Let us derive the relation of electric field intensity at finite distance from transmitter Antenna

If a horizontal Hertzian dipole antenna is used as a transmitter antenna above the horizon, then energy will travel like a wave in free space. Therefore, the amplitude of electric field vector in the radiation field can be given as :

E_q  =60πI_mdl/rλ   (for θ = 90⁰)                            (1)

where r = far field distance

I_m = maximum current in the antenna

dl = length of the dipole

l  = operating wavelength

Also, the power radiated by the dipole is given by

P_t = 80[πI_rmsdl/λ]²                                          …(2)

Taking I_m= Imax/√2, equation (2) becomes

P_t = 80[πI_rmsdl/√2λ]²

P_t = 80[E_θr/60√2]²             (By putting equation 1)

or                          E_q = 60√2(P_t/80)^1/2/r  V/m

For example, take P_t = 1 kW and distance from the T_X r is 2 km (i.e. receiver). Then,

E_q = = 60√2(1000/80)^1/2/2000  V/m

= 15 m V/m

The above is the derivation and relation of electric field intensity at finite distance from transmitter Antenna