Specifications

98 MDS 4790/9790 Series I/O Guide MDS 05-3438A01, Rev. E
Calculating Path Loss
Assuming that we have satisfied the line-of-sight and first Fresnel zone
clearance requirements, we can calculate the path loss. At 450 MHz, the
loss between two isotropic radiators (0 dBi antennas) that are 1 km apart
is 86 dB. For every doubling of distance, the loss increases by an addi-
tional 6 dB. Knowing this, the output power (+37 dBm), and the receiver
sensitivity (–115 dBm), we can calculate antenna size and tower height
requirements to cover any desired distance.
10.3 Formulas for System Planning
The following standard formulas are provided for assistance in deter-
mining system installation parameters.
Free Space Path Loss
where:
α
fs
= free space loss in dB
d = path distance in kilometers
ƒ = frequency in GHz
Fresnel Zone Boundary
where:
F
n
= Fresnel zone boundary in meters
d
1
= distance from one end of the path to the Fresnel zone boundary (in
kilometers)
d
2
= distance from the other end of the path to the Fresnel zone
boundary (in kilometers)
D = total path distance (d
1
+d
2
) in kilometers
ƒ = frequency in GHz
n = Fresnel zone, 1 (for 1st) is used here
Theoretical Signal Strength
where:
RSSI = signal strength at the receiver in dBm
EIRP = RF power output in dBm + G
ta
–L
tfl
α
fs
= free-space path loss in dB
G
ra
= receive antenna gain in dBi
L
rfl
= receive feedline loss in dB
L
tfl
= transmit feedline loss in dB
G
ta
= transmit antenna gain in dBi
α
fs
92.4 20
10
flog 20
10
dlog++=
F
n
nd
1
d
2
⋅
fD
--------------------
17.3
=
RSSI EIRP α–
fs
G
ra
L
rfl
–+=