# Electric field intensity at finite distance from transmitter Antenna

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 :

Eq  =60πImdl/rλ   (for θ = 900)                            (1)

where r = far field distance

Im = maximum current in the antenna

dl = length of the dipole

l  = operating wavelength

Also, the power radiated by the dipole is given by

Pt = 80[πIrmsdl/λ]2                                          …(2)

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

Pt = 80[πIrmsdl/√2λ]2

Pt = 80[Eθr/60√2]2             (By putting equation 1)

or                          Eq = 60√2(Pt/80)1/2/r  V/m

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

Eq = = 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