Antenna parameters (e.g. input impedance, radiation pattern) can be measured in Full Anechoic Chambers (FAC). The simulated input impedance is depending on the applied feed model. The most frequently used models are the current probe model, the voltage gap generator, the magnetic frill generator and the waveguide port which are presented in this paper focusing on a simple example, a monopole antenna situated above a ground plane. The Finite Element Method (FEM) has been used in the numerical field analysis.