where E z is the vertical component of the electric field at the receiver in a distance ρ from the transmitter, E o is the electric field of a Hertzian dipole in free space, and ω ω = 2 π π f the angular frequency. In free space, it is T = 1 . Evidently, the Earth–ionosphere waveguide is dispersive because the transfer function depends on frequency. This means that phase- and group velocity of the waves are frequency dependent.