TABLE OF CONTENTS In this section, the polarisation convention used within this text is described. As shown above, in the near zone the electric or magnetic field is completely characterised by specifying three vector components, whilst the far-field is defined unambiguously by specifying two transverse vector components since the vector component in the direction of propagation is identically zero. Although in what follows, the definitions are presented in terms of the electric field, similar definitions can be constructed to specify the magnetic field.
It is customary to define cross-polarisation as 'the polarisation orthogonal to a reference polarisation'. Unfortunately, this leaves the direction of the reference polarisation undefined and thus is ambiguous for all but circularly polarised waves. The definitions presented below can be used to resolve this ambiguity. These transformations are only valid in the true far-field or in the special case where the field has been sampled using an infinitesimal Hertzian dipole.
The electric field is resolved onto three unit vectors, one aligned to each of the three Cartesian axes. When the electric field is decomposed onto these three unit vectors the total field can be expressed mathematically as follows.
| (5.145) |  |
This definition is valid in the near and far zones. Figure 5.21 (see p. C2) shows the copolar (magenta) and cross-polar (red) directions superimposed on a spherical (blue) grid. This clearly shows that this definition is effectively defining the polarisation on a flat screen normal to the boresight...
