B Wireless Technology Basics - HMS Networks Anybus Wireless Bolt CAN User Manual

Appendix B: Wireless Technology Basics
B 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.
Fig. 31 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.
® ™
Anybus Wireless Bolt CAN User Manual
Fresnel zone radius (r)
2.4 GHz (WLAN or Bluetooth)
1.7 m
2.5 m
3.0 m
3.5 m
5 GHz (WLAN)
1.2 m
1.7 m
2.1 m
2.4 m
scm-1202-175 1.1 en-US
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