11.3.9 Radio Path Loss Assessment

The foregoing information can be used to assess the RPL for a particular link. The RPC for the path is normally known in advance for a given system. The RPL is calculated from Equation (11.20) to be the sum:

R P L = \underset{P E L}{\underset{⏟}{F S L + R L}} + D L + C L + A L (d B)

(11.50)

Then, if RPL<RPC, the link is workable; if RPL≈RPC, the link is marginal, or if RPL>RPC, the link is unworkable.

Egli’s formula. Adding all the constituent losses can take some considerable time. A faster approximation can be made by a useful heuristic formula developed by Egli. For propagation frequencies between 40 MHz and 1 GHz over irregular terrain (with about 15-m variation between hills and valleys) the received power can be approximated to be:

P_{r} = P_{a m p} G_{t} L_{t} L_{a} G_{r} L_{r} \underset{P E L}{\underset{⏟}{{(\frac{h_{t} h_{r}}{d^{2}})}^{2}}} \underset{\begin{matrix} E g l i' s e s t i m a t e \\ o f d i f f r a c t i o n l o s s \end{matrix}}{\underset{⏟}{{(\frac{40}{f})}^{2}}}

(11.51)

where the range (d) and antenna heights are in meters, powers in W, and the frequency is in MHz. Note that the first part of Egli’s formula is identical to the plane-earth loss formula from Equation (11.47). The last frequency-dependent term is empirically developed and accounts for diffraction loss over the stated terrain for the given frequency range. Care must therefore be taken to apply Egli’s formula only within the stated constraints of frequency and terrain.