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In a wireless communication, the Free Space Path Loss (FSPL) (also known as Free Space Loss, FSL) is the attenuation of radio energy between the Tx and Rx antennas.
The path of propagation of EM wave must be obstacle-free with line-of-sight (LoS) through free space (usually air)
The free space path loss is used to predict the strength of a RF signal at a particular distance. This is a theoretical value, as in the real world, there are many obstacles, reflections and losses which need to be accounted for when estimating the signal at a location. However, the FSPL is a good approximation for estimating the loss of signal when propagating through free space.
Free Space Loss increases with the 1) square of distance between the antennas because the radio waves spread out by the inverse square law and 2) decreases with the square of the wavelength of the radio waves. The FSPL is rarely used standalone, but rather as a part of the Friis transmission formula, which includes the gain of antennas. It is a factor that must be included in the power link budget of a radio communication system, to ensure that sufficient radio power reaches the receiver such that the transmitted signal is received intelligibly.
The Free Space Path Loss formula derives from the Friis transmission formula. This states that in a radio system consisting of a transmitting antenna transmitting radio waves to a receiving antenna, the ratio of radio wave power received Pr to the power transmitted Pt is:
Beside the assumption that the antennas are lossless, this formula assumes that the
Polarization (Vertical or Horizontal) of the antennas is the same
No multipath effects
Radio wave path is sufficiently far away from obstructions that it acts as if it is in free space.
An ellipsoidal area around the line of sight out to 0.6 of the Fresnel zone be clear of obstructions.
The Fresnel zone increases in diameter with the wavelength of the radio waves. Often the concept of free space path loss is applied to radio systems that don't completely meet these requirements, but these imperfections can be accounted for by small constant power loss factors that can be included in the link budget.
A radio communication system has operating at 400 MHz and has a transmit power of 25W and has an effective range of 2000 m. The transmitting and receiving antenna gains are 25dB and 3 dB respectively.
State the condition that must be valid to apply the free space path loss model.
Transmitter and receiver must have a clear, unobstructed line of sight path between them.
Assuming no other losses, calculate the free space loss
Free space path loss LP (dB) = 32.44 + 20 log d (km) + 20 log f (MHz)
= 32.44 + 20 log 2 + 20 log 400
= 90.5 dB
22 ноя 2023
