On the excitation of VLF waves

The ratio between the excitation factors of vertically and horizontally polarized sources is equal to the squared ratio between the characteristic impedance of free space and the ground level coupling impedance of TE and TM field components. This relation provides an analytical check of numerically calculated excitation factors. Also, the need of directly calculating the two excitation factors of an anisotropic environment may be obviated if the excitation factor of an isotropic environment is used for estimating the dominant field components of a given quasi-TM or quasi-TE mode.