Fade Margins - GE MDS 4790 Series Installation & Operation Manual

MDS 05-3438A01, Rev. F
RSPSupply - 1-888-532-2706 - www.RSPSupply.com
http://www.RSPSupply.com/p-8457-MDS-UP-9790X.aspx
Figure 50 shows a representation of the 4/3 earth "radio
horizon." This figure shows that under normal radio propagation condi-
tions, a station with its antenna 15 meters above flat terrain will have a
radio horizon approximately 15 kilometers away, well beyond the visual
horizon.
76
(250)
61
(200)
46
Antenna
(150)
Height in
Meters
30
(100)
(Feet)
15
(50)
0
(0)
0 3 6 10
(0) (2) (4) (6)
Radio Horizon Over Level Terrain in Kilometers (Miles)
Figure 50. 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, 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.
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.
MDS 4790/9790 Series I/O Guide
Invisible place holder
13 16 19 23 26 32 29 35
(8) (10) (12) (14) (16) (18) (20) (22) (24) (26) (28) (30) (32) (34)
Normal Conditions
39 42 45 48 52 55 58
(36)
101