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Balun transformers are used in front of network analyzers to measure the balanced- and common-mode characteristics of transmission lines and electronic networks. The nonideal nature of real baluns will cause measurements made with this technique to be in error. This paper shows that by following a calibration procedure similar to the conventional network analyzer calibration of Kruppa and Sodomsky, where known standards are measured, and these measurements are used to correct measurements of unknown devices, one can compensate for the nonideal nature of the measurement baluns. In this procedure, a conventional two-port network analyzer is connected to the balun transformer and, without disconnecting the network analyzer, inaccuracies in the balun can be calibrated out. Balanced mode, common mode, and mode conversion parameters can be accurately measured. This paper applies the previously published method of Silvonen for conventional network analyzer calibration to correct the measurement errors when using balun transformers.