ZyXEL Communications XMG3512-B10A, Руководство пользователя

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Appendix B Wireless LANs
XMG3512-B10A User’s Guide
267
Antenna Overview
An antenna couples RF signals onto air. A transmitter within a wireless device sends an RF signal to the
antenna, which propagates the signal through the air. The antenna also operates in reverse by
capturing RF signals from the air.
Positioning the antennas properly increases the range and coverage area of a wireless LAN.
Antenna Characteristics
Frequency
An antenna in the frequency of 2.4GHz (IEEE 802.11b and IEEE 802.11g) or 5GHz (IEEE 802.11a) is needed
to communicate efficiently in a wireless LAN
Radiation Pattern
A radiation pattern is a diagram that allows you to visualize the shape of the antenna’s coverage area.
Antenna Gain
Antenna gain, measured in dB (decibel), is the increase in coverage within the RF beam width. Higher
antenna gain improves the range of the signal for better communications.
For an indoor site, each 1 dB increase in antenna gain results in a range increase of approximately 2.5%.
For an unobstructed outdoor site, each 1dB increase in gain results in a range increase of approximately
5%. Actual results may vary depending on the network environment.
Antenna gain is sometimes specified in dBi, which is how much the antenna increases the signal power
compared to using an isotropic antenna. An isotropic antenna is a theoretical perfect antenna that
sends out radio signals equally well in all directions. dBi represents the true gain that the antenna
provides.
Types of Antennas for WLAN
There are two types of antennas used for wireless LAN applications.
• Omni-directional antennas send the RF signal out in all directions on a horizontal plane. The coverage
area is torus-shaped (like a donut) which makes these antennas ideal for a room environment. With a
wide coverage area, it is possible to make circular overlapping coverage areas with multiple access
points.
• Directional antennas concentrate the RF signal in a beam, like a flashlight does with the light from its
bulb. The angle of the beam determines the width of the coverage pattern. Angles typically range
from 20 degrees (very directional) to 120 degrees (less directional). Directional antennas are ideal for
hallways and outdoor point-to-point applications.
Positioning Antennas
In general, antennas should be mounted as high as practically possible and free of obstructions. In
point-to–point application, position both antennas at the same height and in a direct line of sight to
each other to attain the best performance.