User's Manual

Appendix D Wireless LANs
B222s User’s Guide
241
Antenna Characteristics
Frequency
An ant enna in t he frequency of 2.4GHz ( I EEE 802.11b and I EEE 802.11g) or 5GHz ( I EEE 802.11a)
is needed t o com m unicat e efficiently in a wir eless LAN
Radiation Pattern
A radiat ion pat t er n is a diagram t hat allows you t o visualize t he shape of t he ant enna’s coverage
area.
Antenna Gain
Ant enna gain, m easured in dB ( decibel), is t he increase in coverage wit hin t he RF beam widt h.
Higher antenna gain im proves t he range of the signal for bet ter com m unications.
For an indoor site, each 1 dB increase in antenna gain result s in a range increase of approxim at ely
2.5% . For an unobstructed outdoor site, each 1dB increase in gain results in a range increase of
approxim at ely 5% . Actual result s m ay vary depending on the network environm ent .
Ant enna gain is som et im es specified in dBi, which is how m uch the ant enna increases t he signal
power com pared to using an isotropic antenna. An isotropic ant enna is a theoretical perfect ant enna
that sends out radio signals equally well in all direct ions. dBi represent s the true gain t hat t he
antenna provides.
Types of Antennas for WLAN
There are two t ypes of antennas used for wireless LAN applications.
• Om ni- direct ional ant ennas send the RF signal out in all direct ions on a horizont al plane. The
coverage area is t orus- shaped ( like a donut ) which m akes these ant ennas ideal for a room
environm ent . With a wide coverage area, it is possible to m ake circular overlapping coverage
areas wit h m ultiple access point s.
• Direct ional antennas concent rat e t he RF signal in a beam , like a flashlight does wit h t he light
from it s bulb. The angle of the beam determ ines t he width of t he coverage pat t ern. Angles
typically range from 20 degrees (very directional) t o 120 degrees (less direct ional) . Direct ional
antennas are ideal for hallways and out door point- t o-point applications.
Positioning Antennas
I n general, ant ennas should be m ount ed as high as practically possible and free of obstructions. I n
point- to–point application, posit ion both ant ennas at t he sam e height and in a direct line of sight to
each ot her t o at tain the best perform ance.
For om ni- direct ional ant ennas m ount ed on a table, desk, and so on, point t he ant enna up. For
om ni- direct ional ant ennas m ount ed on a wall or ceiling, point t he ant enna down. For a single AP
application, place om ni- directional ant ennas as close t o t he cent er of the coverage area as possible.
For direct ional ant ennas, point t he ant enna in t he direction of t he desired coverage area.