Wireless in the Enterprise

A word about antenna strategies

Email from NW on Wireless in Enterprise 6-21-06

By Joanie Wexler

Several readers wrote in following my recent discourse on antenna basics requesting some follow-up on the subject, particularly in the area of antenna polarization. While not a physicist, I can share a few key points that are important to know if you are an IT/network manager finding yourself in the position of designing and managing wireless networks.

The main reasons polarization is important:

* To form a wireless communications link, the polarization of two communicating stations must match. This maximizes received signal strength, while a mismatch will degrade signal strength and, possibly, performance.

* Cross-polarization design strategies might eventually play a role in circumventing interference from neighboring systems as the airwaves grow increasingly populated.

That said, what is antenna polarization?

The polarization of an antenna is the orientation of the electric field of the radio wave with respect to the Earth's surface. FYI, the industry lingo for “electric field” is “E” plane, in case that term should come up. Polarization is affected by reflections.

Most antennas radiate either linear (including horizontal and vertical) polarization or circular polarization. Vertically polarized antennas’ electrical field is perpendicular to the Earth's surface. Horizontally polarized antennas have their electric field parallel to the Earth's surface. So a straight-wire antenna will have one polarization when mounted vertically and a different polarization when mounted horizontally, because its relationship to the Earth’s surface changes with the different mounts.

Circular polarized antennas, by contrast, radiate energy in both the horizontal and vertical planes and all planes in between.

If one station has vertical polarization, others that it communicates with must also have vertical polarization. If the station has a horizontal polarization, its mate must, too. Same with circular.

When two antennas do not have the same polarization the condition is called “cross-polarization.” This can actually be beneficial to your design strategy in some circumstances. If the antennas of link A are cross-polarized to the antennas of nearby link B, where links A and B aren’t meant to communicate with each other, interference between the links is reduced and possibly prevented.