We derive and use analytic formulas to calculate some aspects of the RF behaviour of Proximity Card transponders at the contactless air interface. The aspects we consider cover the minimum operating H-field, thermal loading, card loading and maximum load modulation depending on transponder equivalent circuit parameters. We not only consider results, but also carefully discuss the approximations and simplifications we make, which should give an understanding of the principle and though allow to adapt it for a selected fabrication technology among the variety of existing options.