ZyXEL Communications Corporation NWD271N 用户手册

 Appendix B Wireless LANs

NWD271N User’s Guide

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. 

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

A radiation pattern is a diagram that allows you to visualize the shape of the antenna’s 
coverage area. 

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.   

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.