During radio transmission, the radio signals emitted by an antenna are attenuated by the atmosphere. This attenuation is called free space path loss (FSPL) and is decisive for the bridgeable distance between the transmitter and receiver. Free space path loss is a radio characteristic value that expresses the energy loss experienced by electromagnetic waves in the transmission medium.
The free space loss is the difference between the transmitted power and the received field strength. It depends on the distance between two points and on the wavelength, and is calculated from the logarithm of the ratio of distance to wavelength. This means that as the frequency increases, i.e. at shorter wavelengths, the free space attenuation increases logarithmically.
For example, the level difference between frequencies of 900 MHz and 2.4 GHz is 8.6 dB in favor of the sub-GHz frequency. Therefore, long range, low data rate applications, such as low power WANs( LPWAN), operate in sub-GHz frequencies.
Free-space attenuation is specified dimensionlessly in decibels (dB) and increases by 20 dB with each decade of distance. The free-space attenuation determines, among other things, the range in radio and optical directional radio and in the 60 GHz band. It can only be compensated by a higher antenna gain. For this purpose, antenna arrays are used for millimeter waves.