This contribution deals with the IEC 61000-4-20, the international standard for emission and immunity testing in transversal electromagnetic (TEM) waveguides. The applicability of the IEC standard's Annex C for transient testing with broadband signals is verified by measurements. The measurement results show that the standard can be extended to other fast rising pulses yielding the existing limits and definitions for field homogeneity in the usable testing volume. Additionally some new practical measurement techniques for testing with fast transient signals in GTEM cells are presented.