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Fresnel zone details

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 freqency and the distance between the antennas. For reference here is a table to use as a guideline:


900 MHz 2.4 GHz
Distance between antennas Fresnel zone diameter Freespace loss (dB) Fresnel zone diameter Freespace loss (dB)
1000 ft (300 m) 16 ft (4.9 m) 81 11 ft (3.4 m) 90
1 Mile (1.6 km) 32 ft (9.7 m) 96 21 ft (6.4 m) 104
5 miles (8 km) 68 ft (20.7 m) 110 43 ft (13 m) 118
10 miles (16 km) 95 ft (29 m) 116 59 ft (18 m) 124
20 miles (32 km) 138 ft (42 m) 122 87 ft (27 m) 130
40 miles (64 km) 192 ft (59 m) 128 118 ft (36 m) 136

In order for a wireless link to work, the available system operating margin (TX power - RX Sensitivity + Antenna gains) must exceed the Freespace loss and all other losses in the system.

Example:

The MaxStream X09-009PKC radio modem has the following parameters:

So the system operating margin is 21.5 - (-110) + 4.2 = 135.7 dBm which would be enough to transmit 40 miles if freespace was the only loss in the system. For this to be the case, the antennas must be mounted with a combined height greater than 192ft above all obstructions (including the ground) to keep the fresnel zone clear. In practice however, there are many losses in the system besides just freespace and it is recommended that there be at least 10-20dB of extra system operating margin.

Additional Fresnel Zone resources:


http://www.softwright.com/faq/engineering/Fresnel%20Zone%20Clearance.html

Mobile Radio Plus radio path profile software http://www.cplus.org/rmw/english1.html

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