13
2.1.5 The link budget
For each link, a ‘link budget’ needs to be made. The link budget will calculate the
signal level through the link, and predicts the signal level at the receiver’s side.
Within the ‘link budget’ there are a few parameters that are influenced by the
location of the link. These are humidity and terrain roughness. The length of the
link also has a major influence on the ‘link budget’.
An example:
We want to bridge a point-to-point distance of 2.5 Km over average terrain in a
dry climate. The bridge must perform at 11Mbps.
To be able to do this, we first select the components we need. This is the long-
range bridge and a high gain helix array antenna.
The output power of the long-range bridge is (Pout): 10 dBm
Antenna gain is (Gt):
16 dBic
Cable loss and connector loss is (Cr):
4.5 dB
Therefore, the EIRP (effective isotropic radiated power) is:
EIRP= Pout + Gt - Cr
EIRP= 10 + 16 – 4.5
EIRP= 21.5 dBm
The free space loss is the attenuation of the signal power travelling through air.
FSL= 20*log
10
(F) + 20*log
10
(d) + 92.45
Where:
FSL= Free space loss
F= Frequency (Ghz)
d = Distance between antenna (Km)
Our example gives
FSL= 20*log
10
(2.4) + 20*log
10
(2.5) + 92.45
FSL= 108.01 dB
Then the antenna on the other side of the bridge will receive the signal. This
antenna has the same gain (16 dBic) as the transmitting antenna. The signal
received by the antenna will suffer the same attenuation on the cable, (4.5dB).
So the signal strength at the receiver can be calculated as follow:
Pin= EIRP – FSL + Gr - Cr
This results in:
