[MLB-WIRELESS] [TIB] 25.8 dBi parabolic grid antennas (WAS: TIB...)

[Simon Butcher]

Hi Jason,

> I know this is a pretty broad question and there are a lot of 
> factors that can contribute to the answer...
> 
> I am interested in setting up a point-to-point link and just 
> wondering what kind of distance I would get using two of 
> these antennas.  You may assume there is good line of sight 
> and little interference.  I just won't to get like a minimum 
> and maximum idea.

Well, without getting too technical, I'll consider you'll have
worst-case cabling (3 dB feed-line loss at both sites) and are using
commonly found cards (15 dBm output). I'll also consider you want a very
safe SOM, somewhere around 10 dB or more, so you have room for things
like birds warming their feet on your microwave antenna's dipole in
winter :)

I'll also consider you have excellent line of sight, and will only take
into consideration a 'normal' amount of background noise (I think RF
nerds call it 'noise temperature'?)..

With two of these (one at either end, of course) _theoretically_ you
could run 802.11g (54 Mbps) up to 8 km, 802.11b+ (22 Mbps) up to around
32 km, and 802.11b (11 Mbps) up to nearly 39 km. You could go much
further too, if you dropped to lower speeds - low speed 802.11 (1 Mbps)
theoretically could run up to 92 km, although it's doubtful you'll get
even close to this distance.

These figures are based on the same calculations used by LocFinder, and
using reception sensitivity readings from D-Link hardware (the
802.11g-draft gear is the USA specification). Situations vary
enormously, especially when you start talking about noise floors (more
noise means a poor link).

You're likely to find troubles beyond 30km, I think, since none of this
gear was designed to run any further than that. Most of the problem,
AFAIK, comes from multi-path distortion (such as your beam bouncing off
a building in the middle of the link). 802.11 isn't equipped to deal
with multipath signals being received with fairly 'large' (in the
millisecond range) delays - That's something 802.16 can cope with.

I don't know about minimum distances, but I'd imagine if you put two of
these antennas 1 metre apart the signal would probably distort. Does
anyone know what the maximum RX tolerance of wifi is, if there is one?
I'm too lazy to skim over the 802.11b specs.

Hope this helps,

 - Simon


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