Is 5KM possible?

[garysims]

Hi,

I live in a small village which doesn't have broadband internet. The nearest two towns both have ADSL at 2Mbps and I am thinking of getting connected via a wireless connection.

There are several comanies which sell 2.4Ghz antennas and some claim distances of up 12KM when using directional antennas point to point.

Has anyone tried setting up a reliable 802.11b or g connection over 5KM or more.

Thanks,

Gary

[M/Q]

It is very possible. I suggest you search the forum using "long distance link" Then after getting a better understanding you can ask more specific questions as this subject is rather popular and as to not re invent the wheel so to speak.

[garysims]

Hi,

Thanks for your reply... I have searched the forums as your suggested and this is yielding good information.

My first specific question is: What is the best way to align the antennas?

After setting the direction generally, a friend of mine fine tuned the alignment t by connecting his laptop to the AP and pinging the other end. He would move the antenna and see if the roundtrip ms went up or down.

Is there any other tips on doing this. Over 5KMs I guess small movements of the antenna can make big differences.

Thanks,

Gary

[M/Q]

Thank you for researching that helps a great deal.

Good question as well. First you need to become friends with this website.

http://www.terabeam.com/support/calculations/index.php

The first issue is called the Fresnel Zone, and it is very important in any long distance link. it is where the RF signal zone has to have a clear path. In your case if there is nothing vertically in the way between the two sites your antennas have to be at least 12m above the ground. Now if there is something in the way you have to be 12m above it.

So that makes aligning antennas even more interesting. I typically use GPS and bearings to get initially in the ball park if you can not see the remote location. I am old and eyesight is getting bad. ;-) Then I use a PDA or small notebook with NetStumbler. It is a signal strength application. It is the same approach, except it is a bit more real time then pinging.

And you are correct, with the higher gain antennas the horizontal coverage angle gets quite small. And obviously it is best to be spot on if possible. This also brings up a point about antenna movement and wind loading. I am sure you can see right away what I am getting at.

[garysims]

Hi,

Thanks for your detailed replies... I have asked this and other wifi related questions in other forums and I haven't had an responses, so many thanks.

OK, next question... Due to the Fresnel Zone stuff (which I about half understand now) the height is obviously important. I wanted to transmit from the roof of a four storey building to the roof of a house over this 5KM distance. Today I have heard that the town from which I want to send on the block is 11m lower than the village where I want to receive.

So does that mean (ignoring the difference in the heights of the buildings) that I need to actually get the antenna 23m off the ground at the transmitting end?

Related to this is that there are some tall trees somewhere in the middle, I guess I need to add the height of those to the antenna heights?

Hmmm... This isn't looking good, that would mean I would need to have the antenna at the sending end maybe 28-30m off the ground.

Thanks,

Gary

[M/Q]

To understand it, just think of a ripple in a calm puddle. If you inject a piece of wood in the ripple it will remove that portion of the ripple it blocks. I am sure you have seen this, so that part of the signal is subtracted form the overall gain.

That is what the trees will do plus the fact that they contain water and that will absorb energy just like a microwave heating up a container of vegetables.

So in the best scenario you need to be at least 80% clear of the highest obstruction along the path between you and the remote site.

It is just a good thing that you do not have to worry about the curvature of the earth at this distance. Also this is all theoretical as you know. I just try to give you the best set of variables as I would feel really bad if I did not and you spent a great deal of money based on my advice and it did not work.

So with that in mind, even with the devices only 10m off the ground it may still work, as RF signals are not bound or totally understood by us humans. So, depending on your circumstances, will determine the amount of risk you are willing to take. I never gamble as I have really crummy luck. Just me though.