Line of Sight Calculation

This section contains some useful formulas for data radio "link budget" calculations. Note that the calculations are based on assumptions of certain circumstances and should only be used as a guideline for radio network design. In reality, many factors can have an impact on the radio link. Planning of a radio network is usually done by first simulating the network in a desk top propagation study using a professional software package. The results are then verified by performing a field survey.

However, the formulas below can be used to calculate if the radio link has an acceptable fade margin or, if not, how much antenna gain that needs to be added or if repeaters have to be used.

The known factors are often;

• The distance between two sites
• The (possible) height of the antennas
• The transmit power of the radio
• The receiver sensitivity of the radio
• The antenna gain
  

The formulas below are to be used when there is no terrain or obstacles that can interfere with the radio signal (line of sight).

The first calculation to be made is the FSL, Free Space Loss. The FSL value tells how much the signal strength is decreased due to the distance between the transmitter and the receiver.

FSL - Free Space Loss
FSL (dB) = 20*LOG ( l /4 p *R)     

Where;

R = Distance between Rx and Tx Antenna. (Line of sight) in meters. l = C/f = 300/f    

Example

f = 460 MHz  distance = 10 000 meters (10 kilometres)  (6.2 miles) -> 
l = C/f = 300/460 = 0.65 m     
     

FSL = 20*LOG(0.65/4 p *10 000) = -105.7 dB

Conclusion (using Ex2):

If the transmission power is 33 dBm (2 W) the signal strength at the receiver antenna will be

33db - 105.7db = -72.7 db (Not taking cable losses at the transmitter in consideration).

If the receiver sensitivity is –110 dBm, this is ok. However, there are more factors for the link budget calculation. Those are calculated by using the same formula as for no line of sight links but by replacing the over air attenuation A with Free Space Loss - FSL.