User's Manual

88 MDS Master Station MDS 05-6399A01, Rev. D
Earth Curvature
As the distance of a communication link increases, the clearance problem is compounded by the earth’s
curvature. Radio waves traveling through typical atmospheric conditions bend slightly, which is repre-
sented by treating the earth as though it were slightly flatter than it actually is. Experience has shown that
if we consider the earth’s radius 4/3rds of its actual size, we get good agreement between theory and
measured propagation results.
The figure below shows a representation of the 4/3 earth “radio horizon.” This figure shows that under
normal radio propagation conditions, a station with its antenna 15 meters above flat terrain will have a
radio horizon approximately 15 kilometers away, well beyond the visual horizon.
Figure 8-2. Antenna Height vs. Theoretical Radio Horizon
Fade Margins
Variations in the temperature and humidity of the atmosphere with elevation cause the signals to bend
more or less, resulting in fading at the receiver. The longer the path, the more likely that deep fades will
occur, hence the greater the fade margin required.
Different parts of the world have differing propagation conditions, which can be categorized as favorable,
average, or adverse. In general, mountainous areas have favorable propagation conditions, while tropical
areas and those near large bodies of water have adverse conditions.
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 additional 6 dB. Knowing this,
the output power (+37 dBm), and the receiver sensitivity, we can calculate antenna size and tower height
requirements to cover any desired distance.
8.1.2 Formulas for System Planning
The following standard formulas are provided for assistance in determining system installation parame-
ters.