Friis Formula

Assuming you connect the power source with power of (Pt) to an transmit antenna with gain of Gt. What will the power be received (Pr) at a receive antenna with a gain of Gr?

This is a very important question when designing wireless systems. Following the concepts of the energy conservation and the gain of antenna, the Friis formula provide good estimation of the receive power. Friis formula is expressed in (1)

(1)

Where r is the distance between the transmit and receive antenna and is the wavelength of the RF signal.
It is obvious that the power is inversely proportional to the square of the distance r. And it is also noticed that the power is directly proportional the the square of the wavelength. Therefore, the receive power lower when the frequency is higher.