RF Characteristics and Behavior

0:01Welcome to Life of an RF Signal. Now, RF is radio frequency, so we're talking wireless. And I'm a bit of an RF geek. Now, I started out way back in the day working with wireless and all different kinds of wireless, not just Wi-Fi wireless but radio wireless and satellite communications wireless. So I had to learn all about antennas and how they work and how radio frequencies actually travel along through the atmosphere and interact with different objects.

0:32And that's what we're going to look at, we're going to identify what an RF signal is and then look at the life cycle of that signal. So with no further hesitation, let's jump in. So RF signals, those are wireless signals that are free flowing through space and they are an alternating current signal, which you see here, which just means that they do something called oscillation. That simply means that they go up and down like a waveform, like a sine wave that we see here.

1:05Now, each of these signals actually operates at a different frequency. And in order to understand or receive that signal, you have to be on the correct frequency. Now, what determines the frequency of an RF signal? Well, it comes down to something called CPS or cycles per second. And that is, if we look at this waveform, it's the point for one wave to the next wave in the same point in the waveform.

1:36So here we're looking at the crests of the wave, that's the top of the wave. But we could also look at the trough or the bottom of the wave. So from one point to the exact same point on the next wave is one cycle. And cycles per second is the number of cycles that happens in one second. Pretty simple, right? So these frequencies that are cycles per second are measured in Hertz or we abbreviate it with a capital H lowercase z, Hertz.

2:06So if I have one cycle per second, I'm operating at 1 Hertz. If I have 100 cycles per second, then I'm operating at 100 Hertz. But guess what? When we talk about wireless and Wi-Fi. We're actually working in the gigahertz and that means billions of cycles per second. So that is a whole lot of cycles in one second as you can imagine. So that is how we measure a frequency or determine a signal's frequency.

2:38So let's take a look at some different frequencies. This is what we call the RF spectrum and this is all of the different frequencies that exist in the spectrum, starting way down here at the Hertz. Where we're dealing with 10 Hertz down here as we can see and we go all the way down to exahertz. So we've got hertz, kilohertz, megahertz, gigahertz, terahertz, and pitahertz, and then, of course, exahertz. Though that's a lot of Hertz, isn't it?

3:10But these are all different frequencies and as we go up. So what we're concentrating on in this wireless course is right in here and this is where we're looking at wireless networks, and specifically, we're looking at the 2.4 gigahertz and 5 gigahertz frequency ranges that we'll be working with. So there you go. That is the RF spectrum. So moving on, let's look at the life of an RF signal. Now, if we were in space and we had this transmitter here.

3:40And we go and transmit the signal like we have here, well, in space it's a vacuum, so there's nothing to slow the signal down. So in space, a wireless signal-- this is really cool now-- it goes as fast as the speed of light, 186,000 miles per second. So I want to make sure of this, it's miles, not meters. Miles per second. Now that is super fast, isn't it? Well, it sure is. But if we want to look at meters per second, well, that's 300 million meters per second, which we can represent as 3 times 10 to the eighth.

4:25And that is the speed of light and that's how fast wireless signals travel in space. And this is important because later on, we're going to talk about wavelength and how to determine those wavelengths, which actually have a lot to do with antennas and the communications and how it all happens. This is actually really important, and we're going to look at a formula that tells us how to figure out the wavelength for different frequencies later on. So in space though, there's nothing to slow that signal down, so it's just going to go and go and go and keep going, because there's nothing there, until it runs into something.

5:01And then once it runs into something, well, yeah something's going to happen. It's going to bounce off of it. It's going to go around it. Maybe it'll be absorbed by it. And we're going to talk about all this stuff in some upcoming videos on how RF signals react in different situations. So that is in space. But what about on Earth? Well, on Earth, we have things like trees and buildings and bodies of water and the atmosphere itself. We have rain drops and water moisture in the atmosphere that also affect RF signals and what happens to them.

5:34So what happens is as these radio signals on Earth go through the atmosphere and run into things, these radio signals start to lose power, because they bounce off of things or they get absorbed by things or they get fractured and they break in a whole bunch of different signals. Well, that all causes signals to lose power and that leads us to attenuation. So we have this signal here at the birth and it's a nice, strong waveform signal and it's going along but on Earth, it's running into things.

6:07It's going through the atmosphere and is slowly losing power until it basically flat lines and it becomes so weak that the receiver over here can no longer hear that signal. So that is the end of the signal lifecycle, so we say RIP Mr. or Miss wireless signal. Your life has come to an end. And that is a good thing actually, because if not, they would continue through the atmosphere for eternity and cause all kinds of noise pollution in the air.

6:39So that's what happens to the signal. So they're born a nice strong signal and as they travel through the atmosphere and run into things, they begin to attenuate. That's a very important word, let's write that down, attenuate. And that's where a signal loses power until they're no longer able to be received by a receiver over here. And that is the life of an RF signal. I hope those has been informative for you and I'd like to thank you for viewing.

0:00In RF or radio frequency wavelength is super important. Now, wavelength actually determines the size of antennas, because we base an antenna size off of the wavelength of the frequency that we're dealing with. It also determines how a signal is going to interact with different objects that it runs into. So as you can see, wavelength is very important. So that's why we're doing a whole lesson on wavelength. So without further ado, let's jump in and get started.

0:31What is wavelength? Well, it's the length of a wave. That's pretty straightforward. So from one point in a wave to the exact same point on the next wave is the wavelength however long this is. And we measure this in meters. So a wavelength is measured meters and represented by the Greek symbol lambda. Just like that. So that is wavelength. Now, as I mentioned, wavelength actually affects the dimensions of antennas, like how big they are.

1:08And the different size of the elements that are part of the antennas are all derived from the wavelength of the signal that we're working with. So it comes down to the frequency. Now, it also affects how these RF signals interact with different things they run into, buildings, trees, cars, desks, elevators. You name it as they go through the air. It actually determines how it interacts, which then comes back to attenuation that we talked about and how long that signal can be active in the air.

1:42Now, there is an inverse relationship between wavelength and frequency. So meaning when one goes up, the other goes down. That's the inverse part of it. Now, and that's between frequency and wavelength. Now, let's talk about this. I like to use the example of a slinky. Remember those toys from when you were younger? Or maybe you've seen the commercials for them, it's a slinky, it's a slinky. And then they showed the slinky going down the stairs here. Well, this is a great way to think about the relationship between frequency and wavelength.

2:15Now, if I have a slinky and if you were to stretch it out, it would kind of resemble a wave. But if I mash that together and I end up with something that looks like this, well, what is the wavelength, the size of these wavelengths? Well, they're pretty small, right? They're tiny because those little pieces of plastic or metal in the slinky are pressed up against each other. So we're talking millimeters of wavelengths. So that means I have itty bitty tiny wavelength, which inversely means I have a high frequency and the exact opposite is true.

2:50If I was to stretch that slinky out and stretch it way out, like this. And now, it goes across the room, well, those wavelengths, the distance between them, they might be actually like a whole meter. So that is a big, long wavelength. So that means my frequency inversely is a low frequency and that's the inverse relationship between the two. Just think about a slinky as you're stretching it out and putting it back together, as it goes down the stairs and that's what we mean by the inverse relationship between frequency and wavelength.

3:29Now, let's take a look at the formula for wavelength. Now, here's what it is. It's wavelength equals the speed of light, that's C, over frequency. So if we have a given frequency, we can identify the wavelength from using this formula. Now, let's take a look some examples down here. First of all, AM radio station. So if we are looking at the 800 kilohertz range or frequency like we looked at the spectrum earlier, the RF spectrum, well, that would be a wavelength of 375 meters.

4:03That is a long wave, right? And as we go down here look at the other signals, let's look at the 802.11 Wi-Fi. This is our 2.4 gigahertz range. Well, its wavelength is actually 0.125 meters. And I put it here in inches, so if you're here in the US, then you can kind of get a better idea of how long it is. 4.8 inches. And if we look at the 5 gigahertz Wi-Fi range, that's 0.06 meters or 2.4 inches.

4:33So it's very small. So the higher we go up in frequency here, as we're going up in frequency, what do we see our wavelength doing? Well, of course, it's going down. That's right. And that's that inverse relationship again. So this is how we determine the wavelength for a given frequency. And again, that wavelength has to do with how those signals are going to travel through the air and interact with different things that they run into. Could be trees and bodies of water or desks, elevators, cars.

5:04You name it. Anything that they could run into, as well as the size of the elements or pieces of hardware used in antennas. So again, that's the importance of wavelength. This is how we determine it. And that is wavelength. I hope this has been informative for you and I'd like to thank you for viewing.

0:00As we continue with RF and radio frequency, it's time to continue our study of the signals and look at some of their characteristics such as amplitude and phase. So let's jump in. So we're actually starting off with attenuation and we've talked about this previously. So attenuation is as the lifecycle of a waveform goes on, it slowly loses power and it does so because it runs into things.

0:31And it deals with resistance in the atmosphere. And as it ages, it attenuates or loses power. Now, if I was to take a look at this signal on something like an oscilloscope and measure the power, I would see that on the screen, the signal is getting smaller and smaller. Now, over here on the side, I would have some power rating over here and I would see that this signal at its beginning is strong in power and it begins to lose and gets smaller as it ages.

1:09And that, again, is attenuation and it's because of resistance in the atmosphere and things that it runs into. Now, moving on. We have frequency, which we've talked about measured in Hertz. And again, this is the cycles per second or number of complete waveforms in one second, the number of cycles of waveforms. And we're concentrating specifically in the wireless network area in this course, but you should understand that this is the RF spectrum here and that there are lots of different frequencies within the RF spectrum.

1:51And those are the frequency, and in order to transmit and receive a signal-- So if we have a transmitter over here and it's transmitting at 2.4 gigahertz, in order to receive that signal, we would need a receiver that is also listening at 2.4 gigahertz. So we need to listen at the same frequency as we're transmitting. So it's like if I'm speaking French to you and you don't speak French, well, you're not going to understand my message.

2:21The same thing applies in frequencies and radio signals. It comes down to the frequencies and you have to tune in to the right frequency. Just like when you were a kid and you played with walkie talkies. And before you start, you're like, hey, I'm on channel 12. And you'd have to set your walkie talkie to channel 12 so that you could hear the person you're playing with when they talk across the walkie talkies, which are always fun. Then we move on to amplitude. Now, amplitude is the size of the waveform.

2:54Now, we're not talking about meters. We're not talking about wavelength. Not today. Right now, we're talking about the size, so the height of the waveform. And that's because the bigger the waveform, the more power we have in it. So if we were to measure this, it would be able to provide us with a power rating. So when we have a transmitter over here that's transmitting a signal out of it, well, it's transmitting at a given amplitude or given power.

3:26And the same thing on a receiver. When we have a receiver receiving a signal, so the signal comes all the way over and is received by the receiver over here, then we have a received amplitude or received power. And remember, we talked about the attenuation a minute ago, because we used the example of an oscilloscope and we were measuring the strength of a signal. And when it started out, it was strong and it started to slowly attenuate. Well, as you see here, it's losing power. And that is where the amplitude and power relate.

4:01And lastly, we have something called phase. And I find this to be absolutely amazing. Now, this is one of my favorite things about RF. Now, we have different signals and we've talked about them. So if I show you a signal here and it's flowing through the air, well, that is a single signal. But if I have another signal at the same frequency traveling through the air as well, well, they have a relationship if they share the same space. Now that relationship is called phase.

4:32Now, if I have two signals-- Let me get rid of those last two I drew. There we go. And let's draw a new one here. This is a single signal here and I have another signal and it is traveling along the exact same frequency here, the same amplitude and everything. It's what we call in phase. They match up exactly. And when two signals are in phase, I like that in phase, they actually become additive and they make the signal stronger and more powerful.

5:12Think about it like this when we reference in phase, I want you to think of a choir. Because when you hear a choir singing and all those people are in the phase, they're saying the same words at the same frequencies at the same time, it becomes a really strong voice. And you hear them because they're in phase. Now, the opposite is true as well. If we have two signals that are out of phase-- let's draw another one here.

5:43There we go-- And I have another signal and it's a little bit out of phase-- oh, goodness, gracious-- well, what happens there? Well, they actually work against each other and it reduces the signal strength. So that actually reduces the signal strength, because they're interfering with each other. Think about this. When you go to a restaurant and it's noisy in there, you can hardly hear your own conversation. And if you try to eavesdrop on somebody on the next table over, well, you could barely hear them.

6:15You can't really make it out because there's all this noise and distraction. And that's what this. This is out of phase and that's exactly what you're hearing in that restaurant. Because they're cross talking. They're talking on top of each other. And it's just not a good thing. And here's something really interesting. When you have two signals that cross that are exactly, what we say, 180 degrees out of phase, meaning they're the exact opposite phase, believe it or not, they cancel each other out.

6:52That means you have a loss of signal at that point. So what we have is 180 degrees out of phase cancels. Now, what do I mean by degrees? Well, a waveform in itself is a complete circle. So that's 360 degrees. So like this signal over here that we're looking at could actually be 90 degrees out of phase, meaning that it's 1/4 of a wave cycle out of phase. And it doesn't matter. If it's out of phase, it's out of phase, except when it comes to 180 degrees, because then then they got cancellation.

7:25So keep that in mind as we move forward. We're going to look at some technologies that come into place like MIMO that actually take advantage of this and are able to provide us with better signal strength. So in phase, out of phase, really cool stuff. I hope this has been informative for you and I'd like to thank for viewing.

0:00Well, now it's time to take a look at RF Behavior. So we're going to look at how radio frequency or RF signals behave in different instances, like when they bounce off of things or when they might be absorbed by things or even scattered. So let's jump in and take a look at RF behavior. So starting off, we're talking about something called propagation. And what that is, that's simply how a signal travels, and it's just kind of a generic term. So think of it like this. If I drop a rock into a pond, well, then, I'm going to have the ripple effect.

0:35And that's how that signal or that energy caused by the rock penetrating the surface of the water travels across the water, and it creates that wave. Well, the same thing is true for radio waves. As they propagate to the air, they run into things, they bounce off of things, they get absorbed by things, and that's what we're going to take a look at. So starting off, we're looking at absorption, and that's the loss of an RF signal to a material as the wave passes through it.

1:07So that's as a wave passes through maybe a brick wall, for instance, and it will pass through it depending on the frequency. For example, a 2.4 gigahertz wave in our Wi-Fi spectrum, that goes through a brick wall, will be 1/16 the power it was when it comes out of that brick wall. So it loses a whole lot of power, as you can imagine. Well, it also gets absorbed by water. Water is a great absorber of radio waves.

1:40We're talking trees, shrubs, shrubbery, paper products, and things like trees that are full of water. Because if you try to shoot an RF signal through a tree during the summertime that's in full bloom, it's going to lose a lot of power as it goes through it. And that's absorption. Makes sense, right? Just like a sponge absorbs water, well, different objects absorb RF energy. Then we move to reflection, and this is the bouncing off of an object by an RF signal.

2:13So as a signal flows through the air, it will bounce off of things that are larger than the wavelength of the signal itself. So again, here's where wavelength comes into play. If I have a small wavelength, like we're talking our 2.4 gigahertz frequency range, it had a wavelength of 0.125 meters or 4.8 inches, it's a small wavelength. And if it goes against a building that's bigger than the wavelength, meaning bigger than 4.8 inches, which, of course, all buildings are bigger than that, it's going to bounce off.

2:53It's going to reflect off of it. And that is what reflection is. Now, reflection can lead to something called multipath, and that's because the signal will bounce off of it. And it could actually go in different directions, and it could bounce off a road off of there. And what you end up with is one signal that turns into multiple signals, and it creates multiple different paths of that signal. Now, that can be problematic because if I have a transmitter over here and I shoot a signal out, and it arrives at my receiver as one signal, hey, that's great.

3:36I have one signal to listen to. But if I transmit a signal and it ends up bouncing off of things and ends up at the receiver as multiple signals-- well, remember, we talked about something called phase-- and if those are out of phase, they're going to cause cancellation, and they're going to cause interference. And my receiver is going to have a hard time hearing the signals and discerning between them. So that's where something like multipath can be a problem, and reflection can cause that.

4:06Next up, we have scattering. Now, this is where we have a signal that passes through an object that is smaller than the wavelength of the signal. So again, wavelength comes into play, so when it's smaller. So for example-- we're going to stick with our 2.4 gigahertz frequency, so we're talking 4.8 inch wavelength here-- and if I had a bunch of tiny rocks in a pile and that signal bounced off, it's not a clean reflection, meaning like your reflection in the mirror.

4:42It's not a bouncing off one-off. It's going to scatter. Think of like a-- oh, remember when you were young and you went to the skating rink and they had the disco balls? And when the light hit it, it came off in all different kinds of beams, right? So when you shot one light at it, like a signal, it bounced off in all different kinds of directions. And that is what scattering is. Now, it could be a wire fence. It could be a tree that causes it. It could be rock.

5:13It's something that is smaller than the wavelength of that signal. And that is scattering. Then we come across refraction. And you've seen this before, probably in grade school when you did some science projects and you used a prism, and that did a refraction of light. And that is just what it is. This is a bending of the waves, RF waves, as they pass through an object that has different densities. And some examples of that are clouds and our atmosphere.

5:47And it could be things like changes in air pressure and water vapor. These are things that would cause RF refraction. And that's what that is. Then we have from refraction to diffraction. Now, this is a little different. Remember, refraction was the bending of waves as they go through an object. Well, diffraction is the bending of waves around an object. Just like here.

6:17This is my interpretation of-- no, not a log. [LAUGHS] Good guess, though. [CHUCKLES] This is actually supposed to be a river. And what we have here is a rock in the river. So we've got this rock, and as the water flows down the river, it's going around the rock. And that is diffraction. Now, when it comes to what could cause it for RF signals, it could be things like buildings. It could be hills. It could be lots of different things.

6:50But the idea is diffraction is the bending of radio waves around something. And that is diffraction. Now, we've got our body attenuation that we've talked about before. Now, attenuation again is the losing of RF power throughout the life cycle of a wave, and it could be because it goes through things like a door. Hey, this is a nice-- you like my drawing of a living room here? Well, that's because I was trying to show the drywall, because drywall will absorb signals and cause it to attenuate.

7:22Even going through glass, like a window up here, and definitely a brick wall. Remember, we said that if an RF signal at 2.4 gigahertz, RF signal goes through a brick wall, it's going to come out at 1/16 the original power. So that tells you, wow, that lost a lot of power. And, of course, different types of material, like wood or drywall or glass, all attenuate signals differently. Now, one thing to keep in mind-- the lower the signal, the better it will pass through things.

7:58So for example, a 2.4-gigahertz signal will pass through a brick wall or drywall better than a 5-gigahertz signal. And what that tells us is when we're dealing with Wi-Fi in 2.4-gigahertz frequency range in five years, that if we had an environment that we set up with 2.4 gigahertz access points and we decide that it's time to upgrade and we deploy new 5 gigahertz, then we cannot do a one-for-one replacement most likely because a 2.4 gigahertz passes through objects better than 5 gigahertz.

8:375 gigahertz is going to have less coverage depending on the environment. So we may actually need to deploy additional access points in the 5 gigahertz range. That's just something to keep in mind and to know. And lastly, we have something called free space path loss. Now, that is the attenuation of the signal as it spreads-- basically, through the atmosphere, and losing strength. So as we transmit a signal-- we've got a transmitter here, and here's our antenna-- and we're going to transmit the signal, it's going to go out.

9:12And I want you to think of wirelesses as something-- I like to actually say this is something I came up with, and I think it is. I've never heard of this, but free range-- let's write this out. Free range communications, or comms, because that's what wireless is. If you deal with a wired network, you have a wire, and it is containing the signal that's going through your network. But if you're using wireless, this is my marketing coming out, [CHUCKLES] free range, like free range chickens and free range eggs and free range all those things, well, wireless is free range communications because it is going to do that.

9:52It's going to go wherever it can. It's just going to spread and keep going. And that's what wireless does. And that's where free space path loss comes into play. Now, this is kind of a rule of thumb that every time a signal doubles in distance, it loses 75% of its power. So if we have a signal-- I've got a transmitter here-- and it is shooting a signal out and I'm receiving it at, let's say, 50 meters, and I was to measure the strength at 50 meters, I would have X.

10:32And then if I was to go ahead and as a signal goes out, I measure it at 100 meters, then I should have X minus 75% of its strength. So I would end up with 25% of X. And that is a rule of thumb. So that's something you definitely want to keep in mind when talking about free space path loss. The rule of thumb is every time you double the distance, then you lose 75% of that signal's power.

11:06And that is RF behavior and how RF signals work and react in different environments and some good facts to know. I hope this has been informative for you, and I'd like to thank you for viewing.

0:01Now, we've talked about something called multipath in previous videos, and we know that multipath can cause problems and it's caused by the reflection of RF signals. So we're going to take a look at what causes it, what's happening with it, and how we can address the problems it causes. So multipath, we've talked about this before, and we said that it's caused by reflection. So if I have an RF signal going through the air, and it bounces off of something, well, it can bounce off causing multiple iterations of the same signal.

0:37And that becomes a problem, because when I have a transmitter here and I transmit my signal out, well, when the receiver is over here and it receives a single signal, then it can read that signal. It does something called the modulation. And it works through converting that radio signal into bits and that's what it does and then it sends it on down the line. However, if I have a transmitter transmit a signal and I've got reflexion going on and I have multipath happening.

1:13And I end up with multiple signals arriving at the destination most likely at different times. So something called a delay spread, that is the time between the signals arriving and it's generally in nanoseconds, because it is a very short period. Well, what happens is those signals are probably going to be out of phase. Now, remember we said in phase signals, they work additive, so they actually help strengthen the signal.

1:47But out of phase signals did the exact opposite and they caused degradation. So when we talk about in phase signals, there's a term called up fade. And that references the fact that there is an increase in signal strength when the signals arrive in phase. But the same is true when we have out of phase and the out of phase, we refer to as a down fade and that's because there is a reduction in power.

2:21And there is a loss and there's actually nulling as well. So if I was to say nulling, null usually is a negative annotation, that's when those signals arrive 180 degrees out of phase, which we said caused cancellation, which is also known as nulling. And believe it or not, when the receiver receives these, as it's going through it's the modulation techniques and it's working on those signals, it can lead to data corruption.

2:53And data corruption happens because in our receiver-- this is our receiver here-- once a signal arrives, it goes through a process of demodulation, which is where we take that signal here and we convert it to 1's and 0's and then we send it on down the line to our equipment. Well, during this process of demodulation, bits can actually overlap, and this is known as ISI or inter-symbol interference.

3:27And what we can do is use something called CRC or cyclic redundancy check to address this problem. So the problem is data corruption. When I've got these different signals coming in here and there are at different times, it can actually cause the modulation to overwrite some bit, so that they're not exactly the same. They're not lining up. So we have ISI or inter-symbol interference that is causing our data corruption.

3:59And we address that through our cyclic redundancy check. Now, what that is is when we're sending these data frames through the air, if we were to dissect a frame and we see that there's different information, like a source address, there'd be a destination address, and there's data in a whole bunch of stuff in here. Well, there's also a piece of the frame called the CRC or cyclic redundancy check. And that's where the system before transmitting this wireless frame actually takes a look at the data here and it runs a mathematical algorithm against it to create a value.

4:35And that's known as the CRC. So when the receiving station receives this frame, they're going to take a look at the CRC and they're going to do the same mathematical equation. And if the CRCs equal each other, then we know that there's no corruption. But if there is corruption, then basically, the system's not going to acknowledge it received this frame. So the transmitting station is going to rescind that to address the problem with data corruption.

5:08But when we're dealing with real time data technologies, like voice over IP or streaming videos, well CRCs don't play into that. Because we don't want that corrupted data back. Think about it, if you're talking on the phone with somebody or watching a video and you drop a frame or two because of errors, well, you don't want that video or audio to replay a second later once it's re-transmitted, right? No, not with real time communications. We don't want that. But when we're talking about sending messages or transferring files or browsing the web, all of that does use the CRC.

5:46So again, multipaths can lead to data corruption and we can address that with our cyclic redundancy check. So now that we know what problems multipaths can cause, well, how do we deal with it? Well, there's a couple different ways. First of all, we can use directional antenna. So the idea is let's corale this RF signal. Remember I mentioned earlier that this is like free range communications. Well, let's try to limit where this signal can go, because if I can control that, maybe I can control multipath some.

6:21So instead of sending the signal everywhere it wants to go, maybe I use a directional antenna here to say, signal, you are only going to go in this direction. You're not going to go everywhere. And I can use a directional antenna to help me with that. The other thing is I can use something called antenna diversity. And antenna diversity is where an access point or radio, we call it, like this here, has multiple antennas.

6:53And it has the ability to say, hey, I'm receiving a really good signal on this antenna but the other antennas aren't so good. So what it'll do is simply not listen for the signal on those antennas and only receive the good signal. And that's where our antenna diversity comes into play. That's how it works. So it allows some intelligence on the receiving station to only receive a good, strong signal and to avoid those that are causing interference or that are out of phase and may cause some data corruption.

7:31Another thing is we could try to reduce the transmit power. If I'm sending a signal and it only needs to transmit at 50 milliwatts to get where it's going, well, why should I transmit at 100 milliwatts? That's going to send that signal that much further causing more multipath, so I could reduce the power and that could help with my multipath. The other thing is something called MIMO and that is a multiple input, multiple output and it's antenna diversity that we talked about using something called maximum ratio combining or MRC, that's MRC.

8:12And it does that to take advantage of multipath, so it's listening. It's got all these ears, these antennas-- that's not a very good looking ear-- but these antennas act like ears and they're listening. Well, it can actually select the best antenna-- that's what we talked about, antenna diversity-- but it can also use some technology to combine signals that are out of phase and maybe delay them just a brief nanosecond to get them in phase so that they are combining strength.

8:46So MIMO can actually take advantage of multipath path and provide this better signal strength. And we're going to have a whole video talking about MIMO and some advanced technologies a little later on. But that is one way to deal with multipaths. So again, we can use directional antennas. We can reduce signal strength, use antenna diversity and look at using MIMO. And those are different ways we can deal with multipaths. I hope this has been informative for you and I'd like to thank you for viewing.

0:00When working with RF signals, well, sometimes we need to increase that signal and there's a couple of different ways we can do that. And we do that through a process called gain where we're achieving gain on a signal. So we're going to jump in and take a look at RF gain as well as something called voltage standing wave ratio or VSWR. So let's get started. There are times when we want to increase the strength of an RF signal and we do that by using something called gain.

0:32There's a couple different types. We have active gain and we have something called-- you guessed it-- passive gain. Now, the difference is active game is when we use an amplifier. So we have something that uses an external power source. It plugs in and it amplifies our signal. You're probably familiar with amplifiers. We use amplifiers in electronics. In your stereo system there's amplifiers and such and in cars.

1:02So that is an amplifier. But passive gain, now you might not be familiar with that. Passive gain is when we use an antenna. So yes, we use an antenna to actually increase our signal strength. And that's because as we radiate an RF signal-- I'm going to draw an antenna here and this we refer to as a dipole antenna. It's just one of those you see off of an access point. So if I have an AP and I've got a couple different antennas, well, that's it right there.

1:33Well, this radiates or sends out a radio signal equally in all directions. So it sends it out like this everywhere. Well, I can actually use the design of an antenna to take maybe this RF energy over here and focus it in that direction. So I end up with an antenna that is actually sending out a signal only in this direction. So I've taken all the energy that was over here and I focused it over there.

2:06And that actually increases the signal strength that's over here because I focused that energy. And that is antenna gain. And that is via passive gain and that's really what it is. It's pretty neat stuff. Well, sometimes we need to measure these RF signals. We need to know how strong they are. And there's a couple different tools we use to do that with. One is a spectrum analyzer. And a spectrum analyzer basically allows you to look at an area-- so on the screen you would see it like this-- of frequency.

2:45So down here, it might say 2.4 gigahertz and over here 2.6 gigahertz. There we go. And what you would have in between is 2.5 gigahertz. So you're looking at an area of frequency and it would look like something like this where you are seeing signals that are within the frequency ranges. And you would see down here at the bottom, this is called the noise floor down here where there's not much going on. But then once you get up here and you see an active signal, well, that tells you that there's a signal transmitting at that frequency and that is what a spectrum analyzer does.

3:23And we use those all the time in wireless to do a wireless site survey or troubleshooting to see if there's interference from something else. Because believe it or not, other things use the same frequency as our Wi-Fi networks like microwave ovens, believe it or not. If you turn on a microwave oven and it's not insulated properly and grounded properly, it will actually cause interference in this spectrum. And you will see it on your spectrum analyzer. There's another tool we use also called an oscilloscope.

3:56And an oscilloscope is used to measure a single signal at one time and we measure the power of that signal. So if you look at the screen of the oscilloscope, what you're seeing is not the actual frequency per se like you would on a spectrum analyzer, but you're actually going to be looking at the oscillating signal as it goes along. So you're able to then take a look and measure the power by looking at the-- that's right-- amplitude of the signal that you're analyzing.

4:33So the oscilloscope, let's just take a look at the single signal. And we really don't use this a whole lot in wireless troubleshooting. This would be more for electronics and working with the design of some things like the design of antennas and transceivers, transmitters and receivers. But for wireless networking and troubleshooting, we generally use a spectrum analyzer. Now we have our VSWR, Voltage Standing Wave Ratio. Now what this is is pretty simple. I want you to think of kind of like-- let's think of water pipes for instance.

5:08So if I've got-- I'm going to say down here at the bottom, I've got a pipe here and this is a 3-inch diameter pipe. And I've got water flowing through here and it's coming through a lot. I've got this full power and I'm just blowing water through here. And this is my 3-inch pipe and I choke that down to a 1-inch pipe. What is going to happen? Well, think about all this water when it hits this flat surface. It's going to bounce back. Let's change colors here. It's going to bounce back and it's going to cause something called back pressure.

5:44And what this does is it causes interference. Now, some water will flow through here, but you're going to have this water that's circulating back through here because of the back pressure. So it's going to cause you to be able to send less water through the pipe. And then, of course, we're shrinking the pipes size, so you have that as well. Now I want you to now think about a radio transmitter or RF transmitter like we have right here. And every piece of equipment in this line-- so we go from the transmitter to a connector here to the cable to other connectors to antennas-- all pieces have something called impedance and it's represented by the ohm symbol, O-H-M.

6:33That's actually the omega symbol or we refer to it as ohms, so who put omega. And that symbol is used to represent impedance. Now, impedance is resistance to a signal. So it's resistance to an AC current or a signal. So when we send a signal through equipment or down a wire, there is some inherent resistance in that material and it receives an ohm rating or an impedance rating.

7:06And now that impedance is very important because it's just like these pipes. This could be like a 3 ohm impedance and a 1 ohm impedance. Well, obviously they're different, so we end up with this back pressure thing going on down here. Now, here's the key with VSWR. In order to avoid it-- and we're talking about what it is here in a minute-- but in order to avoid it, we need to practice impedance matching.

7:36And that just means that every piece of equipment we use in the line here-- that means from our transmitter all the way to and including the antenna-- they need to have the same impedance rating. It's fairly simple. A very common rating is 50 ohms impedance. And it just means you want to use a 50 ohm transmitter, a 50 ohm connector, 50 ohm wire, 50 ohm connector, 50 ohm antenna. You kind of get the point. As long as you stay the same, it's like having a water pipe that's all the same size.

8:08You don't get any of that back pressure. And that's what VSWR is like. It's like back pressure and what it does is it causes a loss of power. And we represent VSWR through a ratio. All right, I'm going to clear a little space here. Let's move just a little bit, so we can talk about the VSWR ratio. There we go. All right, now VSWR ratio is how VSWR is represented and it's a ratio between the highest transmit power along the way and the lowest.

8:44So we have the highest transmit and the lowest along the way and it's a ratio. So if I have a 1 to 1 VSWR ratio-- let me put the VSWR in here-- that means the highest and lowest power are the same. So I didn't have any loss. But if I have a 2 to 1 then I have some loss. Or if I have a 6 to 1 then again I have even more loss because the highest power was 6, the lowest power is 1 then I have a-- numerically, I've lost 5 whatevers of power pr here 2 whatevers of power.

9:24Now, the take away from this is you should know what VSWR is that we just talked about. And we avoid it by using impedance matching. Remember, our impedance rating. And we were expressing VSWR in ratios, a perfect VSWR is 1 to 1. Everything else we're losing power, so that's not a good thing. And that is VSWR. We have one more thing in this lesson and this is return loss.

9:55This is the difference between the power sent to the antenna and the power being reflected back to the transmitter. Now, this goes back to, believe it or not, VSWR. Remember, we talked about the water? Pipes and we went from a big pipe to a little pipe. We had that back pressure. Well, the same thing happens with VSWR and the energy because those signals-- if we draw this out a little bigger-- those radio signals are going down through here. And if we have some reflect back, what do you think is going to happen?

10:29Well, remember we talked about phases and signals being in phase, not a phase? Well, it's going to cost signal loss and that signal is going to go back down the line. And that's causing interference and it's causing us to lose. And return loss is the difference-- that's key here-- the difference between power sent and power reflected back. So if we were to take a look and we said we had 100% power sent and we had 0% reflected back, the difference between those is 100, which is great because we don't have any reflected back.

11:10And the higher the number of return loss, the better. So if I had a 100% power transmitted-- which I'm going to have 100%-- and I had a 10% return, then the difference is 90. I mean, that's pretty simple math. The key here is the difference between them, higher is better. So in an ideal situation, we would have a return loss of 100%. Now, when you go and look at return loss, it is rated in decibels, and that is something we're going to take a look at in some upcoming videos.

11:48But for now, just remember the higher the return loss, the better. Because if we don't have anything coming back, meaning we don't have VSWR problems, that's a good thing. Because believe it or not, if your VSWR gets high, that means those signals are coming back against the flow. They can actually damage your transmitter because they can get high enough that they can cause it to overheat. So that is gained VSWR and return loss.

12:19I hope this has been informative for you and I'd like to thank you for viewing.