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Antenna Selection and Positioning Recommendation D-1
Appendix D
Antenna Selection and Positioning
Recommendation
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.
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.