In this page it is explained the theoretical method to calculate the point-to-point transmission distance between two radio devices in optical sight (LOS) without obstacles. In a radio system of the type shown in the figure:
We can hire:
PRx=PTx-CL+GTx-FSLP+GRx-CL
In which :
PRx [dBm] = Potenza ricevuta PTx [dBm] = Potenza trasmessa CL [dB] = Perdita del cavo (se esiste) GTx [dBi] = Guadagno dell'antenna trasmittente GRx [dBi] = Guadagno dell'antenna ricevente FSPL [dB] = Free Space Path Loss, ovvero attenuazione del segnale in spazio aperto (vedi tabella)
It is also remembered that the equation for the calculation of the FSPL is the following:
FSPL = 32.4 + 20*Log F + 20*Log R
In which:
F [MHz] = Frequenza Operativa R [km] = Distanza tra le antenne
In the table below are shown some typical values of attenuation FSPL as a function of distances and frequencies.
| Distance [km] | FSPL [dB] | ||||
| @169MHz | @434MHz | @868MHz | @2.4GHz | @5.4GHz | |
| 0.5 | 70.94 | 79.05 | 85.15 | 93.98 | 101.03 |
| 1 | 76.96 | 85.07 | 91.17 | 100 | 107.05 |
| 2 | 82.98 | 91.09 | 97.19 | 106.02 | 113.07 |
| 4 | 89 | 97.11 | 103.21 | 112.05 | 119.09 |
| 8 | 95.02 | 103.13 | 109.23 | 118.07 | 125.11 |
| 10 | 96.96 | 105.07 | 111.17 | 120.00 | 127.05 |
| 12 | 98.54 | 106.65 | 112.75 | 121.59 | 128.63 |
| 16 | 101.04 | 109.15 | 115.25 | 124.09 | 131.13 |
| 20 | 102.98 | 111.09 | 117.19 | 126.02 | 133.07 |
The same equation is represented in graphical form in the figure below:
From the table and the graph, we can see that for every increase of 6dB FSPL corresponds to a doubling of distance. It is also important in the calculation to maintain a safety margin of at least 5-6dB.
Taking for example a radio link made with Maxstream XStream X24-009 radiomodem equipped with 2.1dBi dipole antennas, we could hypothesize a LOS radio range of:
FSPL = PTx+GTx-PRx+GRx = 17+2.1+105+2.1 = 126.2dB
to this value must be removed 6dB of safety margin:
126.2-6 = 120.2dBm Correspondents in the table and in chart a 10km.