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In this chapter, precision power measurement, which is probably the most important area in RF and microwave metrology, will be discussed. Firstly, the background of RF and microwave power measurements and standards will be introduced. Secondly, the working principle of primary power standard (i.e., microcalorimeter) will be described, followed by the discussions of direct comparison transfer technique. Finally, there will be some discussions about the performance evaluation and uncertainty estimation for microwave power measurements.

Most microwave power standards adopt calorimetry techniques to calibrate the effective efficiency of a thermistor mount as transfer standard. Correction factor of the calorimeter is the key coefficent. To correct the influence of the power loss of the transmission line to the thermopile in the calorimeter, a thin copper flat foil will usually be put before the mount's input port to terminate the signal, as so to separate the transmission line (including the thermal isolation section and the interface flange) and foil dissipate RF power from the DUT. The flat foil short's reflection coefficient measurement is the main uncertainty contributer of the total calorimter measurement uncertainty. In this paper, we introduce a new evaluation technique which will reasonably reduce the correction factor uncertainty. The technique is based on electromagnetic field theory to get the short's RF dissipation power. The detailed analysis of its princple and results are provided.Index Terms -Microcalorimeter, microwave power, millimeter-wave power, primary standards.