Harmonic radar transponders are passive RF tags that produce a harmonic response when illuminated with an RF signal. With a Schottky diode as the main component, the tag current is usually modelled as an exponential function of antenna voltage, resulting in an approximate polynomial function for small signals. The authors introduce the Lambert function that accounts for the resistive load and leads to a two-region tag model: a power series representation for small signals and an asymptotic linear large-signal model. Furthermore, they provide the relationship between the modulated bandpass signals at the input and output of the tag in the two regions by expressing the tag signals with complex envelopes. The derived expressions offer deeper insight of the transponder's impact on the operation of harmonic radar systems.