Power flow and negative wave impedance in the dielectric-rod waveguide

In most waveguides of practical interest the wave impedance in the direction of propagation of the wave is positive and its magnitude does not vary with position. It is shown that, in general, a guiding structure supporting a hybrid mode, i.e. a mode having axial components of both the electric and magnetic fields, can present a negative wave impedance. The case of the EH11 mode on a dielectric rod is then considered, and it is shown that for this mode the wave impedance is negative over part of the transverse plane and hence leads to regions of negative power flow if the usual interpretation of the Poynting vector is employed. The total power flow, however, is still positive, since the negative power density and the negative wave impedance occur only in a restricted part of the cross-section. The orthogonality of the fields is also discussed, and it is shown that for hybrid modes the transverse electric and magnetic fields are not at right angles to each other.