Instruction Guide – Massa M3 Wireless Sensor Evaluation Kit Page of 14
M3 Kit Guide 111017.docx 10-20-2011
12
PRELIMINARY
APPENDIX B
Fresnel Zone and Effect on Range
(See Digi web site for more information.)
It makes sense that obstructions between a transmitter and receiver will reduce the communication
range. In order to obtain the absolute maximum communication range possible, a radio modem
system must be installed such that true RF Line-of-Sight (LOS) conditions exist between the
transmitting and receiving antennas.
RF LOS is different that visual LOS. Visual LOS is present when one can stand next one antenna and
use binoculars to view the other antenna. RF LOS requires not only a visual sight line between the
antennas but it also requires that a football shaped area between the two antennas be free of
obstructions.
This football shaped area is called the Fresnel Zone (pronounced Fernel Zone). The Fresnel Zone is
an area that is larger in diameter at the center and smaller in diameter at either end. Also, the greater
the distance between the antennas, the larger the diameter of the Fresnel Zone in the center.
Any obstructions that enter into the Fresnel Zone will reduce the communication range; including
buildings, vegetation, the ground, etc. As the antennas get further apart and the diameter of the
Fresnel Zone increases, the ground can begin to obstruct the Fresnel Zone. In order to keep the
entire Fresnel Zone free of obstructions it is necessary to raise the antennas. To keep the Fresnel
Zone off the ground the heights of the antennas added together must total more than the diameter
of the Fresnel Zone at the specific distance.
The diameter of the Fresnel Zone is a function of the frequency and the distance between the
antennas. For reference here is a table to use as a guideline:
2.4 GHz
Distance between antennas
Fresnel zone
diameter
Freespace loss (dB)
1000 ft (300 m) 11 ft (3.4 m) 90
1 Mile (1.6 km) 21 ft (6.4 m) 104
Side view of Fresnel Zone
Since the RF signal is omnidirectional, it can
reflect signals from objects back to the sensor
and gateway. This overhead view shows the RF
signal deflecting off of an object that may
reduce the signal of the direct RF path if out of
phase. This example could be the source of
problems of obtaining good signal strength
between the 2 devices and is why it is important
to perform site testing where the gateway and
sensor it to be located. Most cased the
reflected signal would be low in sensitivity and
may not be an issue.
(29 pagine)
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