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ZyWALL 2 and ZyWALL 2WE
Antennas
14
Appendix B
Antennas
This appendix provides information about antenna selection and positioning.
The access points in a wireless LAN send a radio frequency (RF) signal to the antennas, which propagate and
capture the RF signal. Choosing the right antennas and positioning them 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) 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.