56 Appendix
758-919 Wireless Access Point
Manual
Version 1.0.0
6.2
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 32: Fresnel zones
Table 32: Area to keep clear of obstacles (first Fresnel zone)
Distance
Fresnel zone radius (r)
2.4 GHz (WLAN or Bluetooth) 5 GHz (WLAN)
100 m
1.7 m
1.2 m
200 m
2.5 m
1.7 m
300 m
3.0 m
2.1 m
400 m
3.5 m
2.4 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.
