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The radiation resistances of a monopole antenna and a dielectric loaded monopole antenna have precisely been measured on the basis of the Wheeler cap (1947) method. The precision of the measurement has been proved to improve with the use of the stub tuner. To give a theoretical background for the improvement in the measured radiation resistance, the drift error of the network analyzer has mathematically been characterized using the error vector model. Using the model, we have theoretically estimated the enhancement of measurement precision when the antenna is matched to the transmission line. The use of such impedance converters as the stub tuner can generally be applied to measure the radiation resistance of small antennas having a small resistance and a large reactance, where the drift error of the instrument gives a serious degradation to the measurement precision.