Why do Linksys WMP300N wireless cards have three antenna connectors?

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ANSWERED November 2008:
The WMP300N is a draft 802.11n wireless PCI adapter that comes equipped with an external MIMO (multiple input multiple output) antenna. The purpose of that MIMO antenna is to transmit and receive data along more than one spatial path. MIMO antennas let 802.11n products operate at much higher data rates by transmitting different data along different paths (recombined at the receiver). They also let 802.11n products operate more reliably by transmitting the same data redundantly along different paths (reconciled at the receiver).

Precisely how each 802.11n radio uses its MIMO antennas depends on the product and how it's configured. For example, a 3x3 MIMO device has three independent transmit and three independent receive chains. Using two antennas to send different data (spatial streams) yields a maximum data rate of 130 Mbps -- that's twice the data rate of 65 Mbps that can be achieved using a single antenna (assuming 20 MHz channel, 800 ns guard interval). Adding a third spatial stream would crank that data rate up to 195 Mbps. Alternatively, that third transmit chain might be used for redundancy, improving signal-to-noise ratio (SNR) in locations where maximum data rates cannot actually be used because there is too much interference.

Given this, you probably now realize the purpose of those three connectors on your WMP300N card, which do indeed correspond to three directional antenna blades. Disconnecting two of those antennas would be counterproductive: You would defeat MIMO by limiting your card to 1x1 operation at no more than 65 Mbps (assuming that you could even configure the card to operate correctly with only one functional antenna)!

If you still want to replace these factory-supplied MIMO antennas, you must come up with replacement MIMO antennas that can accomplish your goals. For example, TerraWave sells a variety of after-market MIMO antennas for the 2.4 and/or 5 GHz band, including omni-directional and patch antennas. It's not a good idea to simply connect three independent higher-gain directional antennas to those three connectors -- you must use antennas designed for MIMO in order to correctly transmit and receive data over different spatial paths.