Main Beams And Side Lobes; Locating The Main Beam - Alcatel-Lucent 9500 MXC User Manual

Chapter 5. Antenna Alignment

Main Beams and Side Lobes

Locating the Main Beam

Vol. III-5-18
This section describes how to locate the main beam, and typical tracking path
errors. Refer to:
• Locating the Main Beam
• Tracking Path Error
Ensure the antennas are aligned on the main beam, and not a side lobe.
Once a measurable signal is observed, very small alignment adjustments are
required to locate the main beam. For instance, a 1.2m antenna at 23 GHz
typically has 0.9° of adjustment from center of main beam to the first null (0.4° to
the -3 dB point). Antenna movement across the main beam will result in a rapid
rise and fall of signal level. As a guide, 1 degree of beam width is equivalent to
moving approximately 1.0 mm around a standard 114 mm (4.5 in.) diameter O/D
pipe.
Antennas can be verified as being on main beam (as opposed to a side lobe) by
comparing measured receive signal level with the calculated level.
Signal strength readings are usually measurable when at least a main beam at one
end and first side lobes at the other are aligned.
The strongest signal occurs at the center of the main beam. The highest first lobe
signal is typically 20–25 dB less than the main beam signal. When both antennas
are aligned for maximum main beam signal strength, the receive signal level
should be within 2 dB of the calculated level for the path. This calculated level
should be included in the installation datapack for the link.
Figure 5-3 on page 5-19 is an example of a head-on, conceptual view of the beam
signal strength, with concentric rings of side lobe peaks and troughs radiating
outward from the main beam.
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