This paper discusses a field measurement method, based on a two-antenna setup, to qualify the transmission of transient signals inside a GTEM cell. The transmission characteristic of the GTEM1250 is evaluated by the Pearson correlation coefficient (PCC) and is presented with a heatmap. Due to deviations of the uncertainty contribution of the field homogeneity, the frequency band around 100 MHz is evaluated and its effect to the PCC is discussed. Therefore, a comparable narrowband transient signal, a damped sinusoidal (DS) is used. Furthermore, a detailed discussion focussing on nonlinear and distorting effects of the GTEM1250 is performed. The measurements in time domain (TD) identify comparable high secondary E-field components in the propagation direction, which are characteristic for higher order modes. Based on the same setup, another measurement is performed in frequency domain (FD) and relates the phase response of the GTEM cell to the above mentioned effects. According to the measured phase response the propagation time is discussed to investigate the distorting effects caused by higher order modes.