Vector potential is a vector

On either side of a charge-free interface between two media,

The electric field E and the magnetic field H of a short dipole antenna satisfy the condition

A rectangular waveguide with cross-section dimensions 5 cm by 3 cm is filled with a dielectric of relative permittivity 3.
(i) Determine the cutoff frequency of the$$T{E_{11}}$$ mode.
(ii) Determine the frequency at which this / mode has an attenuation of $$3\,\pi $$ nepers/m.