Why TEM wave is not supported by hollow waveguide?

Arif Ullah khan utman kheel

this is because for conductor E parallel is zero this means that the surface of the wave guide is at equipotential and this potential follow the laplace equation .it means that there is no maxima and minima inside the wave guide . this means that the electric field inside zero . hence the TEM do not exist in wave guide only TE and TM can be exist . if we place some conductor in the wave guide then the conductor inside will not be equipotential and the TEM waves can be exist . like in Coaxial cables