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The path difference is the required clearance of the antenna beam from obstacles in its path to avoid loss of radio signal.
Signals reflected from any even-numbered Fresnel Zone result in signal cancellation while the odd-numbered Fresnel Zones
add to the direct path signal.
9.4 Clearance Factor
Clearance Factor is a correction value (in percentage) that should be used in case, where the signal path of your wireless link
does not provide the minimum clearance as listed in the
of the Fresnel Zone and is calculated as:
Clearance Factor = Fresnel Zone x 60%
For optimal performance of your outdoor wireless link, the signal path between the BSU and SU must provide sufficient
clearance. Clearance is interpreted as:
–
The total height above the surface of the earth, in the open areas without obstacles in the signal path. Let's say
antenna is mounted on the roof, then clearance is the total height including the height of the building and the height
of the mast above the rooftop.
–
The height above the highest possible obstacle, in the areas with obstacles along the signal path (path between two
antennas).
–
The height above the rooftop or highest obstacle in the signal path, in dense urban areas.
: An outdoor wireless link that lacks sufficient clearance will exhibit poor performance, which is typically perceived as
slow network response time. However, your radio equipment automatically retransmits every lost data frame due to
an out-of-range situation or frame collision. The larger the number of retransmissions, the lower is the throughput
efficiency of your wireless link.
As shown in the above figure, the clearance required for optimal performance is interpreted as:
– Vertical clearance above the ground and the highest buildings or objects along the signal path
– Horizontal clearance from neighboring buildings and objects along the signal path
For optimal range and throughput performance, you must ensure that your antenna installation provides maximum clearance
in both horizontal and vertical directions.
If the local authorities, proprietor of the premises or other miscellaneous factors do not allow you to set up an antenna mast
for the clearance requirements, then you may not achieve a full line-of-sight clearance. However, if the distance that your
wireless outdoor installation covers is less than the listed maximum range, you don't need full clearance.
To determine the effect of insufficient signal path clearance, you must determine the Clearance Factor and also calculate its
effect on the range for your antenna installation, by using the formula described in
®
ORiNOCO
AP-9200R - Hardware Installation Guide
Figure 9-2 Clearance required for the Optimal Performance
Maximum Range
Table. In general, clearance factor is taken as 60%
Measuring Signal Performance
Determining the
Range.
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