User Manual

www.moxa.com [e-mail] 28
2009 Industrial Wireless Guidebook
Understanding Industrial WLAN – IEEE 802.11
2
To ensure that the Fresnel Zone remains unobstructed, the height of the antennas must exceed that of the
highest obstruction by this radius. The figure below shows the full height of the antenna mast.
Antenna Gains
The gain of each antenna specifies its directionality. In general, the lower the gain, the more evenly
distributed in all directions the radiation will be. High gain antennas, on the other hand, emit radiation in a
more specific direction. The gain defines its power gain or directive gain in terms of the ratio of the intensity,
or power per unit surface. In general, when we choose an antenna, the longer the transmission distance, the
higher the antenna gain must be. At the same time, we must sacrifice omni-directional coverage.
Antenna Alignment for P2P Operations
When there is not enough obstacle-free Fresnel Zone available, you may need to relocate the wireless devices
or elevate the antennas to clear more Fresnel Zone space (see figure below). In long distance applications,
Earth Bulge may need to be taken into consideration as an obstacle.