All TEmnp resonances in a TEM cell have been suppressed by cutting the cell walls longitudinally. The TMmnp resonances and propagating TEM mode were not affected. The suppressed resonances have a residual interaction with a biological body inside the cell. Because of this, the modified cell can only be used for bioeffects dosimetry studies at a small number of frequencies above the upper limit for the original cell. Where an accuracy of only +/- 5 dB is required, such as in some EMC studies, the effective bandwidth of the modified cell is twice that of the original cell.