tec Tip Videos

Wireless Bridging and the Fresnel Zone

Nathan explains some factors involved with wireless bridging, including a quick analysis of the Fresnel Zone.


Hello, my name is Nathan. Today, we're going to be talking a little bit about some of the wireless topics that we frequently are asked in our line of work be that cases or meeting with customers.

One of the questions that we get very frequently has to do with wireless bridging and signals reaching point A to point B and that brings us to the topic for today which is the Fresnel zone.

Fresnel is named for a French scientist and I've seen or heard many pronunciations of his name, "Fernel, Frenel, [Fo Shizzle Fizzle 00:00:41]." I've heard just about everything you can imagine for this guy's name. Wherever you come from, however you pronounce it, feel free.

This diagram here, pretty much describes what the drive of this is today. Here, you'll see there's two antennas crudely drawn, I apologize, with a mountain in the middle. Here is the example of the Fresnel zone. Essentially, when you have two antennas bridging a distance, they will communicate line-of-sight.

The radio waves will travel point A to point B in a straight line. However, along that transmission, the further they are apart, those signals will widen. Not really a big deal as long as those antennas are high enough to accommodate that signal widening.

What tends to happen is that between two bridged antennas, there will be obstructions. That's not the end of the world. Obstructions can be countered for by raising your antennas and there is of course an allowable limit of obstruction which we can get into.

The Fresnel zone just accounts or deals with how much of that signal do we need to let through and how wide will that signal get, the further these antennas are apart. What I have for you over here actually is a math problem that would allow you to figure that out if you're really into it.

We earned a little blue sad face because I'm not a big fan of math, but if you are, feel free to dive into that. At the end of the program though, we'll have a link for you that can do some of that figuring for you, but really the obstruction levels, the max is about 40% obstruction, the recommended is less than 20%.

Really, the point of this is that you want to have your antennas high enough to accommodate any obstructions or you could run in to situations one which happened not that long ago where we had a school that had a bridge link point to point and every day around 3:00 would lose their link and that was due to school buses pulling up in front of the school.

It took us forever to figure out that was what was causing it, but it can be the littlest things, but as long as more of that signal gets through than doesn't, you're going to be able to maintain that connection.

All right, so hopefully you've enjoyed our presentation and hopefully we'll see you back next time where I promise we won't have a sad face, we'll have a happy face and we'll find a reason to smile.