TEM wave is non-dispersion wave, and it can
propagate in GIS at any frequency. When frequency
f>100MHz, rapid attenuation occurs along the propagation
direction; however, TM wave differs from TE wave in that
they have their respective cut-off frequency c f which is
related with the size of GIS. It means that the larger the crosssectional
area of GIS, the lower c f is. If signal
frequency f < c f , the attenuation of signals is so rapid that
they cannot propagate any more; when f > c f , signals can
propagate without loss basically.