Evaluating uncertainties in net power delivery using dual directional couplers

Directional couplers are used extensively to monitor power flow in a transmission line. It is essential to evaluate measurement uncertainties of the power delivery systems for many RF applications. For example, in a field probe calibration process, accurate net power measurements are needed to establish the standard fields. This paper employs the complete model equation (using the full four-port S-parameters of the coupler) to evaluate the measurement uncertainties in the net power. Uncertainties can also be estimated with scalar quantities when vector quantities are not available. In such a case, one can use the scalar coupling coefficients, VSWR and/or directivities of a coupler, and assume random phases to derive an uncertainty budget. On the other hand, when both the vector S-parameters of the coupler and the complex reflection coefficients of the load are known, the net power can be determined exactly using the full model equation.