Why dual WiFi antennas?

[mobileMike]

Can someone explain why many WiFi access points have 2 antennas so close together. I have even seen 2 external antennas mounted closely on the ceiling.

/ mike

[Janak Parekh]

It's a good question. One notable exception to this is the ORiNOCO products, none of which have two antennas; in fact, most of them have internal antennas with an option of a single external antenna, and yet have the best range of 802.11 devices.

Anyone?

--bdj

[kfluet]

If I remember from the antenna theory I took many years ago, two antennas are a dipole antenna and they create an additive interference pattern which increases the signal in two directions (forward and backward) perpendicular to the antenna pair. It allows you to aim the signal.

Along the same lines, it allows you to tune the direction of reception much like old fashioned rabbit ears on a TV.

Someone who remembers more radio theory please correct me if I'm wrong.

-- Kevin

[Dave Beauvais]

Can someone explain why many WiFi access points have 2 antennas so close together. ...
This is called "diversity" and is used to help overcome the signal loss that occurs as an RF signal passes through (or is blocked by) objects in the environment. Depending on the location of your laptop/Pocket PC/access point/whatever, a signal may reach one antenna on your access point sooner than the other due to the difference in position relative to obstacles in the environment. Having dual antennae helps improve coverage by providing a second possible path for a signal to take in order to reach its destination.

Useful sources of info on this subject:
Dell Whitepaper for Deploying Wireless LANs (http://www.dell.com/us/en/biz/topics/vectors_2001-wireless_deployment.htm)
The Personal Telco Project - Diversity Antenna (http://www.personaltelco.net/index.cgi/DiversityAntenna)
A post to the Bay Area Wireless Users Group mailing list (http://lists.bawug.org/pipermail/wireless/2001-August/002145.html)--Dave