Adtran TRC4206 Manual Do Utilizador
Section 2, Microwave Path Engineering Basics
TRACER 4206 System Manual
18
© 2002 ADTRAN, Inc.
612804206L1-1A
shown in Figure 2 on page 18. The antenna alignment step to setting up a microwave link is in fact steering
the main lobes of both antennas until the main lobe of one transmitter is centered on the receiving element
of the receiving antenna.
the main lobes of both antennas until the main lobe of one transmitter is centered on the receiving element
of the receiving antenna.
Figure 2. Typical Antenna Beam Pattern
Antennas are also designed to radiate RF energy efficiently for a specific range of frequencies. Please
consult the data sheet for your particular antenna make and model to ensure that it is specified to operate in
the 5725 MHz to 5850 MHz frequency band.
consult the data sheet for your particular antenna make and model to ensure that it is specified to operate in
the 5725 MHz to 5850 MHz frequency band.
Fresnel Zones, Earth Curvature, & Antenna Heights
The Fresnel zones correspond to regions in the microwave path where reflections of the intended signal
occur and combine in both constructive and destructive manners with the main signal, thereby either
enhancing or reducing the net power at the receiver.
occur and combine in both constructive and destructive manners with the main signal, thereby either
enhancing or reducing the net power at the receiver.
In general, the odd numbered Fresnel zones (1, 3, 5, ...) add constructively at the receiver, while the even
numbered Fresnel zones (2, 4, 6, ...) add destructively at the receiver.
numbered Fresnel zones (2, 4, 6, ...) add destructively at the receiver.
The first Fresnel zone corresponds to the main lobe, and must be at least 60% free of physical obstructions for
the path calculations to be valid. Since the main lobe contains the vast majority of the microwave energy, this
zone is typically used to determine proper antenna heights when placing antennas on towers or buildings.
the path calculations to be valid. Since the main lobe contains the vast majority of the microwave energy, this
zone is typically used to determine proper antenna heights when placing antennas on towers or buildings.
The curvature of the Earth becomes a legitimate obstruction for path lengths of 7 miles or greater, and must
also be accounted for when determining minimum antenna heights.
also be accounted for when determining minimum antenna heights.
The aggregate expression for minimum antenna height that incorporates both the 60% first Fresnel zone
and the Earth curvature is given by
and the Earth curvature is given by
where f is in GHz and d is in miles.
Table 3 tabulates minimum antenna heights for given path lengths.
main lobe
side lobes
h
72.1
d
4f
-----
0.125d
2
+
=
(feet)