Making Sure Your Fresnel Zone Is Clear – New Wireless Installs

When you are looking at a point to point (P2P) wireless link you will hear people mention “line of sight”. As in, if I stand at one point can I see where the other antenna will be mounted. I once thought “as long as I can see the other antenna, I’m good…right?” Not exactly. There is this tiny thing called the Fresnel zone.

Wikipedia will give you the complete scoop, so I will paraphrase 🙂 The Fzone is the shape of the wireless signal between two radios. What most people don’t realize is that the wireless Fzone between two radios is shaped like a football. It is actually fatter in the middle and tapers down towards the ends. What this means is that if you have a large obstruction in the middle of your Fzone, you will get degraded signal. Degraded signal means lower throughput and possibly no connection at all! So, how does one figure all this mess out?

Find point A’s elevation, point B’s elevation and the elevation of point M, which is the halfway point between A and B. A great place to find this info is here. This is Daft Logics altitude finder. It will give you the altitude for a point on a google map.
Lets say that site A is 100′, B is 200′ and M is 120′.

Our setup

What we want to do is take the site with the lowest elevation and make it 0 by subtracting itself:
Site A now: 100 – 100 = 0

Then subtract Site A’s original value from the other sites:
Site B now: 200 – 100 = 100
Site M now: 120 – 100 = 20

Site A = 0, site B = 100 and site M = 20.

Leveling the playing field.

Then find the distance between point A and B “as the crow flies”, aka a straight line. You can again use Daft Logic for this.

We will say that our distance is 10 miles.

Now visit an Fzone calculator like this one. Put in your distance and frequency and it will give you the height of the Fzone at midpoint. I’m saying my distance is 10 miles and my frequency is 5800. This gives me an Fzone height of 46′ at 5 miles (the halfway point).

So using all of this information I know that my Fzone midpoint will be 50′ at 5 miles(site A elevation + site B elevation / 2). This is the center of the Fzone at 5 miles, and we know that from our Fzone calc that the Fzone is 46′. If we divide that in half, we get 23′. So now we take our 50′ midpoint for the Fzone, subtract 23′ and last subtract the mid point M’s elevation which is 20 and we end up with 7′.

That's a spicy meatball.

It looks like the earth isn’t hitting our Fzone, but if anything stands higher than 7′ at that mid point, it will start to eat into our Fzone. What can we do? We can raise up our antennas to increase our distance from earth.

I’m sure that was very confusing, but perhaps it made sense to a few 😛