Wireless Antennae Diversity

Nathan explains the advantage of diversity - utilizing multiple wireless antennae in an access point to ensure that a throughput signal is strong enough to avoid multipath propagation.

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Hello, this is Nathan. Thanks for joining us. Today, we're going to be talking about diversity. I don't mean racial diversity, as good a thing that is. We're actually going to be talking about antennae diversity as it relates to wireless. I have another cruelly drawn diagram. I apologize, but hopefully this will get the point across.

Essentially diversity is when you utilize more than one antenna in an access point to make sure that the throughput or the transmitted signal is good enough that we don't end up with what's called multipath propagation. I will explain what multipath propagation is. That is when signals are transmitted into a room or an environment, and because of obstacles, be that the ceiling or maybe a metal filing cabinet, those signals are bounced around the room and arrive at the antenna at different intervals. This can cause retransmits and throughput issues.

This being a perfect environment, what you want to do is, you have more than one antenna, which is why there could be two antennas on say, a Cisco 1242 where you'd have two antennas for the A radio and two for the G radio. Let's assume this is a G radio scenario. You have your two antennas.

When any radio signal is transmitted, it widens. As it widens, that signal can then bounce off any number of things in the room, be they a wall, a pane of glass, the ceiling, like I said, or a metal filing cabinet, or even people or other things in the room. As those reflect, what you want to have happen is that most of the signal or as much of it as possible, gets to one of these two antennas. That's where diversity comes in.

In this case, you can see that signal is reflecting, but it's hitting antenna 1, the receiver 1 on the AP and not 2. Receiver 1 is the antenna that the AP will use for that transmission. There could be another scenario with, because of maybe the way the room is laid out, because of something or an event going on in the room, or maybe in a warehouse where stock may change or stock levels rise, maybe receiver 2 would be used maybe later in the week or another time. Either way, it's going to pick one of these two antennas to utilize.

We have a number of customers who used to believe or think that what they could do is take these two antennas and separate them by large distances in two different rooms just to spread coverage. It's really not what diversity is for. Diversity is to keep those antennas close together so that you can make sure that you maintain a great connection from your transmitters to your receivers.

Today, we were talking about diversity and I wanted to wrap with a couple of key points. We talked about those antennas and how we have two of them. What I wanted to bring to your attention is that, it's important to space these antennas five inches apart. When you are dealing with, say a Cisco 1242 or 1231, and yes those are Legacy AP's, but when you are dealing with a scenario like that, you want to keep those antennas spaced at five inches at a max of 20. You can do five inch increments, but the match really is 20 inches apart. Sticking them in two different rooms obviously, is not going to be an option.

Also, your internal antennas on your access points like your 1142's or 1131's, those are already pre-spaced, so you don't have to worry about those. That information earns us the happy smiley face. Thank you for joining us today and hopefully you learned something.