Wireless Access

Hi all,

Just got a question regarding antenna spacing for outdoor coverage using 1x 175AP and 2x 2.4ghz 8dbi patch antennas. What is the maximum distance between antennas and still stay within MIMO spec? Is MIMO going to be of value if there is just single antenna clients with minimal bandwidth (hand held barcode scanners)? The main aim we have is to try and provide an altenative rf path if something large gets in the way of the client an AP. Also no plans to use any 5ghz clients.

Cheers

Hey Max,

First things first, In order for MIMO to work for 11n you need to have your patch antennas installed so that one is vertically polarized and the other is horizontally polarized - otherwise you are just providing diversity which in itself isn't a bad thing.  If your clients are 11n then it will matter, you will not see 11n rates with your antennas set up in the same polarization. It really is not a good idea to spread your antennas apart very far at all for client coverage, this is likely to cause issues with hidden node / multipath fade.

Thanks for that insight mwehmeyer, I don’t think bandwidth is going to be something the clients will need much of and at present and there isn’t any 11n clients (none yet anyway) so I am not sure if MIMO is of much value for this install but something I wanted to keep in mind.  The key thing is providing an alternative physical path between client and AP. Each antenna is pointing around each side of the RF obstacle. In this case it is a Boeing 747. AP is at the nose and need to cover each side around to the tail so ideally need to spread them as far apart as possible if that makes sense.

Max,

You do not want to split antennas from a single radio this way as it will definitely cause hidden node issues.  Two clients will try to talk to the AP but will not be able to hear the other transmitting because the 747 is in between them.  Your best bet is to set up two different APs on each side of the 747 and use a directional antenna if possible. 

Cheers!

Marcus Wehmeyer