CWNP CWNA – RF Signal and Antenna Concepts Part 2
Semidirectional Antenna Now, a semidirectional antenna is trying to direct that signal in a specific direction. You might remember that I gave the example of this light pole, right, which sticks out of the ground and then has a piece that hangs over and then has the light that shines down and covers a pretty good area. but it’s certainly not omnidirectional. It’s just trying to light up an area, like I said, of a parking lot or a road or something else, maybe the front of your house, to…
Now, a semidirectional antenna is trying to direct that signal in a specific direction. You might remember that I gave the example of this light pole, right, which sticks out of the ground and then has a piece that hangs over and then has the light that shines down and covers a pretty good area. but it’s certainly not omnidirectional. It’s just trying to light up an area, like I said, of a parking lot or a road or something else, maybe the front of your house, to try to deter crime. So it works pretty well, but it’s not as focused as other types of antennas. But it does direct, as it says here, a signal in a specific direction. We can use it for short- and medium-distance communications.
And when we talk about the medium distance because of the directionality, remember that was called the passive gain that we got, then we do pretty well at getting more coverage over a longer distance than if it was omnidirectional. Patch panels and the yogi are the three different types of antennas that we’ll discuss in this category. And I’m sure that if you’ve ever had to run copper cable for a network, you’ve seen a patch panel like this one where you connect wires. And that’s not exactly what we mean by that, but you’ll see some examples of these different types of antennas.
So a patch antenna is usually used for outdoor point-to-point communications for up to almost a mile. What you’re seeing here in this first one is that the antenna itself looks very flat. And remember, this antenna is separate from the actual access point. Right. We still need to determine whether or not the access point is the actual AC transmitter. The cable would then connect to this, which would be mounted on the side of a wall. So if I had, let’s say, Building A—I hope that looks like a building—I just wouldn’t want to put windows in it. But here are the floors and building B, and if we wanted them to be able to talk to each other, then we would mount these antennas onto the walls, and they would be semidirectional, so that they’re trying to make sure they have coverage from one antenna to the other. And not too shabby point for point.
That means we’re pretty much bridging networks. We’re not creating an area where we could have people outside sitting on a bench and still have WiFi coverage. Now it’s designed to connect one wired network to another wired network. This is useful because there may be a city street, a freeway, or something similar between these two buildings. And many different ordinances would not allow me to string a cable because there are fibre optic cables that can certainly go beyond a single mile. They usually don’t let you use the telephone poles on the sides of the street any longer. You go to a lot of older communities, and you still see them.
So they wouldn’t let you string a cable across the street because it looks awful. And many times the city won’t let you come over here and dig a big trench under their road to be able to put the cable in. So it solves a lot of problems in many situations where we need to be able to connect one wired network to the other without having to pay the extra expense of, say, a service provider. Now the other one, of course, right? Still, you can see the antenna mounted on a wall, and the antenna is still going to be directional as far as the transmission of its signal. So they are commonly used as a central device, like I said, for coverage from the access point to another device—basically another antenna.
A planar antenna is also designed to provide coverage for things like long hallways with offices on each side. Or just imagine, if you will, a large city library, and when you go into that library, you see rows of books. Do people still go to libraries, or do we just all download them now and read them on our laptops? But anyway, so I’m making some rows of books, and we’re talking a significant-sized library here. And what I might do with a planar — again, it’s semidirectional — is position the antennas so that they cover one row and possibly place one on the other side so that it covers that row. And by doing this, I would be able to get coverage in that specific area or, like it says, in long hallways with offices on each side. That might work for you as well. It is determined by how semidirectional, or what the beam width of that access point is. So that’s where we might see the use of a simpler type of access point.
Now, the Yogi antenna, a little different type of creature, is used for short to medium-distance point-to-point communications for up to about 2 miles. Now, I realise if your experience with these antennas has been home use and you’re thinking, “2 miles,” I could stay online for a lot longer while I’m walking down the road. Now, even though they’re designed to have a high gain, they can be used for, like I said, a longer distance.
And one of the benefits is that you can instal it high on a wall, and you could tilt it toward the area because, remember, it’s like that street light. It’s designed to ventilate an area. So, again, if I think of having multiple buildings, maybe one building is really tall and one building is really small, and I need to make that coverage instead of putting my antenna in that direction where it’s going to miss the antenna on the other building, then you just give this thing a little tilt, right? You just take it out, tilt it, and change the direction so you can get to the other antenna. And likewise, you can tilt the other one back up. So that’s another reason why they talk about it being tilted downward to get to the area that needs to be covered.
Highly directional antennas are again used for point-to-point communications to provide “network bridging,” as we would call it, between two buildings. They truly provide what we would call the most focused service. What I said was like a spotlight with a very narrow beam width, which also means a big increase in gain, doesn’t it, as we constrict that radiofrequency and more of any of the other types? So one of them could be like a parabolic dish antenna.
Well, you know, we have antennas like that. Many of you in your house have paid a lot of money for somebody to put holes in your roof. So you can have that parabolic antenna come up here and get to your satellite coverage in the sky, so you can get your TV shows right, very narrowly. That’s why, I suppose, having someone instal them makes it easier to get the right aim. But we see those. They could also be put on very tall towers. One of the problems with putting these types of antennas on towers is that even though we can go a great distance with them, and we really can, by the way, the distance is somewhat affected by things like mountains and hills and right.
We still need what we call the line of sight for those communications. In fact, if they are too far apart, the curvature of the earth may act as an interference. Again, as an example, if you’re sitting on a flat area of ground and you’re looking off to the horizon, your visible distance is about 2 miles, right? When you’re down here, you can see maybe two miles if you’ve got good eyes. and then you can’t see anything else. Like, if there’s a city over here with a lot of skyscrapers, you’re not going to see them. But if I elevate myself to a higher height, then your line of sight becomes longer. And so those are other things we have to plan. When we use these types of equipment or antennas, how high up do they have to be? How far is the distance? Are there forests and trees and other obstacles that might be in your way? Now, beyond that, the grid antenna, basically what we see as a definition, has the spacing of wires determined by the wavelength of the frequencies that the antenna is designed for. And I’ve got a better image to show you in a slightly different example.
Now, an antenna array, we are told, is a group of two or more antennas that are integrated to provide coverage. And they operate together to do what? Beam forming. Now, one great example might be if you were out in—well, that’s not exactly the picture I wanted to show you here. My birth town is this city called Seattle, and they have this strange thing there called the Space Needle. But even though the Space Needle is not designed for this function, I’m just using it as an example that we could have an antenna pointing in one direction and another antenna pointing in another direction, and they would work together. But again, they’re covering more areas.
But it just makes more sense to have the somewhat directional quality of those antenna arrays most of the time. If I could have drawn the British Telecom tower, I would have had the opportunity to work with British Telecom. But what you see on this tower is that you see these dishes all over the place, and surprisingly, they actually don’t do anything anymore, but you still look at them. I don’t know if it’s an eyesore or not, but because of the elevation of the multiple antennas, they were able to broadcast, like, TV shows and frequencies, all to the different parts of London, and they worked together to broadcast that same signal.
We’re going to take a look at this concept called beam forming. It’s a method, basically, of concentrating your RF energy. Concentrating the signal, remember, means that the power output is going to be greater, and we want to maybe use this to make sure that it’s greater than the existing S and R that the receiver has. In other words, to be able to get over that noise floor and have a signal that provides great transmissions. So we’re going to take a look at some other options here for beam forming.
So one type of beam forming is what we call “static beam forming.” And it’s used again where we have a tower and many different types of directional antennas to give a fixed radiation pattern. Now, the static beam formation is going to use multiple directions clustered together, aimed away from the center. But you might notice that it may vary in terms of which channel and which frequency they’re using. Because one of the problems we have, as you saw with many of those polar charts, is that even though we have this beam width, remember we talked about how to determine what the beam width is?
We might still have some overlap with that beam that could interfere with the neighbouring antenna. So what we’re doing is making sure we don’t interfere, at least in this example, not only on the channels but also on the frequencies that we’re using. And we have them again on a pole, a tower, or wherever they are, aimed away from the centre point so that we can cover a large geographical area.
Dynamic beam forming is a little more interesting. We still have that centre tower, and we still have multiple types of antennas. But what we’re trying to do here—and, by the way, it says it’s focusing the RF energy in a specific direction and in a particular shape—is So, instead of having all of the antennas working at the same time, as with static beam forming, it will change the direction of the radiation pattern frame by frame. In other words, if I’ve got one antenna sending out a signal, it might be doing it because it determined that there was a receiver out there who wanted to hear that. If the receiver moves, then this dynamic beamforming will use the lobe to again be able to get to that user as they’re moving. So they make that change frame by frame.
Now, transmit beam forming is, as it says here, performed by transmitting a lot of phase-shifted signals with the hope and intention that they’re going to arrive in phase at the location where the transmitter believes the receiver is located. So it’s similar to the dynamics. But when we’re looking at these signals, and again, if I had this tower, I prefer drawing my towers vertically rather than looking down on them. And I’ve got all these antennas everywhere, little round antennas. At least that’s what it looks like from the front. What it’s going to do is send these signals in different shifts. Remember what the phase shift was? Right. 90-degree offset, 180-degree offset Hopefully not. But what it’s doing is trying to find the best shift, if you will, and sometimes something we haven’t talked about yet—polarisation—helps you get to the actual receiver that you’re trying to communicate with.
Antenna polarisation is a phenomenon of looking at the amplitude of the signal being sent. So, when a wave radiates from an antenna, its amplitude will oscillate, either straight up and down, vertically or horizontally. By the way, drawing that in a picture can be difficult at times.
The polarisation is designed to help us in our communications. If I’m sending something in horizontal polarisation and the receiving antenna is aligned vertically, we’re not going to have as good a receive frequency because we’re not matching the antenna. And so it’s important to have transmitting and receiving oriented in the same way to get the best possible signal. Now, indoors, we don’t worry about it as much because when things like reflection occur, that might change the polarisation of the actual radio frequency. And also, we’re very close to the antenna in an indoor type of setting, so we often don’t worry too much about that polarization, but certainly it’s more important for you when you’re thinking about doing outdoor coverage or travelling long distances.
We mentioned this in some of our other calculations when talking about these access points. And yes, I’m going to keep using external antennas like they used to be. And we asked the question, “Why do we need the two antennas?” Or, like with N, why are we using three antennas? But when we get to high throughput, we’ll talk about why we need a third or even more antennas and the distance between these two antennas, which, like I said, is usually around five inches. They didn’t have to be very far apart. But do you remember why I said that was important to us? One of the reasons was, of course, that we can receive signals at different times and separate them using something like the 2.4 GHz band.
And again, that could happen from my laptop because its transmission might bounce off of a wall before it comes in, and I might have another one that goes right there. And so we had this problem that we called “multi path,” and the goal of the access point was on a frame-by-frame basis: as it receives these signals and they’re not at the exact same time, it’s going to figure out which of those two antennas is the best one to use. And then it’s going to say, “All right, I’ll accept that frame on this antenna, and then drop the frame on the other one.” So it’s constantly comparing the signals from both antennas, and it’ll use whichever one of them has the best signal strength to receive that frame of data. And it’s amazing to me that they’re doing this frame by frame, choosing whichever one has the strongest signal strength. And it’s what we called “switch diversity.” And it was something we saw before the 811 standard came in, where we actually wanted to use multiple Nintendo’s to improve the performance.
So speaking of the eight or 211 Nor 8 or 211 NAC; they use a technology called multiple input, multiple output, or MIMO. I’ve heard some people call it MIMO, but I like MIMO better. And it’s a more sophisticated form of antenna diversity, where we’re actually going to take advantage of multipath. That is, when I look at my access point, especially now that we have that third antenna with N, I’m going to try to take advantage of having multiple antennas to receive a signal on and multiple antennas to transmit my signal on, in the hopes of increasing throughput significantly. And so that’s what we do—we want to use them concurrently. So the techniques basically are to send data, and we use multiple simultaneous RF signals. That’s a different set of signals.
And then the receiver, getting all those extra signals coming from these different antennas, would be able to get more data quickly. And of course, most of the time, your laptops aren’t quite as well designed as this. You usually have one radio, but that doesn’t mean your radio couldn’t also use multiple signals at the same time. So by getting more signals, basically getting more data channels, you could say that, in essence, it sounds like we’re doubling the effect of bandwidth. That’s certainly what we were trying to do with eight zero eleven N and eight two eleven AC, which, by the way, is our current best standard for high speed. I believe it was once referred to as “one gigabit per second” wireless, as many of the others, such as N, were. At best, I believe I’ve seen it at around 350 megabits, though you might read some documents that say it can be a little bit higher. But the reason they’re able to get that higher throughput is because of the MIMO technology.
So in this module, we looked a lot at information about antennas, about the frequency or actual beam forming, and about the way in which we can rate the area of coverage and why it was important to us. So we looked at those asthma and elevation charts.
We talked about omnidirectional, semidirectional, and highly directional antennas. Some of those we looked at were the antenna arrays and how we could use those arrays with things like static beam forming, dynamic beam forming, and even transmit beam forming. And then we looked at some of the other types of possibilities with technologies like MIMO, which take advantage of multiple radio frequencies.