where r is the distance between the observation and source points, λ<sub>0</sub> is the free space wavelength and D is the largest dimension of the radiating structure. In [[EM.Cube]], the far-zone electric fields '''E<sup>ff</sup>'''(θ, φ) are functions of the spherical observation angles only and are defined as
:<math>\begin{align} & \mathbf{E(r) = E} (r,\theta,\phi) = \frac{e^{-jk_0 r}}{r} \mathbf{E^{ff}}(\theta,\phi) \\ & \mathbf{H(r) = H} (r,\theta,\phi) = \frac{1}{\eta_0} \mathbf{ \hat{r} \times E^{ff}(r) } \end{align} \quad k_0 r >> 1 </math>
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