The field uniformity and cross-polar performance inside a GTEM 1750 have been measured from 0.08 GHz to 6 GHz, and the results show that propagation above 3 GHz is no longer predominantly TEM mode. A computer simulation of the GTEM confirmed this. Further, the fields generated by a quad-ridged lens horn placed inside the GTEM showed that a well polarized transmitter inside the GTEM will generate significant cross-polar fields. These results have significant implications for EMC testing in the GTEM above 3 GHz.