FRIIS FREE SPACE EQUATION
Today we will discuss a very important relation related to wave propagation. The equation is known as Friis free space equation.
As only a small fraction of radiated power is received at the receiver from an isotropic radiator in free space, but the received signals, must be 10-20 dB above the receiver noise to complete the link between transmitter (TX) and receiver (RX) antenna. The amount of received power depends on transmitted power, gains of transmitter and receiver antennas and separation between them, operating frequency and path attenuation. Thus in order to describe the characteristics of wave propagation, it is necessary to derive equation relating to these parameters. The expression relating these parameters is known as Friis free space wave equation.
Let us take an isotropic radiator transmitting power in free space, so the medium surrounding is homogeneous and non absorbing with dielectric constant unity. The power density at distance d from the centre of the radiator will be
PD = Pt/4πd2 …(1) (a)
where PD = Power density (W/m2)
Pt= Transmitted power (W)
d = Distance between transmitter and receiver (km)
4Πd2 = Spherical surface area (m2)
If a directional antenna is used at receiver, the receiving density will increase by the multiple of gain of the transmitting antenna. i.e.,
PD = Pt Gt /4πd2
where Gt = maximum directive gain of the TX antenna and
= 6(R/λ)2 in the case of microwave dish antenna
in which R is larger aperture of antenna and l is operating wavelength.
If a is attenuation of the medium the power density is modified to
PD = PtGt/4πd2a
As the transmitted power spreads over a spherical area of many kilometers, the receiving antenna picks up only a small fraction of the radiated power. The amount of power at the RX antenna will be area of the receiving antenna (A) times the power density at the TX antenna.
PD= PtGtA/4πd2a …(1) (b)
As the gain of receiving antenna is
Gr = 4πA/λ2
then A = Gr λ2/4π …(2)
The power received at the receiver will be
Pr = PDA
Put equation (1.b) and (2)
Pr = PtGtGra( λ2/4πd2) …(3)
or Pr = Pt Gt Gr a 1/Lp
Where LP = Lp = 4πd2/ λ2 = Free space path loss. This may be defined as the ratio of antenna area one wavelength square to area over which the transmitted power has been spread.
In equation (3) all the parameters can be determined easily except attenuation a. The a depends upon atmospheric conditions that vary with time and local weather.
Equation 3 is known as fundamental equation of free space propagation or Friis free space wave equation.