The free space path calculator provides the attenuation or loss of a signal as it propagates through free space.
To calculate the FSPL, enter the following:
- Frequency of operation f
- Transmit Antenna Gain GTx
- Receive Antenna Gain GRx
- Distance d
Formula
FSPL = 20*Log10(d) + 20*Log10(f) + Lc + 20*Log10(4π/c) – GTx – GRx
★ This formula only applies when the distance (d) between the antennas must be large enough that the antennas are in the far field of each other. In other words d >> λ where λ = c/f. It’s sufficient if the distance is at least 10 times larger than the wavelength.
Definition of Terms
Below is a list of the terms used in the calculator and what each of them mean.
Frequency of operation
This is the frequency at which the communication system operates.
Transmit Antenna Gain
This depends on the type of antenna used and is expressed in dBi (dB relative to isotropic antenna).
Receive Antenna Gain
Depends on antenna used and is expressed in dBi. In cases where the signal is from a particular direction, a high gain antenna (8 dBi for instance) can be used. This allows focusing of energy instead of receiving from all directions. It can be calculated from the Antenna Factor and frequency of operation
Example Calculation
At a frequency of 100 kHz and a distance of 100 km, the attenuation or FSPL = 52.45 dB.
At a frequency of 100 kHz and distance of 100 meters, the FSPL = -7.55 dB which is clearly an erroneous result as the attenuation cannot be negative.
To understand this, let’s take into consideration the requirement that for the equation to be valid the distance d has to be at least 10*λ = 2998 meters (using the frequency to wavelength calculator). The calculator is therefore applicable at distances greater than 29980 meters or 30 km.