RF Characteristics and Behavior

0:01Welcome to Life of an RF Signal.

0:03Now, RF is radio frequency, so we're talking wireless.

0:07And I'm a bit of an RF geek.

0:10Now, I started out way back in the day working with wireless

0:13and all different kinds of wireless,

0:15not just Wi-Fi wireless but radio wireless and satellite

0:20communications wireless.

0:21So I had to learn all about antennas and how they work

0:24and how radio frequencies actually travel along

0:28through the atmosphere and interact

0:30with different objects.

0:32And that's what we're going to look at,

0:34we're going to identify what an RF signal is and then look

0:38at the life cycle of that signal.

0:40So with no further hesitation, let's jump in.

0:45So RF signals, those are wireless signals

0:48that are free flowing through space

0:51and they are an alternating current signal,

0:54which you see here, which just means that they do

0:56something called oscillation.

0:59That simply means that they go up and down like a waveform,

1:02like a sine wave that we see here.

1:05Now, each of these signals actually

1:07operates at a different frequency.

1:09And in order to understand or receive that signal,

1:12you have to be on the correct frequency.

1:16Now, what determines the frequency of an RF signal?

1:20Well, it comes down to something called

1:23CPS or cycles per second.

1:26And that is, if we look at this waveform,

1:29it's the point for one wave to the next wave in the same point

1:35in the waveform.

1:36So here we're looking at the crests of the wave, that's

1:40the top of the wave.

1:42But we could also look at the trough

1:44or the bottom of the wave.

1:45So from one point to the exact same point on the next wave

1:48is one cycle.

1:50And cycles per second is the number

1:53of cycles that happens in one second.

1:55Pretty simple, right?

1:56So these frequencies that are cycles per second

1:59are measured in Hertz or we abbreviate it with a capital H

2:04lowercase z, Hertz.

2:06So if I have one cycle per second,

2:09I'm operating at 1 Hertz.

2:12If I have 100 cycles per second, then I'm

2:16operating at 100 Hertz.

2:18But guess what?

2:19When we talk about wireless and Wi-Fi.

2:21We're actually working in the gigahertz

2:25and that means billions of cycles per second.

2:28So that is a whole lot of cycles in one second

2:31as you can imagine.

2:33So that is how we measure a frequency

2:35or determine a signal's frequency.

2:38So let's take a look at some different frequencies.

2:41This is what we call the RF spectrum

2:44and this is all of the different frequencies that

2:47exist in the spectrum, starting way down here at the Hertz.

2:51Where we're dealing with 10 Hertz down here as we can see

2:55and we go all the way down to exahertz.

2:59So we've got hertz, kilohertz, megahertz, gigahertz,

3:03terahertz, and pitahertz, and then, of course, exahertz.

3:07Though that's a lot of Hertz, isn't it?

3:10But these are all different frequencies and as we go up.

3:13So what we're concentrating on in this wireless course

3:16is right in here and this is where

3:18we're looking at wireless networks,

3:20and specifically, we're looking at the 2.4 gigahertz

3:24and 5 gigahertz frequency ranges that we'll be working with.

3:29So there you go.

3:30That is the RF spectrum.

3:32So moving on, let's look at the life of an RF signal.

3:37Now, if we were in space and we had this transmitter here.

3:40And we go and transmit the signal

3:42like we have here, well, in space it's a vacuum,

3:46so there's nothing to slow the signal down.

3:49So in space, a wireless signal-- this is really cool

3:52now-- it goes as fast as the speed of light,

3:55186,000 miles per second.

4:00So I want to make sure of this, it's miles, not meters.

4:03Miles per second.

4:04Now that is super fast, isn't it?

4:07Well, it sure is.

4:08But if we want to look at meters per second,

4:11well, that's 300 million meters per second,

4:19which we can represent as 3 times 10 to the eighth.

4:25And that is the speed of light and that's

4:27how fast wireless signals travel in space.

4:30And this is important because later on, we're

4:32going to talk about wavelength and how

4:35to determine those wavelengths, which actually have a lot

4:38to do with antennas and the communications

4:41and how it all happens.

4:42This is actually really important,

4:44and we're going to look at a formula that tells us

4:46how to figure out the wavelength for different frequencies

4:50later on.

4:51So in space though, there's nothing

4:53to slow that signal down, so it's just

4:55going to go and go and go and keep going,

4:58because there's nothing there, until it runs into something.

5:01And then once it runs into something,

5:02well, yeah something's going to happen.

5:03It's going to bounce off of it.

5:05It's going to go around it.

5:06Maybe it'll be absorbed by it.

5:07And we're going to talk about all this stuff in some upcoming

5:10videos on how RF signals react in different situations.

5:14So that is in space.

5:15But what about on Earth?

5:16Well, on Earth, we have things like trees and buildings

5:20and bodies of water and the atmosphere itself.

5:24We have rain drops and water moisture

5:27in the atmosphere that also affect RF signals

5:32and what happens to them.

5:34So what happens is as these radio signals on Earth

5:37go through the atmosphere and run into things,

5:40these radio signals start to lose power,

5:41because they bounce off of things

5:44or they get absorbed by things or they get fractured

5:48and they break in a whole bunch of different signals.

5:51Well, that all causes signals to lose power

5:53and that leads us to attenuation.

5:57So we have this signal here at the birth

6:00and it's a nice, strong waveform signal and it's going along

6:04but on Earth, it's running into things.

6:07It's going through the atmosphere

6:09and is slowly losing power until it basically flat lines

6:14and it becomes so weak that the receiver over here

6:17can no longer hear that signal.

6:19So that is the end of the signal lifecycle,

6:22so we say RIP Mr. or Miss wireless signal.

6:27Your life has come to an end.

6:29And that is a good thing actually,

6:31because if not, they would continue

6:33through the atmosphere for eternity

6:35and cause all kinds of noise pollution in the air.

6:39So that's what happens to the signal.

6:41So they're born a nice strong signal

6:43and as they travel through the atmosphere and run into things,

6:47they begin to attenuate.

6:48That's a very important word, let's write that down,

6:51attenuate.

6:52And that's where a signal loses power

6:53until they're no longer able to be received

6:57by a receiver over here.

6:58And that is the life of an RF signal.

7:02I hope those has been informative for you

7:03and I'd like to thank you for viewing.

0:00In RF or radio frequency wavelength is super important.

0:05Now, wavelength actually determines the size

0:09of antennas, because we base an antenna

0:12size off of the wavelength of the frequency

0:16that we're dealing with.

0:17It also determines how a signal is

0:20going to interact with different objects that it runs into.

0:23So as you can see, wavelength is very important.

0:25So that's why we're doing a whole lesson on wavelength.

0:28So without further ado, let's jump in and get started.

0:31What is wavelength?

0:33Well, it's the length of a wave.

0:35That's pretty straightforward.

0:37So from one point in a wave to the exact same point

0:42on the next wave is the wavelength

0:45however long this is.

0:47And we measure this in meters.

0:50So a wavelength is measured meters and represented

0:53by the Greek symbol lambda.

0:56Just like that.

0:57So that is wavelength.

1:00Now, as I mentioned, wavelength actually

1:04affects the dimensions of antennas,

1:07like how big they are.

1:08And the different size of the elements

1:10that are part of the antennas are all

1:12derived from the wavelength of the signal

1:15that we're working with.

1:16So it comes down to the frequency.

1:19Now, it also affects how these RF signals interact

1:23with different things they run into, buildings, trees, cars,

1:27desks, elevators.

1:29You name it as they go through the air.

1:31It actually determines how it interacts,

1:33which then comes back to attenuation

1:36that we talked about and how long that signal

1:39can be active in the air.

1:42Now, there is an inverse relationship between wavelength

1:46and frequency.

1:47So meaning when one goes up, the other goes down.

1:50That's the inverse part of it.

1:52Now, and that's between frequency and wavelength.

1:54Now, let's talk about this.

1:56I like to use the example of a slinky.

1:59Remember those toys from when you were younger?

2:01Or maybe you've seen the commercials for them,

2:03it's a slinky, it's a slinky.

2:05And then they showed the slinky going down the stairs here.

2:08Well, this is a great way to think

2:10about the relationship between frequency and wavelength.

2:15Now, if I have a slinky and if you were to stretch it out,

2:19it would kind of resemble a wave.

2:21But if I mash that together and I end up

2:25with something that looks like this,

2:27well, what is the wavelength, the size of these wavelengths?

2:30Well, they're pretty small, right?

2:32They're tiny because those little pieces

2:33of plastic or metal in the slinky

2:35are pressed up against each other.

2:37So we're talking millimeters of wavelengths.

2:40So that means I have itty bitty tiny wavelength, which

2:44inversely means I have a high frequency

2:48and the exact opposite is true.

2:50If I was to stretch that slinky out and stretch it way out,

2:55like this.

2:57And now, it goes across the room, well, those wavelengths,

3:02the distance between them, they might be actually

3:05like a whole meter.

3:06So that is a big, long wavelength.

3:10So that means my frequency inversely is a low frequency

3:15and that's the inverse relationship between the two.

3:18Just think about a slinky as you're

3:19stretching it out and putting it back together,

3:22as it goes down the stairs and that's

3:25what we mean by the inverse relationship between frequency

3:28and wavelength.

3:29Now, let's take a look at the formula for wavelength.

3:33Now, here's what it is.

3:34It's wavelength equals the speed of light,

3:37that's C, over frequency.

3:40So if we have a given frequency, we

3:42can identify the wavelength from using this formula.

3:47Now, let's take a look some examples down here.

3:50First of all, AM radio station.

3:52So if we are looking at the 800 kilohertz range or frequency

3:56like we looked at the spectrum earlier,

3:58the RF spectrum, well, that would

3:59be a wavelength of 375 meters.

4:03That is a long wave, right?

4:06And as we go down here look at the other signals,

4:08let's look at the 802.11 Wi-Fi.

4:11This is our 2.4 gigahertz range.

4:14Well, its wavelength is actually 0.125 meters.

4:19And I put it here in inches, so if you're here in the US,

4:22then you can kind of get a better idea of how long it is.

4:254.8 inches.

4:27And if we look at the 5 gigahertz Wi-Fi range,

4:30that's 0.06 meters or 2.4 inches.

4:33So it's very small.

4:35So the higher we go up in frequency here,

4:38as we're going up in frequency, what

4:40do we see our wavelength doing?

4:42Well, of course, it's going down.

4:44That's right.

4:44And that's that inverse relationship again.

4:47So this is how we determine the wavelength for a given

4:51frequency.

4:52And again, that wavelength has to do

4:54with how those signals are going to travel through the air

4:57and interact with different things that they run into.

5:00Could be trees and bodies of water or desks,

5:03elevators, cars.

5:04You name it.

5:04Anything that they could run into, as well

5:06as the size of the elements or pieces of hardware

5:11used in antennas.

5:14So again, that's the importance of wavelength.

5:16This is how we determine it.

5:18And that is wavelength.

5:19I hope this has been informative for you

5:21and I'd like to thank you for viewing.

0:00As we continue with RF and radio frequency,

0:03it's time to continue our study of the signals

0:06and look at some of their characteristics

0:08such as amplitude and phase.

0:11So let's jump in.

0:13So we're actually starting off with attenuation

0:16and we've talked about this previously.

0:18So attenuation is as the lifecycle of a waveform

0:23goes on, it slowly loses power and it does

0:28so because it runs into things.

0:31And it deals with resistance in the atmosphere.

0:34And as it ages, it attenuates or loses power.

0:39Now, if I was to take a look at this signal on something

0:43like an oscilloscope and measure the power,

0:45I would see that on the screen, the signal

0:51is getting smaller and smaller.

0:53Now, over here on the side, I would have some power rating

0:57over here and I would see that this signal at its beginning

1:02is strong in power and it begins to lose and gets smaller as it

1:08ages.

1:09And that, again, is attenuation and it's

1:11because of resistance in the atmosphere and things

1:16that it runs into.

1:18Now, moving on.

1:19We have frequency, which we've talked about measured in Hertz.

1:24And again, this is the cycles per second or number

1:28of complete waveforms in one second, the number of cycles

1:33of waveforms.

1:34And we're concentrating specifically

1:37in the wireless network area in this course,

1:41but you should understand that this is the RF spectrum here

1:45and that there are lots of different frequencies

1:49within the RF spectrum.

1:51And those are the frequency, and in order

1:53to transmit and receive a signal--

1:56So if we have a transmitter over here

1:58and it's transmitting at 2.4 gigahertz,

2:03in order to receive that signal, we

2:05would need a receiver that is also

2:09listening at 2.4 gigahertz.

2:12So we need to listen at the same frequency

2:14as we're transmitting.

2:15So it's like if I'm speaking French to you

2:18and you don't speak French, well,

2:19you're not going to understand my message.

2:21The same thing applies in frequencies and radio signals.

2:26It comes down to the frequencies and you

2:28have to tune in to the right frequency.

2:30Just like when you were a kid and you

2:31played with walkie talkies.

2:33And before you start, you're like, hey, I'm on channel 12.

2:36And you'd have to set your walkie talkie to channel 12

2:40so that you could hear the person you're playing with when

2:44they talk across the walkie talkies, which are always fun.

2:47Then we move on to amplitude.

2:50Now, amplitude is the size of the waveform.

2:54Now, we're not talking about meters.

2:56We're not talking about wavelength.

2:58Not today.

3:00Right now, we're talking about the size,

3:02so the height of the waveform.

3:05And that's because the bigger the waveform,

3:08the more power we have in it.

3:11So if we were to measure this, it

3:13would be able to provide us with a power rating.

3:17So when we have a transmitter over here

3:20that's transmitting a signal out of it, well,

3:23it's transmitting at a given amplitude or given power.

3:26And the same thing on a receiver.

3:29When we have a receiver receiving a signal,

3:32so the signal comes all the way over

3:34and is received by the receiver over here,

3:36then we have a received amplitude or received power.

3:40And remember, we talked about the attenuation a minute

3:43ago, because we used the example of an oscilloscope

3:47and we were measuring the strength of a signal.

3:49And when it started out, it was strong

3:51and it started to slowly attenuate.

3:53Well, as you see here, it's losing power.

3:55And that is where the amplitude and power relate.

4:01And lastly, we have something called phase.

4:04And I find this to be absolutely amazing.

4:07Now, this is one of my favorite things about RF.

4:10Now, we have different signals and we've talked about them.

4:12So if I show you a signal here and it's

4:14flowing through the air, well, that is a single signal.

4:18But if I have another signal at the same frequency traveling

4:23through the air as well, well, they

4:26have a relationship if they share the same space.

4:29Now that relationship is called phase.

4:32Now, if I have two signals--

4:34Let me get rid of those last two I drew.

4:37There we go.

4:38And let's draw a new one here.

4:41This is a single signal here and I have another signal

4:47and it is traveling along the exact same frequency here,

4:53the same amplitude and everything.

4:54It's what we call in phase.

4:57They match up exactly.

4:59And when two signals are in phase,

5:02I like that in phase, they actually become additive

5:07and they make the signal stronger and more powerful.

5:12Think about it like this when we reference in phase,

5:16I want you to think of a choir.

5:18Because when you hear a choir singing and all those people

5:22are in the phase, they're saying the same words

5:26at the same frequencies at the same time,

5:30it becomes a really strong voice.

5:33And you hear them because they're in phase.

5:35Now, the opposite is true as well.

5:38If we have two signals that are out of phase--

5:41let's draw another one here.

5:43There we go--

5:44And I have another signal and it's a little bit

5:47out of phase--

5:49oh, goodness, gracious-- well, what happens there?

5:52Well, they actually work against each other

5:55and it reduces the signal strength.

5:57So that actually reduces the signal strength,

6:02because they're interfering with each other.

6:04Think about this.

6:05When you go to a restaurant and it's noisy in there,

6:08you can hardly hear your own conversation.

6:11And if you try to eavesdrop on somebody on the next table

6:14over, well, you could barely hear them.

6:15You can't really make it out because there's

6:17all this noise and distraction.

6:19And that's what this.

6:22This is out of phase and that's exactly what you're

6:26hearing in that restaurant.

6:28Because they're cross talking.

6:29They're talking on top of each other.

6:31And it's just not a good thing.

6:33And here's something really interesting.

6:36When you have two signals that cross that are exactly,

6:41what we say, 180 degrees out of phase, meaning they're

6:45the exact opposite phase, believe it or not,

6:49they cancel each other out.

6:52That means you have a loss of signal at that point.

6:56So what we have is 180 degrees out of phase cancels.

6:59Now, what do I mean by degrees?

7:01Well, a waveform in itself is a complete circle.

7:05So that's 360 degrees.

7:08So like this signal over here that we're looking at

7:12could actually be 90 degrees out of phase,

7:15meaning that it's 1/4 of a wave cycle out of phase.

7:19And it doesn't matter.

7:20If it's out of phase, it's out of phase,

7:21except when it comes to 180 degrees,

7:23because then then they got cancellation.

7:25So keep that in mind as we move forward.

7:27We're going to look at some technologies that

7:29come into place like MIMO that actually take advantage of this

7:35and are able to provide us with better signal strength.

7:38So in phase, out of phase, really cool stuff.

7:43I hope this has been informative for you

7:44and I'd like to thank for viewing.

0:00Well, now it's time to take a look at RF Behavior.

0:04So we're going to look at how radio frequency or RF

0:07signals behave in different instances,

0:10like when they bounce off of things

0:11or when they might be absorbed by things or even scattered.

0:15So let's jump in and take a look at RF behavior.

0:19So starting off, we're talking about something

0:21called propagation.

0:22And what that is, that's simply how a signal travels,

0:27and it's just kind of a generic term.

0:28So think of it like this.

0:29If I drop a rock into a pond, well, then, I'm

0:33going to have the ripple effect.

0:35And that's how that signal or that energy caused

0:39by the rock penetrating the surface of the water

0:43travels across the water, and it creates that wave.

0:46Well, the same thing is true for radio waves.

0:49As they propagate to the air, they run into things,

0:52they bounce off of things, they get absorbed by things,

0:54and that's what we're going to take a look at.

0:56So starting off, we're looking at absorption,

1:00and that's the loss of an RF signal to a material

1:04as the wave passes through it.

1:07So that's as a wave passes through maybe a brick

1:10wall, for instance, and it will pass through it

1:12depending on the frequency.

1:14For example, a 2.4 gigahertz wave in our Wi-Fi spectrum,

1:20that goes through a brick wall, will

1:22be 1/16 the power it was when it comes out of that brick wall.

1:29So it loses a whole lot of power, as you can imagine.

1:33Well, it also gets absorbed by water.

1:36Water is a great absorber of radio waves.

1:40We're talking trees, shrubs, shrubbery, paper products,

1:44and things like trees that are full of water.

1:48Because if you try to shoot an RF signal through a tree

1:52during the summertime that's in full bloom,

1:55it's going to lose a lot of power as it goes through it.

1:58And that's absorption.

2:00Makes sense, right?

2:01Just like a sponge absorbs water,

2:03well, different objects absorb RF energy.

2:06Then we move to reflection, and this

2:08is the bouncing off of an object by an RF signal.

2:13So as a signal flows through the air,

2:16it will bounce off of things that

2:20are larger than the wavelength of the signal itself.

2:24So again, here's where wavelength comes into play.

2:28If I have a small wavelength, like we're talking our 2.4

2:31gigahertz frequency range, it had

2:34a wavelength of 0.125 meters or 4.8 inches,

2:41it's a small wavelength.

2:43And if it goes against a building that's

2:45bigger than the wavelength, meaning bigger than 4.8 inches,

2:49which, of course, all buildings are bigger than that,

2:52it's going to bounce off.

2:53It's going to reflect off of it.

2:56And that is what reflection is.

2:58Now, reflection can lead to something called multipath,

3:04and that's because the signal will bounce off of it.

3:08And it could actually go in different directions,

3:10and it could bounce off a road off of there.

3:12And what you end up with is one signal

3:15that turns into multiple signals,

3:19and it creates multiple different paths of that signal.

3:23Now, that can be problematic because if I

3:25have a transmitter over here and I shoot a signal out,

3:30and it arrives at my receiver as one signal, hey, that's great.

3:36I have one signal to listen to.

3:37But if I transmit a signal and it

3:40ends up bouncing off of things and ends up at the receiver as

3:44multiple signals--

3:46well, remember, we talked about something called phase--

3:48and if those are out of phase, they're

3:50going to cause cancellation, and they're

3:53going to cause interference.

3:54And my receiver is going to have a hard time hearing the signals

3:58and discerning between them.

4:00So that's where something like multipath can be a problem,

4:04and reflection can cause that.

4:06Next up, we have scattering.

4:09Now, this is where we have a signal that

4:12passes through an object that is smaller

4:15than the wavelength of the signal.

4:17So again, wavelength comes into play, so when it's smaller.

4:22So for example-- we're going to stick with our 2.4 gigahertz

4:27frequency, so we're talking 4.8 inch wavelength here--

4:33and if I had a bunch of tiny rocks in a pile

4:36and that signal bounced off, it's

4:38not a clean reflection, meaning like your reflection

4:42in the mirror.

4:42It's not a bouncing off one-off.

4:45It's going to scatter.

4:46Think of like a--

4:47oh, remember when you were young and you

4:49went to the skating rink and they had the disco balls?

4:52And when the light hit it, it came off in all different kinds

4:56of beams, right?

4:58So when you shot one light at it, like a signal,

5:01it bounced off in all different kinds of directions.

5:04And that is what scattering is.

5:08Now, it could be a wire fence.

5:10It could be a tree that causes it.

5:12It could be rock.

5:13It's something that is smaller than the wavelength

5:18of that signal.

5:19And that is scattering.

5:22Then we come across refraction.

5:24And you've seen this before, probably

5:26in grade school when you did some science projects

5:28and you used a prism, and that did a refraction of light.

5:33And that is just what it is.

5:35This is a bending of the waves, RF waves,

5:39as they pass through an object that has different densities.

5:42And some examples of that are clouds and our atmosphere.

5:47And it could be things like changes in air

5:51pressure and water vapor.

5:53These are things that would cause RF refraction.

5:58And that's what that is.

5:59Then we have from refraction to diffraction.

6:03Now, this is a little different.

6:05Remember, refraction was the bending of waves

6:09as they go through an object.

6:11Well, diffraction is the bending of waves around an object.

6:17Just like here.

6:17This is my interpretation of-- no, not a log.

6:22[LAUGHS] Good guess, though.

6:23[CHUCKLES]

6:24This is actually supposed to be a river.

6:27And what we have here is a rock in the river.

6:31So we've got this rock, and as the water flows down the river,

6:35it's going around the rock.

6:38And that is diffraction.

6:40Now, when it comes to what could cause it for RF signals,

6:44it could be things like buildings.

6:46It could be hills.

6:47It could be lots of different things.

6:50But the idea is diffraction is the bending of radio

6:53waves around something.

6:55And that is diffraction.

6:57Now, we've got our body attenuation

6:59that we've talked about before.

7:01Now, attenuation again is the losing of RF power

7:06throughout the life cycle of a wave,

7:08and it could be because it goes through things like a door.

7:11Hey, this is a nice-- you like my drawing

7:13of a living room here?

7:15Well, that's because I was trying

7:16to show the drywall, because drywall will absorb signals

7:20and cause it to attenuate.

7:22Even going through glass, like a window up here,

7:25and definitely a brick wall.

7:26Remember, we said that if an RF signal at 2.4 gigahertz, RF

7:31signal goes through a brick wall,

7:34it's going to come out at 1/16 the original power.

7:38So that tells you, wow, that lost a lot of power.

7:42And, of course, different types of material,

7:45like wood or drywall or glass, all attenuate

7:48signals differently.

7:49Now, one thing to keep in mind--

7:52the lower the signal, the better it will pass through things.

7:58So for example, a 2.4-gigahertz signal will pass through

8:03a brick wall or drywall better than a 5-gigahertz signal.

8:09And what that tells us is when we're dealing with Wi-Fi

8:13in 2.4-gigahertz frequency range in five years,

8:16that if we had an environment that we set up with 2.4

8:20gigahertz access points and we decide that it's time

8:24to upgrade and we deploy new 5 gigahertz,

8:27then we cannot do a one-for-one replacement most likely

8:31because a 2.4 gigahertz passes through objects better than 5

8:36gigahertz.

8:375 gigahertz is going to have less coverage depending

8:40on the environment.

8:42So we may actually need to deploy additional access points

8:46in the 5 gigahertz range.

8:47That's just something to keep in mind and to know.

8:50And lastly, we have something called free space path loss.

8:54Now, that is the attenuation of the signal as it spreads--

8:58basically, through the atmosphere,

9:00and losing strength.

9:02So as we transmit a signal-- we've got a transmitter here,

9:05and here's our antenna--

9:07and we're going to transmit the signal, it's going to go out.

9:12And I want you to think of wirelesses as something--

9:15I like to actually say this is something I came up with,

9:19and I think it is.

9:20I've never heard of this, but free range--

9:24let's write this out.

9:25Free range communications, or comms,

9:28because that's what wireless is.

9:30If you deal with a wired network, you have a wire,

9:34and it is containing the signal that's

9:36going through your network.

9:38But if you're using wireless, this

9:41is my marketing coming out, [CHUCKLES] free range,

9:44like free range chickens and free range eggs

9:46and free range all those things, well,

9:48wireless is free range communications

9:51because it is going to do that.

9:52It's going to go wherever it can.

9:55It's just going to spread and keep going.

9:57And that's what wireless does.

9:59And that's where free space path loss comes into play.

10:03Now, this is kind of a rule of thumb that every time a signal

10:07doubles in distance, it loses 75% of its power.

10:16So if we have a signal--

10:18I've got a transmitter here--

10:19and it is shooting a signal out and I'm receiving it

10:24at, let's say, 50 meters, and I was

10:27to 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,

10:35I measure it at 100 meters, then I

10:39should have X minus 75% of its strength.

10:46So I would end up with 25% of X. And that is a rule of thumb.

10:51So that's something you definitely

10:52want to keep in mind when talking about free space path

10:56loss.

10:56The rule of thumb is every time you double the distance,

11:01then you lose 75% of that signal's power.

11:06And that is RF behavior and how RF signals work and react

11:11in different environments and some good facts to know.

11:14I hope this has been informative for you,

11:16and I'd like to thank you for viewing.

0:01Now, we've talked about something called multipath

0:03in previous videos, and we know that multipath

0:06can cause problems and it's caused by the reflection of RF

0:11signals.

0:12So we're going to take a look at what causes it,

0:15what's happening with it, and how we can

0:18address the problems it causes.

0:21So multipath, we've talked about this before,

0:24and we said that it's caused by reflection.

0:26So if I have an RF signal going through the air,

0:29and it bounces off of something, well, it

0:32can bounce off causing multiple iterations of the same signal.

0:37And that becomes a problem, because when

0:39I have a transmitter here and I transmit my signal out, well,

0:45when the receiver is over here and it

0:49receives a single signal, then it can read that signal.

0:53It does something called the modulation.

0:56And it works through converting that radio signal into bits

1:01and that's what it does and then it sends it on down the line.

1:04However, if I have a transmitter transmit a signal and I've got

1:08reflexion going on and I have multipath happening.

1:13And I end up with multiple signals

1:15arriving at the destination most likely at different times.

1:22So something called a delay spread,

1:24that is the time between the signals arriving

1:27and it's generally in nanoseconds,

1:30because it is a very short period.

1:33Well, what happens is those signals are probably

1:37going to be out of phase.

1:39Now, remember we said in phase signals,

1:43they work additive, so they actually

1:46help strengthen the signal.

1:47But out of phase signals did the exact opposite

1:52and they caused degradation.

1:54So when we talk about in phase signals,

1:58there's a term called up fade.

2:01And that references the fact that there

2:04is an increase in signal strength

2:06when the signals arrive in phase.

2:09But the same is true when we have out of phase

2:13and the out of phase, we refer to as a down fade

2:17and that's because there is a reduction in power.

2:21And there is a loss and there's actually nulling as well.

2:25So if I was to say nulling, null usually

2:27is a negative annotation, that's when those signals arrive

2:32180 degrees out of phase, which we said

2:35caused cancellation, which is also known as nulling.

2:40And believe it or not, when the receiver receives

2:44these, as it's going through it's the modulation techniques

2:48and it's working on those signals,

2:50it can lead to data corruption.

2:53And data corruption happens because in our receiver--

2:58this is our receiver here--

2:59once a signal arrives, it goes through a process

3:03of demodulation, which is where we take that signal here

3:07and we convert it to 1's and 0's and then

3:12we send it on down the line to our equipment.

3:15Well, during this process of demodulation,

3:19bits can actually overlap, and this

3:21is known as ISI or inter-symbol interference.

3:27And what we can do is use something

3:30called CRC or cyclic redundancy check to address this problem.

3:35So the problem is data corruption.

3:36When I've got these different signals coming in here

3:39and there are at different times,

3:41it can actually cause the modulation

3:44to overwrite some bit, so that they're not exactly the same.

3:50They're not lining up.

3:51So we have ISI or inter-symbol interference that

3:56is causing our data corruption.

3:59And we address that through our cyclic redundancy check.

4:02Now, what that is is when we're sending these data

4:05frames through the air, if we were to dissect a frame

4:08and we see that there's different information,

4:11like a source address, there'd be a destination address,

4:16and there's data in a whole bunch of stuff in here.

4:18Well, there's also a piece of the frame called the CRC

4:21or cyclic redundancy check.

4:23And that's where the system before transmitting

4:25this wireless frame actually takes a look at the data here

4:29and it runs a mathematical algorithm against it

4:34to create a value.

4:35And that's known as the CRC.

4:37So when the receiving station receives this frame,

4:41they're going to take a look at the CRC

4:43and they're going to do the same mathematical equation.

4:45And if the CRCs equal each other,

4:50then we know that there's no corruption.

4:52But if there is corruption, then basically, the system's

4:55not going to acknowledge it received this frame.

4:58So the transmitting station is going

5:01to rescind that to address the problem with data corruption.

5:08But when we're dealing with real time data technologies,

5:11like voice over IP or streaming videos,

5:16well CRCs don't play into that.

5:18Because we don't want that corrupted data back.

5:21Think about it, if you're talking

5:23on the phone with somebody or watching a video

5:25and you drop a frame or two because of errors,

5:29well, you don't want that video or audio

5:31to replay a second later once it's re-transmitted, right?

5:34No, not with real time communications.

5:36We don't want that.

5:38But when we're talking about sending messages

5:40or transferring files or browsing the web, all of that

5:44does use the CRC.

5:46So again, multipaths can lead to data corruption

5:50and we can address that with our cyclic redundancy check.

5:54So now that we know what problems multipaths can cause,

5:58well, how do we deal with it?

6:00Well, there's a couple different ways.

6:02First of all, we can use directional antenna.

6:05So the idea is let's corale this RF signal.

6:09Remember I mentioned earlier that this is like free range

6:11communications.

6:12Well, let's try to limit where this signal can go,

6:16because if I can control that, maybe I

6:18can control multipath some.

6:21So instead of sending the signal everywhere it wants to go,

6:25maybe I use a directional antenna here to say, signal,

6:29you are only going to go in this direction.

6:33You're not going to go everywhere.

6:35And I can use a directional antenna to help me with that.

6:38The other thing is I can use something

6:40called antenna diversity.

6:44And antenna diversity is where an access point

6:49or radio, we call it, like this here, has multiple antennas.

6:53And it has the ability to say, hey,

6:57I'm receiving a really good signal on this antenna

7:01but the other antennas aren't so good.

7:04So what it'll do is simply not listen for the signal

7:08on those antennas and only receive the good signal.

7:13And that's where our antenna diversity comes into play.

7:16That's how it works.

7:17So it allows some intelligence on the receiving station

7:21to only receive a good, strong signal

7:23and to avoid those that are causing interference

7:26or that are out of phase and may cause some data corruption.

7:31Another thing is we could try to reduce the transmit power.

7:34If I'm sending a signal and it only

7:38needs to transmit at 50 milliwatts

7:42to get where it's going, well, why should I

7:45transmit at 100 milliwatts?

7:46That's going to send that signal that much further causing more

7:49multipath, so I could reduce the power

7:52and that could help with my multipath.

7:55The other thing is something called

7:56MIMO and that is a multiple input, multiple output and it's

8:03antenna diversity that we talked about using

8:06something called maximum ratio combining or MRC, that's MRC.

8:12And it does that to take advantage of multipath,

8:16so it's listening.

8:18It's got all these ears, these antennas--

8:20that's not a very good looking ear--

8:22but these antennas act like ears and they're listening.

8:25Well, it can actually select the best antenna-- that's

8:28what we talked about, antenna diversity--

8:30but it can also use some technology

8:34to combine signals that are out of phase

8:38and maybe delay them just a brief nanosecond

8:41to get them in phase so that they are combining strength.

8:46So MIMO can actually take advantage of multipath path

8:51and provide this better signal strength.

8:54And we're going to have a whole video talking

8:56about MIMO and some advanced technologies a little later on.

8:59But that is one way to deal with multipaths.

9:02So again, we can use directional antennas.

9:05We can reduce signal strength, use antenna diversity

9:10and look at using MIMO.

9:12And those are different ways we can deal with multipaths.

9:15I hope this has been informative for you

9:16and I'd like to thank you for viewing.

0:00When working with RF signals, well, sometimes we

0:04need to increase that signal and there's

0:06a couple of different ways we can do that.

0:08And we do that through a process called gain where we're

0:11achieving gain on a signal.

0:14So we're going to jump in and take a look at RF gain

0:17as well as something called voltage standing wave

0:19ratio or VSWR.

0:22So let's get started.

0:24There are times when we want to increase the strength of an RF

0:28signal and we do that by using something called gain.

0:32There's a couple different types.

0:33We have active gain and we have something called--

0:37you guessed it--

0:38passive gain.

0:40Now, the difference is active game

0:42is when we use an amplifier.

0:45So we have something that uses an external power source.

0:49It plugs in and it amplifies our signal.

0:53You're probably familiar with amplifiers.

0:54We use amplifiers in electronics.

0:57In your stereo system there's amplifiers and such

1:00and in cars.

1:02So that is an amplifier.

1:03But passive gain, now you might not be familiar with that.

1:07Passive gain is when we use an antenna.

1:10So yes, we use an antenna to actually increase

1:14our signal strength.

1:16And that's because as we radiate an RF signal--

1:21I'm going to draw an antenna here

1:23and this we refer to as a dipole antenna.

1:26It's just one of those you see off of an access point.

1:28So if I have an AP and I've got a couple different antennas,

1:31well, that's it right there.

1:33Well, this radiates or sends out a radio signal

1:37equally in all directions.

1:39So it sends it out like this everywhere.

1:42Well, I can actually use the design of an antenna

1:46to take maybe this RF energy over here

1:50and focus it in that direction.

1:53So I end up with an antenna that is actually

1:57sending out a signal only in this direction.

2:01So I've taken all the energy that was over here

2:04and I focused it over there.

2:06And that actually increases the signal strength

2:10that's over here because I focused that energy.

2:13And that is antenna gain.

2:16And that is via passive gain and that's really what it is.

2:20It's pretty neat stuff.

2:22Well, sometimes we need to measure these RF signals.

2:26We need to know how strong they are.

2:27And there's a couple different tools we use to do that with.

2:30One is a spectrum analyzer.

2:34And a spectrum analyzer basically

2:38allows you to look at an area--

2:41so on the screen you would see it like this--

2:43of frequency.

2:45So down here, it might say 2.4 gigahertz and over here

2:492.6 gigahertz.

2:52There we go.

2:53And what you would have in between is 2.5 gigahertz.

2:56So you're looking at an area of frequency

2:59and it would look like something like this

3:02where you are seeing signals that are within the frequency

3:07ranges.

3:08And you would see down here at the bottom,

3:09this is called the noise floor down here where

3:12there's not much going on.

3:14But then once you get up here and you see an active signal,

3:17well, that tells you that there's

3:19a signal transmitting at that frequency

3:21and that is what a spectrum analyzer does.

3:23And we use those all the time in wireless

3:25to do a wireless site survey or troubleshooting

3:28to see if there's interference from something else.

3:30Because believe it or not, other things

3:33use the same frequency as our Wi-Fi networks

3:36like microwave ovens, believe it or not.

3:40If you turn on a microwave oven and it's not

3:42insulated properly and grounded properly,

3:45it will actually cause interference in this spectrum.

3:48And you will see it on your spectrum analyzer.

3:51There's another tool we use also called an oscilloscope.

3:56And an oscilloscope is used to measure a single signal

4:01at one time and we measure the power of that signal.

4:05So if you look at the screen of the oscilloscope,

4:08what you're seeing is not the actual frequency

4:12per se like you would on a spectrum analyzer,

4:15but you're actually going to be looking

4:17at the oscillating signal as it goes along.

4:22So you're able to then take a look

4:24and measure the power by looking at the--

4:27that's right-- amplitude of the signal that you're analyzing.

4:33So the oscilloscope, let's just take

4:35a look at the single signal.

4:36And we really don't use this a whole lot

4:38in wireless troubleshooting.

4:40This would be more for electronics

4:41and working with the design of some things

4:44like the design of antennas and transceivers, transmitters

4:48and receivers.

4:49But for wireless networking and troubleshooting,

4:52we generally use a spectrum analyzer.

4:56Now we have our VSWR, Voltage Standing Wave Ratio.

5:01Now what this is is pretty simple.

5:03I want you to think of kind of like-- let's think of water

5:06pipes for instance.

5:08So if I've got--

5:09I'm going to say down here at the bottom,

5:11I've got a pipe here and this is a 3-inch diameter pipe.

5:15And I've got water flowing through here

5:18and it's coming through a lot.

5:21I've got this full power and I'm just

5:22blowing water through here.

5:24And this is my 3-inch pipe and I choke that down

5:28to a 1-inch pipe.

5:30What is going to happen?

5:32Well, think about all this water when it hits this flat surface.

5:35It's going to bounce back.

5:37Let's change colors here.

5:38It's going to bounce back and it's

5:40going to cause something called back pressure.

5:44And what this does is it causes interference.

5:48Now, some water will flow through here,

5:51but you're going to have this water that's

5:53circulating back through here because of the back pressure.

5:56So it's going to cause you to be able to send

5:59less water through the pipe.

6:00And then, of course, we're shrinking the pipes size,

6:02so you have that as well.

6:04Now I want you to now think about a radio transmitter or RF

6:09transmitter like we have right here.

6:11And every piece of equipment in this line--

6:15so we go from the transmitter to a connector here to the cable

6:20to other connectors to antennas--

6:23all pieces have something called impedance

6:27and it's represented by the ohm symbol, O-H-M.

6:33That's actually the omega symbol or we refer to it as ohms,

6:36so who put omega.

6:39And that symbol is used to represent impedance.

6:44Now, impedance is resistance to a signal.

6:49So it's resistance to an AC current or a signal.

6:54So when we send a signal through equipment or down a wire,

6:57there is some inherent resistance in that material

7:01and it receives an ohm rating or an impedance rating.

7:06And now that impedance is very important

7:10because it's just like these pipes.

7:13This could be like a 3 ohm impedance and a 1 ohm

7:17impedance.

7:18Well, obviously they're different,

7:20so we end up with this back pressure thing going on

7:23down here.

7:24Now, here's the key with VSWR.

7:27In order to avoid it--

7:29and we're talking about what it is here in a minute--

7:31but in order to avoid it, we need

7:33to practice impedance matching.

7:36And that just means that every piece of equipment we

7:40use in the line here--

7:43that means from our transmitter all the way to

7:45and including the antenna--

7:46they need to have the same impedance rating.

7:49It's fairly simple.

7:51A very common rating is 50 ohms impedance.

7:55And it just means you want to use a 50 ohm transmitter, a 50

7:58ohm connector, 50 ohm wire, 50 ohm connector, 50 ohm antenna.

8:02You kind of get the point.

8:03As long as you stay the same, it's

8:05like having a water pipe that's all the same size.

8:08You don't get any of that back pressure.

8:11And that's what VSWR is like.

8:13It's like back pressure and what it does

8:16is it causes a loss of power.

8:19And we represent VSWR through a ratio.

8:24All right, I'm going to clear a little space here.

8:27Let's move just a little bit, so we

8:29can talk about the VSWR ratio.

8:33There we go.

8:34All right, now VSWR ratio is how VSWR is represented

8:39and it's a ratio between the highest transmit power

8:42along the way and the lowest.

8:44So we have the highest transmit and the lowest along the way

8:51and it's a ratio.

8:52So if I have a 1 to 1 VSWR ratio--

8:57let me put the VSWR in here--

8:59that means the highest and lowest power are the same.

9:01So I didn't have any loss.

9:03But if I have a 2 to 1 then I have some loss.

9:08Or if I have a 6 to 1 then again I have even more loss

9:12because the highest power was 6, the lowest power

9:15is 1 then I have a--

9:17numerically, I've lost 5 whatevers of power pr

9:22here 2 whatevers of power.

9:24Now, the take away from this is you

9:26should know what VSWR is that we just talked about.

9:29And we avoid it by using impedance matching.

9:34Remember, our impedance rating.

9:36And we were expressing VSWR in ratios,

9:41a perfect VSWR is 1 to 1.

9:45Everything else we're losing power,

9:47so that's not a good thing.

9:49And that is VSWR.

9:51We have one more thing in this lesson and this is return loss.

9:55This is the difference between the power sent

9:58to the antenna and the power being reflected back

10:02to the transmitter.

10:03Now, this goes back to, believe it or not, VSWR.

10:07Remember, we talked about the water?

10:08Pipes and we went from a big pipe to a little pipe.

10:11We had that back pressure.

10:13Well, the same thing happens with VSWR and the energy

10:17because those signals--

10:19if we draw this out a little bigger--

10:22those radio signals are going down through here.

10:25And if we have some reflect back, what do you think

10:28is going to happen?

10:29Well, remember we talked about phases and signals

10:32being in phase, not a phase?

10:34Well, it's going to cost signal loss

10:36and that signal is going to go back down the line.

10:39And that's causing interference and it's causing us to lose.

10:42And return loss is the difference--

10:45that's key here-- the difference between power

10:48sent and power reflected back.

10:54So if we were to take a look and we said we had 100% power sent

10:59and we had 0% reflected back, the difference between those

11:05is 100, which is great because we

11:08don't have any reflected back.

11:10And the higher the number of return loss, the better.

11:15So if I had a 100% power transmitted--

11:19which I'm going to have 100%--

11:20and I had a 10% return, then the difference is 90.

11:27I mean, that's pretty simple math.

11:28The key here is the difference between them, higher is better.

11:33So in an ideal situation, we would

11:35have a return loss of 100%.

11:40Now, when you go and look at return loss,

11:42it is rated in decibels, and that

11:45is something we're going to take a look at in some upcoming

11:48videos.

11:48But for now, just remember the higher

11:51the return loss, the better.

11:53Because if we don't have anything coming back,

11:56meaning we don't have VSWR problems, that's a good thing.

11:59Because believe it or not, if your VSWR gets high,

12:04that means those signals are coming back against the flow.

12:08They can actually damage your transmitter

12:12because they can get high enough that they

12:13can cause it to overheat.

12:15So that is gained VSWR and return loss.

12:19I hope this has been informative for you

12:21and I'd like to thank you for viewing.