HMS Networks Anybus Wireless Bolt Serial AWB2010 User Manual page 59

Wireless Technology Basics
11.2. Wireless Technology Basics
Wireless technology is based on the propagation and reception of electromagnetic waves. These waves respond
in different ways in terms of propagation, dispersion, diffraction and reflection depending on their frequency and
the medium in which they are travelling.
To enable communication there should optimally be an unobstructed line of sight between the antennas of the
devices. However, the so called Fresnel Zones should also be kept clear from obstacles, as radio waves reflected
from objects within these zones may reach the receiver out of phase, reducing the strength of the original signal
(also known as phase cancelling).
Fresnel zones can be thought of as ellipsoid three-dimensional shapes between two wireless devices. The size
and shape of the zones depend on the distance between the devices and on the signal wave length. As a rule of
thumb, at least 60 % of the first (innermost) Fresnel zone must be free of obstacles to maintain good reception.
Figure 40. Fresnel zones
Area to keep clear of obstacles (first Fresnel zone)
Distance (d)
100 m
200 m
300 m
400 m
The wireless signal may be adequate even if there are obstacles within the Fresnel zones, as it always depends
on the number and size of the obstacles and where they are located. This is especially true indoors, where
reflections on metal objects may actually help the propagation of radio waves. To reduce interference and phase
cancelling, the transmission power of the unit may in some cases have to be reduced to limit the range.
It is therefore recommended to use a wireless signal analysis tool for determining the optimal placement and
configuration of a wireless device.
SCM-1202-143 2.0
Fresnel zone radius (r)
2.4 GHz (WLAN or Bluetooth)
1.7 m
2.5 m
3.0 m
3.5 m
® ™
Anybus Wireless Bolt Serial
5 GHz (WLAN)
1.2 m
1.7 m
2.1 m
2.4 m
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