How To Orient WiFi Antennae

by @edent | 2 comments | Read ~21,123 times.

There's a lot of snake-oil in the technology world. From gold plated fibre optic cables to anti-radiation phone cases - there are a whole lot of people willing to exploit technical ignorance in return for money.

There's also a lot of misinformation, folklore, and crazy thinking which confuses us when we try to interact with "simple" technology. For example, I was in Waterloo Tube station when I saw this

Waterloo Wifi

That's one of the WifI boxes which power Internet access underground. I took a look at the antennae on the box and was shocked at what I saw...

Waterloo Wifi Detail

Look! All of them pointing in exactly the same direction! Surely everyone knows that each antenna should be pointing in a different, preferably perpendicular, direction. That way you... err... avoid interference and... stuff...

At this point, my brain gave up on me. I was fairly sure sure that I had been told how to correctly align WiFi aerials... but I had no knowledge to back up this information. I fired up my trusty WiFi Analyzer and took a look at the signals around me.

Wifi Signal Strength

Actually... that looks pretty good! The clustering is showing the same access point broadcasting multiple SSIDs. I can pick up three distinct networks - probably spaced further down the platform. There is a little bit of interference on channel 1 - and that looks like it's picking up some stray signal from another platform.

So, what's going on? Why is received wisdom so wrong on this matter?

Partly, it's because the science of multiple input multiple output radios is hard.

The ergodic channel capacity of MIMO systems where both the transmitter and the receiver have perfect instantaneous channel state information is
MIMO formula

...Right...

Secondly, your WiFi performance is as much about your physical environment as it is about your technology. If you live in a flat, you don't need those radio waves to travel vertically, if you live in a building with thick walls, there may be nothing you can do to improve performance.

London Underground platforms are fairly benign radio environments. Not a lot of interference down there - compare that to my experience at Mobile World Congress last year:
MWC12 Wifi

In a normal urban home, you're likely only to have to deal with half a dozen neighbours trying to cram their WiFi into the same crowded spectrum.

So, given all the variables, and taking in to account all the myths, what should one do?

Experiment.

Seriously, that's it. Unless you have the money to do a detailed electromagnetic survey, really all you can do is try putting your wifi router in different positions, angle the antennae, move your furniture, and see what gets you the fastest and most reliable connection.

And make sure to remove all amethyst crystals from your house, obviously...

2 thoughts on “How To Orient WiFi Antennae

  1. Roy Badami says:

    The way that access point you show is set up is the 'obvious' way from a radio engineering viewpoint. So called omni-directional antennae are not truly omni-directional. This kind of antenna is designed to be (roughly) vertical, and when so positioned they actually focus the transmission sigtnificantly in the horizontal plane, at the expense of transmitting less vertically. Generally this is what you want - your access point is typically trying to reach stations elsewhere on the same floor of the building (or in this case on the same platform) and not trying to use it to connect users on other floors of the building, which would be better served by an access point on that floor than by signals that would anyway have to pass through lots of concrete and metal.

    This kind of antenna will have a quoted gain of maybe 3db-4db. What does this mean? It means the signal is twice as strong or even a bit more _in the direction(s) it focusses the transmission_ compared with what would you expect from a perfectly omni-directional antenna (a so-called isotropic radiator). How is this possible? Obviously the antenna is just a piece of metal and plastic, it can't actually make radio energy out of nothing! So how does it achieve this seemingly magical field of doubling the amount of radio power? Ofcourse, it doesn't. It's transmitting twice as much power as you would expect in the horizontal plane because it's transmitting a lot _less_ than you would expect above and below the horizontal.

    So given you know that, you now understand why the first instinct of a radio engineer is to make the antennae vertical, And vertical is certainly the right thing to do if you only have one antenna.

    But what do you do when you have 6 antennae? And why do we have six antennae anyway? Well, the first part of the answer is that this will almost certainly be a simultaneous dual band access point. It was two independent radios, one for 2.4GHz and one for 5GHz. These are completely independent. But each radio still has three antenna. Why? Well, one reason is diversity. The vagaries of radio propagation mean that for any particular staton the access point is communicating with, one antenna might pick up a better signal than the others. But there's more. 802.11n and 802.11ac use MIMO, which actually relies on the fact that the signals from the different antennae will take different paths to the station. So for both these reasons, it makes sense to vary the angles of the antennae somewhat, to exagerate the differences in signal picked up by each antenna. You certainly don't want any of the antennae horizontal, because then the blackspots, where the antenna transmits a very much weaker signal, would be on the floor you are trying to serve rather than above and below. (Don't worry too much about someone directly below the AP, they will be so close they will get a good signal no matter what.) But is *probably* does make sense to angle them away from the vertical (in different directions) so as to maximize the effects of diversity and to help MIMO do its job. So the picture that TP-link posts here makes sense.

    http://www.tp-link.com/no/FAQ-455.html

    So, perpendicular doesn't make much sense, but angled away from the vertical does. Probably. But as you say, the best way to find out what works best is really to experiment.

    roy

    1. Terence Eden says:

      Thanks Roy for a very comprehensive answer!

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