Specifications

MDS 05-3438A01, Rev. E MDS 4790/9790 Series I/O Guide 97
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 actu-
ally is. Experience has shown that if we consider the earth’s radius to be
4/3rds of its actual size, we get good agreement between theory and
measured propagation results.
Figure 49 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.
Invisible place holder
Figure 49. Antenna Height vs. Theoretical Radio Horizon
Fade Margins
Variations in the temperature and humidity of the atmosphere with ele-
vation cause the signals to bend more or less, resulting in fading at the
receiver. The longer the path is, 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 trop-
ical areas and those near large bodies of water have adverse conditions.
(0)
0
(2) (4) (6) (8) (10) (12) (14) (16) (18) (20) (22) (24) (26) (28) (30) (32) (34)
(0)
0
(50)
15
(100)
30
(150)
46
(200)
61
(250)
76
(36)
Radio Horizon Over Level Terrain in Kilometers (Miles)
Normal Conditions
Antenna
Height in
Meters
(Feet)
585552484542393532 292623191610 1363