Jeff Duntemanns Drive-By Wi-Fi Guide

Diversity Reception and Dual Antennas

Most (though by no means all) wireless access points and residential gateways have two antennas. Two antennas allow diversity reception, a feature that improves the reliability of your Wi-Fi connection through the access point or gateway.

Microwaves bouncing around a room sometimes reflect back to their source and interfere with themselves, creating 'ghost signals' that weaken the link and reduce its quality. It's analogous to 'ghost images' on a broadcast TV signal, which happen when the signal arrives at the TV's rabbit-ears antenna twice: once on a direct path, and another on a path that bounces off a hill or tall building or even a jetliner flying overhead.

For technical reasons that I can't explain in detail here, a pair of antennas located one wavelength apart will rarely both experience this sort of interference at the same time. When one antenna is beset by reflective ghosting, the other will typically not be. Diversity reception allows the access point to choose which antenna to take its signal from. The access point's radio subsystem continually samples the signal strength from both antennas, and it takes the stronger of the two signals.

One wavelength at Wi-Fi frequencies (2.4 GHz) is a little under five inches, and if you measure the distance between the two antennas on an access point, that's how far apart they'll be, or a little more.

Note well that diversity reception is about reception, not transmission. The signal transmitted from an access point or gateway always goes out through the same antenna. Many access points allow you to select which antenna is the transmit antenna, but in operation, the transmit antenna never changes, even if the receive antenna does. (Wi-Fi gear is 'half-duplex,' meaning that a device will either transmit or receive at a given time, but will never transmit and receive simultaneously.) The only exception I know of is the Cisco Aironet 350 product line, which offers a diversity option on the transmit side. The documentation says little about it and it's unclear what diversity on transmit accomplishes.

All else being equal, an access point or gateway with two antennas will have a greater range than similar devices with only one antenna. This is not because both antennas transmit at once, but because the signals received by the access point are less subject to ghosting interference.

Some client adapters also have dual antennas, but that's much less common. The Cisco Aironet 350 and the Asanté AL-1511 'X-Wing' PCMCIA cards are the only ones I'm sure of, and because there isn't room on a PCMCIA card to put 5" between antennas, the effectiveness of their diversity reception isn't optimal.