Considerable inaccuracy can be introduced in RF current measurements through loading effects from the transformer-type RF current probes. A physical model, which consists of a lumped impedance in series with the flow of the current under measurement, is obtained. The correlation between the loading (insertion) impedance and the geometrical and physical characteristics of the probe is clearly described through formulas that are useful for design purposes. The model is compared with those existing in the technical literature, and the main differences are highlighted and discussed. The theoretical predictions are experimentally confirmed. A direct application of the concepts discussed here to the improvement of the accuracy of the V/I -type vector impedance measurements is also shown. This paper is an extended version of the work by Carobbi and Millanta, including a more detailed description of mathematical derivations and experimental results and an updated list of references.