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_{m}dl/rλ (for θ = 90^{0}) (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

_{rms}dl/λ]

^{2}…(2)

Taking I_{m}= Imax/√2, equation (2) becomes

P* _{t}* = 80[πI

_{rms}dl/√2λ]

^{2}

P* _{t}* = 80[E

_{θ}r/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