Antenna spacing and interference

[oneamp]

I have a wifi card for a cisco router that has two antennas spaced about an inch apart. The antennas can pivot so that the distance between the same point on either antenna can change. Since the antennas don't remain a wavelength multiple apart at all points on them, why is it that they don't destructively interfere with each other?

Thank you

 

[berkeman]

I have a wifi card for a cisco router that has two antennas spaced about an inch apart. The antennas can pivot so that the distance between the same point on either antenna can change. Since the antennas don't remain a wavelength multiple apart at all points on them, why is it that they don't destructively interfere with each other?

Thank you

They aren't driven at the same time. Dual antennas in WiFi are generally used for "spatial diversity" in RX. The RX circuit uses whichever RX signal is the strongest. Because of multipath, one antenna can be in a deep null, but the other antenna spaced a bit away will generally not be in the same null...

 

[oneamp]

So the system just picks one to transmit?

 

[berkeman]

There are different ways to use the two antennas, but that's one of the simpler ones -- TX on one antenna, and RX on both (either at the same time if your PHY chip can handle it, or switch sequentially between them if the TX device is retrying because you can't hear it on your primary antenna).

A more complicated system can use the two antennas to query a node it wants to reach, to see which TX antenna gives a better receive signal strength indicator (RSSI) reading at the RX device. But in systems/channels where the multi-path can change dynamically, that kind of adaptive learning may not be worth it.

Here's a wikipedia article that discusses antenna diversity and multipath...

http://en.wikipedia.org/wiki/Antenna_diversity

 

[oneamp]

Thank you