As part of a collaboration with Rai WAY, the electromagnetic behavior of an open GTEM cell at different operating frequencies (from 250 MHz to 1 GHz) has been analyzed, implementing an accurate finite-element model of the device. The analysis, conducted both from a simulation and an experimental point of view, has allowed to characterize this electromagnetic structure as to the field uniformity, a quality necessary to carry out field probes calibration or electromagnetic compliance tests.