microcalorimeter, the dry-load calorimeter, and in some cases the flow calorimeter continue to serve as the main primary instruments. These have been refined and extended in frequency coverage, resulting in uncertainties in effective efficiency in the range 0.1-0.2 percent up to 40 GHz and 0.5 percent or better above this frequency. Techniques for comparing power meters are also discussed. They include the use of tuned reflectometers, network analyzers, reflectometers of the six-port and multistate kind, and resistive power splitters.