The power density ${S}_{i}$ radiated from an isotropic radiator is homogeneously distributed over a spherical surface (with the radiator in the center of the spherical surface). With the power ${P}_{t0}$ fed to the antenna, this leads to the following power density ${S}_{i}$ in a distance $d$ :
(1) ${S}_{i}=\frac{{P}_{\text{\hspace{0.17em}}t0}}{4\text{\hspace{0.17em}}\pi \text{\hspace{0.17em}}{d}^{2}}$