Mimo Antennas far/near field

[biff44]

I have a 2 X 2 beamforming mimo 2.4 GHz WLAN card. I want to come up with a repeatable performance test in an enclosed space. The antennas are shortened monopoles, 1" long. They are spaced around 10" apart. I know from theory that if I placed a test antenna some distance L away from the unit, that the wlan card can do its beamforming as long as I am in the Fraunhoffer region. The Fraunhoffer region is partway between the near and far fields apparently.

So, for the above situation, where does the fraunhoffer region start? Do I use the 1" monopole length to calculate it, or do I use the 10" spacing between the monopoles (what I think)as an effective array "aperture" width?

Thanks for the help.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[Higgler]

If you place your test antenna at some distance away in a room, and anything reasonably close to these antennas in the room changes, won't that change your results?

I'd say you might want to change your environment so that the RF is controlled. i.e. make your own short and wide waveguide environment, or add a metal cap over your set of antennas and confine your test area. Leave two ends open, add absorber on the open ends.

All depends on how sensitive your MIMO equipment is to slight amplitude and phase changes of the incoming signal.

I'm curious how the "beamforming" is done, could you post a link please.

kch

 

[biff44]

I am talking about a shielded box with all the stuff inside of it. I just don't know if the box is 2' long or 10'long!

Apparently, the latest WLAN chipsets can do some sort of "beamforming". They have 2 or 3 antennas, and can form a crude antenna array with them to center the radiation pattern to their destination lan card. They do so by up/downconverting the 3 antenna paths, digitizing them, and then doing the amplitude/phase stearing of the antenna pattern digitally. Here is one example:

http://rfdesign.com/mag/511RFDF1.pdf

That is all a little superfluous to the question at hand, though. Assume I have a two element array (1" high elements, spaced 10" apart)...how close can I get to it with a single test antenna and still allow it to be beam steered?

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[biff44]

Empirical data says that the beamforming for a 2 element array holds fairly well over a 5 to 32"+ range, so I guess you can get very close.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting