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Understanding wireless antennas - part 1

by Lisa Phifer, VP, Core Competence

Many WLAN users take antennas for granted. They just use the antenna supplied with their 802.11 PC card or router and quietly curse the manufacturer when speed drops and signal fades. But they don't know what WLAN designers do: upgraded antennas can improve performance, reach and security.

Types of antennas
Wireless radios generate signal on a given frequency. Antennas distribute (propagate) that signal through the air in a particular pattern.

The antennas commonly included with 802.11 products are omni-directional antennas that radiate signal in all directions around the antenna's axis. After-market directional antennas focus transmissions more narrowly by radiating in a certain direction.

How does that help?
Picture a 6" water-filled balloon. Put the balloon on a table. The bottom flattens and the circumference expands slightly. Squash the balloon with your hand. By concentrating the same volume of water in the horizontal direction, the balloon can now touch objects 8" apart. Squeeze the balloon into a 4x10" shoebox. By losing its shape and shrinking in one dimension (width,) the balloon is able to reach even further in the other dimension (length.)

The same principal applies to antennas. They take a given power output and make it reach further by reducing direction(s) along which signal is radiated. This is useful because buildings are not spherical - they're like that shoebox. Concentrating signal on your workspace

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makes better use of your AP's power output. Stations inside your workspace get stronger coverage and therefore higher speed. Directing signal where you want it also means less signal where you don't want it. Stations outside your workspace - i.e., war drivers - get little or no coverage.

What kind of antenna do you have?
An omni-directional monopole would radiate signal in a uniform sphere. But that's not your average 802.11 AP antenna.

Most APs come with omni-directional dipole antennas - those flexible black plastic sticks commonly called rubber duckies. Dipole coverage looks like a squashed donut, with the antenna sticking through the center. Power does not radiate out of the top (or bottom) of the antenna; it radiates horizontally.

If you put an AP with a dipole on a table at the center of the second floor, stations on that floor will get the strongest signal. Stations on the first and third floors will get some signal, depending upon the AP's output and the antenna's gain (i.e., how flat the donut is.) You can often improve results by repositioning the AP. For example, in a two-story home, try mounting the AP on the first floor ceiling.

When viewed from the top, dipole coverage is circular. When the AP is centrally-located, stations in distant corners will have the weakest coverage. Multiple APs may therefore be required to provide adequate coverage for the entire floor - in other words, overlapping circles. Speed is higher for stations closer to the AP, so you may be tempted to add APs to boost performance - more overlapping circles. But as APs are positioned closer to exterior walls, the more signal is going to leak outside.

Directional antennas can improve performance without increasing risk and waste. In Part 2, I will describe directional antennas, explaining what Patch, Panel, Yagi and Parabolic mean and where they can be helpful. About the author: Lisa Phifer is vice president of Core Competence Inc., a consulting firm specializing in network security and management technology. Phifer has been involved in the design, implementation, and evaluation of data communications, internetworking, security, and network management products for nearly 20 years. She teaches about wireless LANs and virtual private networking at industry conferences and has written extensively about network infrastructure and security technologies for numerous publications. She is also a site expert to SearchMobileComputing.com and SearchNetworking.com.

Do you have comments about this article, or suggestions for Lisa to write about in future columns? Let us know!

This was first published in March 2007

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