Explore the antenna concepts used in wireless 802.11 devices.
- If you're dealing with radios, you're dealing with antennas. The 80211 standard is radio waves so, let's talk about antennas. Now as a computer person, I'm not an antenna expert. So if you're some radio person out there you're probably going to get a little upset at some of the stuff I say. What I'm gonna say in response to you is the CompTIA exams don't cover radio as deeply as you know it and somebody who really knows radios will come into the CompTIA exam and they actually know too much and end up getting questions wrong because they know too much.
So, keeping it simple, let's talk about the different types of antennas that we use on 80211 devices. Now I'm gonna start off with an antenna that for all practical purposes doesn't exist in the 80211 world. This type of antenna is called an Omni antenna. An Omni antenna is a single piece of metal that goes up in the air to a point. Omni antennas send out a radiation pattern that's shaped like a sphere. So if we have a Omni antenna that radiation pattern is going to emanate all directions equally from that sphere.
The reason I'm bringing that particular one up is because the CompTIA exams use the word Omni antenna. Now if you take a look on this table here you'll see like these antennas right here. Now if you look at this, you'll say well Mike, this is just one antenna sticking straight up. Is that an Omni antenna? And the answer is no. So what I've got here is a little antenna. And if you look at it real careful it looks like just one piece sticking straight up, right? Well, watch this. I've actually broke this one just for you.
So that you can see what's really happening inside. So what you're seeing is one piece of antenna that goes up, and an exact equal length of antenna goes straight down. What you're looking at here where you have two antennas that are opposite of each other, we call this a Dipole antenna. Dipoles are extremely common in the world of 80211. This guy here has three dipoles on him. With a dipole antenna, you get a very interesting radiation pattern.
So imagine taking a bagel and then stomping the bagel really hard so you end up with this flat donut shape radiation pattern and this radiation pattern works perfectly for, for example you want to just send 80211 on one particular floor in a building. This is the best way to do it because the Dipole's radiation pattern really leans toward and that's why you see so many wireless access points so many wireless network cards using a Dipole.
Because it's a very, very common thing to do. And it works great. So Dipoles are by far the most common. But they're not the only thing out there. The next one I want to talk about is this guy right here. This is what we call a Patch antenna. You'll see it's designed to be mounted on a wall, see those two wall mounts there? Patch antennas have a very interesting radiation pattern. It is exactly half of a sphere. So if I were to take this guy and put him up on a wall, he would radiate all this direction but he wouldn't radiate behind him.
So this could be a real benefit if you're have an outside wall you wanna have everybody get 80211 in the house but you don't want to project out on to the street, for example. Patch antennas are extremely common in enterprise environments. Now you can't just go by shape all the time. For example, this little wireless access point here he is a Patch. The reason I know he's a Patch isn't because is the shape, it's because I read the manual.
This little wireless access point right here is actually three Dipole antennas but they're mounted internally. This guy is electronically equivalent to this guy right here. Except they're mounted on the inside. So don't let the shape always be the definer. It is, you have to read the instructions to be able to figure out what it is. All right, so we've done Omni, we've done Dipole, we've done Patch and now I want to do the last one, and it's this guy right here.
This is what we call a Directional or more commonly called a Yagi, Y-a-g-i. Yagi antennas are extremely directional. If I were to define the radiation pattern for this guy what I'd have to do is take a football and stretch it (grunts) 150 feet apart. So you get what's know as a fresnel lens. And that is the actual shape. So guys like this, this type of Yagi are extremely popular.
If say I want to interconnect two buildings. I can have one on one building and one on another and we can shoot them together to be able to get really, really good connections that way. There is one more type of directional antenna I want to show you, it's right over here. Ah! This is a Parabolic. Parabolics are also directional antennas. They tend to be even stronger than Yagis.
I've actually got two of these. I can take these two antennas and assuming I can point them at each other very, very accurate I can shoot an 80211 signal about eight miles. That is nothing compared to what other people can do. So these are the different types of antenna shapes. Now what's actually interesting is let me show you something right here. So you can see this is a little Patch antenna on here. If I unscrew this what you're looking at there is what we call an SMA connector.
This is a very common type of connector. If I want to I'm not saying all wireless devices use that exact same connector, but most do. So it's fairly trivial for me to change, whoop! (laughs) Make it a little tighter. To change out antennas depending on what my particular needs are. Even with devices like these, if I want to I can tap into this and shoot off another antenna. So you're not stuck with using the antennas that came with the wireless device.
If you're in a situation that you need otherwise. I can actually put this guy right here onto my laptop and I can pick up every SSID within miles (laughs) with a Yagi like this. The thing to keep in mind though when we're dealing with all these different types of antennas is that we can change the size of the radiation pattern by increasing or decreasing the gain on the antenna. Gains are measured in decibels, or we say DBI. So we often have a set amount of gain, usually for Dipoles and such it's gonna be or Patches, even, it's gonna be a fairly small number.
3 DBI, 5 DBI. These are designed to shoot not that terribly far. A couple hundred feet at best. With highly directional antennas, you'll start to see decibel ratings with gains in the 30 range, maybe a little less than that, maybe 15, 25, 30 DBI range. And it's very important that we take these into consideration when we're putting these guys together. Because it's easy for us to get a big antenna and overshoot it.
But there's also a lot of situations where people are actually getting smaller antennas cause they don't want to bleed out so that people can sit on their front sidewalk and, you know get on Google or whatever it might be. So when we're talking about the relative strength of an antenna, make sure you're comfortable with the word gain. You will see situations on the exams where they're gonna ask you the type of antenna you need to place and what the relative gain would be and what kind of coverage it would be.
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