The electromagnetic field radiating from a wire can be calculated by solving Maxwell’s equations for a short current element, and then for the whole wire by putting the current elements end-to-end.  This involves complex vector algebra.  It can be shown that the resulting fields can be grouped into three categories:

From the equations, it can be seen that these fields have the same magnitude when the distance from the current element is λ/2π.  This distance is termed the boundary between what is called the near- and far-fields.

When these, rather theoretical, current elements are placed end-to-end to simulate the real-world conditions of a wire aerial, the charges on the neighbouring current elements will cancel out the electrostatic term leaving only the induction and radiation fields.

It is generally well known that the far-field is predominantly a radiation field; this is certainly true when the distance is greater then 10 λ/2π.  At that distance the contribution from the other two field types is negligible and the ratio between E and H fields verges increasingly to what is known and the free-space impedance Zo, with its value of 377ohms.  This is very convenient as it allows one to readily find either the E or H-field magnitudes.

The conditions in the near-field are less understood, and the type of induction field depends on the type of aerial.  Most people know that aerials like verticals and dipoles fundamentally generate  E fields, and that loops by contrast fundamentally develop H fields.   When the distance from the current element is very much less than λ/2π the magnitude of the induction field is very large in relation to the radiating field and the ratio of the E and H fields is far from the free-space impedance.  For the condition where the induction field is an E-field the H-field component of the wave impedance comes from the very much smaller radiating field.  In this situation the wave impedance is very much greater than the free-space impedance.  When the induction field is magnetic then the opposite is true, the wave impedance is very much lower than the free-space impedance. Because the wave impedance in the near-field is not known, or perhaps more accurately it is not easily calculated, it follows that one cannot then readily convert ERP, measured in the far-field, to the equivalent E or H field in the near-field.  The main way of establishing field-strengths in the near-field is to use computer modelling, where the individual contributions from infinitesimally small current elements are summed [or potentially by measurement with a calibrated antenna].  The result, of course is only as good as one is able to model the wire aerial and, importantly the environment in the near-field area around the aerial.