Implementation Considerations - Wi-LAN Hopper Plus 120-24 Installation And Configuration Manual

Parameter Description
XPD • Polarity and Cross-Polarization Discrimination (XPD)
• Antennas have an associated polarity, which is the orientation of the radiating
element with respect to earth
• Antennas are usually described as vertical, horizontal, or circularly polarized. The
polarity of all antennas used in a system must be the same
• Cross-Polarization Discrimination specifies the signal isolation achieved when the
receiving element is perpendicular to the radiating element. Can be advantageous
when co-locating radio systems
VSWR • Voltage standing wave ratio
• VSWR is the voltage ratio of minimum to maximum across a transmission line
• A VSWR of 2.0:1 or less in an antenna is considered effective. Most antennas have a
VSWR of 1.5:1
• For example, when using a radio with a 4 W output with an antenna VSWR of 1.5:1,
the reflected power will be 160 mW

Implementation Considerations

Some key items to consider when selecting and installing antennas for your wireless network follow.
Item Description
Absorption • Antennas mounted too close to "soft" objects, such as trees, may experience a
reduction in signal strength due to absorption
• Absorption is most often encountered in applications installed during the fall or
winter months, and the problem does not become evident until the spring
Diffraction • Diffraction occurs when a radio signal reflects or bounces off of a solid object.
• Level of diffraction could lead to connectivity problems if the remaining signal level is
too low
• Two types of diffraction are shadowing and multipath
Shadowing • Shadowing is a form of diffraction that is typically caused when antennas are mounted
too close to a structure and they lose a portion of the signal lobe due to reflection
The receive antenna is in a shadowed area
• To minimize shadowing, ensure that there is adequate height above structures when
mounting antenna equipment
Multipath • Multipath is a form of diffraction in which the reflected signal arrives at the receiver
Interference at different times which confuses the receiver
• Multipath may be interpreted as interference by the receive antenna and can result in
bit errors and processing delays
DEC 2000 Rev 02
Antenna Basics
115