MASSA PRODUCTS CORPORATION
Product Manual- April 24, 2012 24 M3 Wireless Ultrasonic Level Sensor
APPENDIX D
Fresnel zone and effects on it (see Digi web site for more info)
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 omni-directional, 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.
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