Radio Wave Propagation
Path-loss

A crucial factor in the success or failure of a communications system is how much power from the transmitter actually reaches the receiver. Many different ways that EM waves can be affected have been discussed, including reflection, diffraction, and scattering. These different effects can be combined and described by what are known as path loss calculations. Path loss calculations determine how much power is lost along the communications path.

Free-space loss (FSL) is the signal attenuation that would result if all absorbing, diffracting, obstructing, refracting, scattering, and reflecting influences were sufficiently removed so as to have no effect on propagation. The formula is as follows:

FSL (in dB) = 20 log10(f) + 20 log10(d) + 36.6

Every time the distance from the transmitter to the receiver is doubled, the signal level is lowered (or increased) by 6 dB. Also, for each frequency, there is a series of wavelengths, where energy will escape out of the transmission line and enter the surrounding space. This is called the launch effect. The launch effect typically occurs at multiples of half-wavelengths of the signal. This is illustrated in Figure .

Use the path loss calculator in the interactive activity to gain familiarity with this calculation. Enter any two of the values and the third one will be calculated. Then, use the simulator in the second interactive activity to see how this can affect WLAN communications.


Interactive Media Activity

Interactive Activity: The Free-Space Loss (FSL) Equation

This activity calculates free space loss (FSL) in dB, frequency in MHz, and transmitter-receiver separation distance in km.

Interactive Media Activity

Interactive Activity: Free Space Loss Simulation

This applet demonstrates the concept of FSL (free space loss) between wireless devices.