# Step 4-A: Field Strength of Isotropic Radiator (Reference Value)

The field strength of the isotropic radiator at the point P is used as a reference. The
isotropic radiator is located at *M _{geo}* and has the coordinates
(

*x*,

_{geo}*y*,

_{geo}*z*).

_{geo}To get the field strength, the following steps are required:

- Distance d between
*P*and*M*(using Pythagoras)_{geo}$$d=\sqrt{{\left({x}_{1}-{x}_{geo}\right)}^{2}+{\left({y}_{1}-{y}_{geo}\right)}^{2}+{\left({z}_{1}-{z}_{geo}\right)}^{2}}$$ - Computation of power density
*S*at point_{iso}*P*$${S}_{iso}\text{\hspace{0.17em}}=\frac{{P}_{{t}_{0}}}{4\pi {d}^{2}}\text{\hspace{0.17em}}$$with*P*describing the sum of all transmitted antenna powers._{t0} - Computation of the effective electric field strength
*E*at the pixel_{iso}*P*based on the power density*S*and the impedance of the free space_{iso}*Z*= 120 πΩ_{F0}$${\left|{\underset{\_}{E}}_{iso}\right|}^{2}\text{\hspace{0.17em}}={S}_{iso}\times {Z}_{F0}$$