How to test line of site

We are looking at implementing a building to building long range wireless connection between our head office and a second site. the site is 7.5 KM (5 miles) away we are looking at using a 3com wireless device with a rated range of 15 KM, But I need to determine if I have line of site between the 2 locations to ensure a reliable connection.
Does anyone have any eperience with this? I need to know how one would go about determining if they have line of site. Is there a company one can call who will come out and do a test with special equipment. or is going on the roof with a pair of high power binoculars and simply seeing if I can see the roof of the other building good enough.
I have never connected two sites over this kind of distance before and would like some input from someone who may have done this before.

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Line of sight is exactly that.

Take for instance a mountain top repeater or tv station that beams signals down into a valley. That is a good example of line of sight in that if one can see the antenna, that is line of sight.

If you get on the roof of building A and can see the roof of building B, you will have great line of sight and a good wireless connection. If you have a building in the way, you won't. Some trees are OK, but noting solid.

At night, I have taken a laser pointer and set it on a tripod aimed at my other location and have been able to easily see that coherent light if it is not obscured and "aimed" closely.

There are companies that do have modulated lasers and and special IR laser receivers and can check line of sight points.

But, hopefully you can do it without spending a few hundred more dollars.


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>>going on the roof with a pair of high power binoculars and simply seeing if I can see the roof of the other building good enough.

This is the ultimate definition of Line-of-Site, and will work if you can see it.  

I have heard of folks having a person at each end with binoculars and laser pointers at night. Coordinated via cellphone.  You are only likely to see a red flash at either end.

A topographical map is also a good choice, since the important thing is not having a mountain between the sites.

A building between the sites is not likely to be a problem unless it is huge and close to either end.

As a ham operator, I would probably get a ham friend on one roof, with myself on the other and see if we could communicate with ht's using yagis.  So, if you have any ham friends its worth asking, since it seems like an interesting project...

Good Luck !
Looks like John and I were typing at the same time.
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We know it works anyway! And how to check line of sight!  :-)

Well, being able to see the other site through binoculars would be a good start. :-)
If you can do that, it will just be a matter of figuring out how tall is the highest obstruction between them.

For that distance, if using 802.11a, you'll want your antennae 20 feet higher than the tallest obstruction.
For 11b/g you'll want them 28 feet higher than the tallest obstruction. That should give 60% clearance of the first fresnel zone for each band. So even with clear line of sight, if there's a 25 foot tall house right in between them, you'll want both (highly directional) antennae either 45 feet (11a) or 53 feet (11b/g) up in the air.
Those are good rules of thumb for heights, but if they are tall buildings at each end, I have seen it work with antennas mounted on the edge of the roof or on the side of the roof access square.
Yes, I didn't say they had to be on towers. Lots of commercial buildings are over 40' tall.  I'm sure there are lots of houses more than 40' tall.  With no obstructions between them, 25' high on each end should be fine. But no lower. And with obstructions they need to be higher. If the obstruction is closer to one end just that one end can be raised, too... they don't both have to be the same height.
A critical factor in evaluating any line of sight path is calculating the Fresnel Zone (pronounced 'fre-nel' the "s" is silent). This is cone of space around the visual line-of-sight that radio waves spread out into after they leave the antenna. Signal waves spread out as they propagates; the individual waves that make up the signal don't move at a constant velocity; as a result, the antennas at each end of the signal path actually has a three-dimensional elliptical area between them that circumscribes the signal.This elliptical area
is divided into several zones based on the phase and speed of the propagating waves.This looks kind of like the Fresnel Lens you might see in an overhead projector; hence the name.

Radio line-of-sight is not the same as visual line-of-sight. A visual line of sight i a straight line between two points. An RF line of sight is not a straight line between the antennas; it is the elipse of the Fresnel zone. In a good point-to-point design, you should calculate this elipse to determine its horizontal and vertical size; you can then verify that the elipse will clear  obstacles to provide a good signal. This means ensuring enough height to provide vertical clearance at the halfway point, and enough clear horizontal space to provide that clearance at the same midpoint.

The Fresnel Zone must be clear or else signal strength will degrade. The Fresnel Zone is an issue in both  2.4 and 5.x GHz wireless systems. Although these signals readily penetrate building walls (that don't contain a lot of metal), they have a tough time passing through trees. That's because trees have a high water content, which attenuates any radio signals passing through them.

It's also why microwave ovens operate at 2.4 GHz -- they heat food by vibrating water molecules in the food, which absorbs RF energy and converts it to heat.

I've seen many times installers put in a system where they can clearly make out an antenna a mile distant, but only by peering through foliage or past tall buildings. This sort of antenna placement won't work very well. Plan for the Fresnel and save yourself a lot of grief!

A cool tool that calculates Fresnel zones, effective radiated power (ERP), and even takes into consideration the curvature of the Earth, is this one at a limey site:

A couple of things to consider in your decision and I will sign off, are:

If you use Yagi antennas with all the reflectors and active elements exposed, they can lose some signal strength over time due to the  connections and elements being in the open and birds, depending on your location, love them.  Nice perch.

Lastly, for your distance I might consider parabolic antennas that are vary narrow beam, generally higher gain but need more precise aiming. Using a parabola will, wihtout a doubt, lessen any worries you may have about the fresnel zone. This zone does exists with a parabola, just like any directional antenna, but again it is lessened.

Here's a link to a pretty good company that sells antennas of all sorts, amplifiers, and etc.


mattolanAuthor Commented:
Thats allot of good information. Thanks everyone. I hadn't considered fresnel zones.
We are up on top of a ridge at our main building and according to google earth the second site is 20 meters lower than we are. I am hoping there is nothing tall enough in between to cause an issue.

I think I am going to start by getting a set of binoculars and trying to pick our our second building from the edge of the ridge here. If I can do that I will have an idea of whats in between us for obstacles that might affect the signal
Has anyone helped you? Did you get the information needed?

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