Antenna Polarization - ZyXEL Communications ZyAIR G-5100 User Manual

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.

Antenna Polarization

The orientation of the antenna will change the orientation of the signal. The transmitting and
receiving antennas should be both polarized either horizontally or vertically. Adjacent
antennas on different frequencies can be cross-polarized to help reduce interference between
the two, if your operating license permits this.
Towers
When planning antenna placement, it might be necessary to build a freestanding tower for the
antenna. Regulations and limitations define the height and location of these towers with
respect to airports, runways, and airplane approach paths. The Federal Aviation
Administration (FAA) controls these regulations. In some circumstances, the FAA, the FCC,
or both, must approve the tower installations.
To ensure compliance, review the current FCC regulations regarding antenna structures. These
regulations (along with examples) can be viewed on the FCC web site at http://www.fcc.gov/
antenna.
Path Planning
To get the most value from a wireless system, path planning is essential. In addition to the fact
that radio signals dissipate as they travel, many other factors operate on a microwave signal as
it moves through space. All of these must be taken into account, to avoid attenuation of the
signal by path obstruction.
Appendix G Outdoor Site Planning
ZyAIR G-5100 User's Guide
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