Wi Lan Inc EB03 Manuale Utente

Antenna Basics

JUL 2001 Rev 03

121

Some key items to consider when selecting and installing antennas for your wireless network follow.

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

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 
Interference

• Multipath is a form of diffraction in which the reflected signal arrives at the receiver 

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