The main purpose of the System Fidelity Factor (SFF) is to incorporate frequency and time domain characteristics of an antenna system into a comparison method for ultrawideband (UWB) antennas. The SFF is an interesting tool because both simulations and measurements can be done in a simple and straight-forward manner. Simulations of a single antenna are combined into a two-antennas system analysis by means of a simple post-processing, where the transfer function of the transmitting and receiving antennas are calculated. Measurements of the SFF are done using a two port Vector Network Analyzer (VNA). The polar representation of the SFF allows an equitable comparison between antennas. The procedure to derive the SFF is described in detail in the paper. Two examples are given where the UWB performance of three antenna systems are compared. In the first example antenna systems of two identical monopoles are studied. In the second example the transmitting antenna is a Vivaldi and the receiving antenna a monopole.