Relating Power to Electric Field
There are two basic fields associated with every antenna; an induction field and a radiation field.This field is said to provide no part in the transmission of electromagnetic energy through free space.
Figure.10.2 Radiation field
- Consider a small linear radiator of length L, that is placed coincident with the z-axis and has its center at the origin, L=Linear radiator length; d =Distance; I0=Current; r =Radius; θ ,Ð¤ =Angle
- If a current flows through such an antenna, it launches electric and magnetic fields that can be expressed as
- In the above equations, all I /d terms represent the radiation field component, all I/ d2 terms represent the induction field component, and all I /d3 terms represent the electrostatic field component
- The electrostatic and inductive fields decay much faster with distance than the radiation field
- At regions far away from the transmitter (far-field region), the electrostatic and inductive fields become negligible and only the radiated field components of Eθ and HÐ¤ need be considered.
- In free space, the powerflux density is given by the equation
where is theRfs intrinsic impedance of free space given by Æž = 120π ohm (377ohm).