WEBVTT 0:00:02.980000 --> 0:00:08.000000 Hello and welcome to this video that I've titled Radio Frequency Propagation. 0:00:08.000000 --> 0:00:11.860000 The main focus of this video is to talk about the various things that 0:00:11.860000 --> 0:00:17.180000 could affect a radio frequency signal and degrade it, change it, or just 0:00:17.180000 --> 0:00:18.980000 completely block it. 0:00:18.980000 --> 0:00:22.420000 And so we're going to go over terms as we see here from absorption all 0:00:22.420000 --> 0:00:27.920000 the way down to multipath. 0:00:27.920000 --> 0:00:33.180000 So as radio frequency propagates or transmits away from the receiver there 0:00:33.180000 --> 0:00:38.120000 are several situations which could cause it to severely attenuate, be 0:00:38.120000 --> 0:00:43.840000 completely blocked, or be even reflected to a different direction. 0:00:43.840000 --> 0:00:51.880000 So you might recall that if this right here represents a cable and you're 0:00:51.880000 --> 0:00:54.540000 looking at the cable. 0:00:54.540000 --> 0:00:58.260000 So electrons are moving back and forth through this copper wire, you're 0:00:58.260000 --> 0:01:00.400000 looking at the outside of it. 0:01:00.400000 --> 0:01:05.180000 You might recall that radio waves are propagated 90 degrees away from 0:01:05.180000 --> 0:01:10.200000 that cable. So they're going out like this in all directions away from 0:01:10.200000 --> 0:01:13.520000 that cable. That's where the electromagnetic radiation is going. 0:01:13.520000 --> 0:01:20.540000 And as it keeps going and going and going, it's like waves and a pond. 0:01:20.540000 --> 0:01:24.940000 Now your antenna might be right here. 0:01:24.940000 --> 0:01:27.560000 It's on your laptop. 0:01:27.560000 --> 0:01:31.840000 Oops, forget my terrible spelling, but that's your laptop right there. 0:01:31.840000 --> 0:01:36.620000 Now in order for you to pick up that Wi-Fi signal, number one, it has 0:01:36.620000 --> 0:01:42.180000 to reach you. So we're going to talk about in this video that there are 0:01:42.180000 --> 0:01:52.720000 some cases where that Wi-Fi signal might reach you, but it might be severely 0:01:52.720000 --> 0:01:56.900000 degraded to the point where you can't even tell what originally was. 0:01:56.900000 --> 0:02:02.320000 Or in some cases, that Wi-Fi signal could be reflected by things and it 0:02:02.320000 --> 0:02:05.200000 could come in this direction as well. 0:02:05.200000 --> 0:02:07.640000 Or maybe this direction as well. 0:02:07.640000 --> 0:02:10.260000 And so your antenna might think, what is going on? 0:02:10.260000 --> 0:02:14.600000 I'm getting radio frequencies from all different directions. 0:02:14.600000 --> 0:02:16.340000 Some of them are canceling each other out. 0:02:16.340000 --> 0:02:18.580000 Some of them are making each other stronger. 0:02:18.580000 --> 0:02:21.700000 How do I interpret this into ones and zeros? 0:02:21.700000 --> 0:02:24.660000 So that's what we're talking about here as far as propagation problems, 0:02:24.660000 --> 0:02:28.520000 things that could affect the Wi-Fi signal so that by the time it reaches 0:02:28.520000 --> 0:02:33.640000 the receiver, the receiver might have some problems understanding it. 0:02:33.640000 --> 0:02:40.460000 So let's talk about our first and probably worst problem, which is absorption. 0:02:40.460000 --> 0:02:46.520000 Absorption is the characteristic of a material to absorb the RF energy. 0:02:46.520000 --> 0:02:50.560000 And by absorbing it, I mean it might completely absorb it to where when 0:02:50.560000 --> 0:02:54.560000 the RF energy hits this material, it doesn't come out on the other side. 0:02:54.560000 --> 0:02:56.520000 That energy, sort of like a sponge, right? 0:02:56.520000 --> 0:03:00.360000 If I have a little puddle of water on the top of my oven and I put a sponge 0:03:00.360000 --> 0:03:03.660000 on top of it, it completely soaks up that water. 0:03:03.660000 --> 0:03:06.020000 So when I lift the sponge away, the water is gone. 0:03:06.020000 --> 0:03:10.400000 So similar, there's some materials out there that might radio frequency 0:03:10.400000 --> 0:03:15.440000 energy hits that material, it completely absorbs it and nothing pops through 0:03:15.440000 --> 0:03:17.280000 on the other side. 0:03:17.280000 --> 0:03:23.220000 Or there's some other material that might partially absorb the energy. 0:03:23.220000 --> 0:03:26.640000 Maybe the RF energy goes through, but once it comes out the other side, 0:03:26.640000 --> 0:03:28.180000 it's a lot weaker. 0:03:28.180000 --> 0:03:29.880000 We call that attenuation. 0:03:29.880000 --> 0:03:32.040000 The radio frequency has been attenuated. 0:03:32.040000 --> 0:03:36.260000 It's a lot weaker coming out than when it first hit that material. 0:03:36.260000 --> 0:03:43.200000 And so we can see here some examples of this are rock and stone, water, 0:03:43.200000 --> 0:03:46.740000 drywall, even the human body. 0:03:46.740000 --> 0:03:52.060000 So we can see here a chart that gives some examples of this in absorption 0:03:52.060000 --> 0:03:56.720000 rates. Now the way that you would interpret this chart is that the higher 0:03:56.720000 --> 0:04:02.860000 the number, the higher the decibel rate, the more the absorption of that 0:04:02.860000 --> 0:04:09.000000 material. So for example, compare a plaster or drywall wall, which has 0:04:09.000000 --> 0:04:14.440000 a three to five decibel absorption rate with a concrete wall, which could 0:04:14.440000 --> 0:04:18.560000 have up to a 15 decibel absorption rate. 0:04:18.560000 --> 0:04:24.900000 Now without going too far into the physics of this here, this is not linear. 0:04:24.900000 --> 0:04:28.660000 In other words, a loss of one decibel. 0:04:28.660000 --> 0:04:34.680000 So a loss of one decibel in power means that you've lost 20% of the original 0:04:34.680000 --> 0:04:41.240000 RF energy. So if I have a radio frequency transmitter that's right here, 0:04:41.240000 --> 0:04:45.320000 something in the middle that's absorbing some of that energy, and here's 0:04:45.320000 --> 0:04:49.560000 the receiver on the other side, and the amount of energy is one decibel 0:04:49.560000 --> 0:04:51.720000 less than what was originally transmitted. 0:04:51.720000 --> 0:04:56.940000 That means that receiver is receiving only 80%, 20% of it's been lost 0:04:56.940000 --> 0:04:59.860000 with just a one decibel level loss. 0:04:59.860000 --> 0:05:03.520000 Now if we just go up to a three decibel level loss, that might not sound 0:05:03.520000 --> 0:05:07.560000 a lot like a difference of one dB to a three dB, but we're actually talking 0:05:07.560000 --> 0:05:12.320000 about a 75% loss in power if it's a three dB. 0:05:12.320000 --> 0:05:17.060000 So we can see here, looking at this chart, that something going through 0:05:17.060000 --> 0:05:29.240000 drywall, which might have a 15 decibel loss, that is a significant loss 0:05:29.240000 --> 0:05:32.700000 in power. That means that the signal that went through the concrete wall 0:05:32.700000 --> 0:05:38.080000 by the time it reaches you is a tiny, tiny fraction, like maybe a thousandth 0:05:38.080000 --> 0:05:42.260000 of the power that if that same signal had gone through drywall. 0:05:42.260000 --> 0:05:44.540000 So that's what we call absorption. 0:05:44.540000 --> 0:05:48.000000 And certainly there are definitely things in your environment which can 0:05:48.000000 --> 0:05:50.780000 absorb your radio frequency. 0:05:50.780000 --> 0:05:51.680000 What's another thing that's going to be a little bit more important than 0:05:51.680000 --> 0:05:53.820000 the other issue that you could come up with? 0:05:53.820000 --> 0:05:58.680000 Well, in the world of RF, you can have reflection and refraction. 0:05:58.680000 --> 0:06:03.000000 So reflection like it sounds is a material that actually reflects the 0:06:03.000000 --> 0:06:08.580000 RF energy into a different direction or even back to the source. 0:06:08.580000 --> 0:06:13.140000 So in this case, the RF energy is not absorbed into the material, it is 0:06:13.140000 --> 0:06:14.680000 reflected off of it. 0:06:14.680000 --> 0:06:18.800000 For example, metal, even bodies of water can do this. 0:06:18.800000 --> 0:06:26.120000 So to draw an analogy of this, if this is my access point and here's his 0:06:26.120000 --> 0:06:30.760000 radio, I should say here's his antenna, and the RF energy is going in 0:06:30.760000 --> 0:06:36.440000 this direction, and over here is my receiver. 0:06:36.440000 --> 0:06:39.360000 Here's the antenna that's trying to pick that up. 0:06:39.360000 --> 0:06:44.920000 But right here is some sort of metal that's at an angle. 0:06:44.920000 --> 0:06:51.360000 Well, when that RF energy hits that, depending on the reflection index, 0:06:51.360000 --> 0:06:55.640000 it could actually be reflected in this direction and never even make it 0:06:55.640000 --> 0:06:57.560000 to the receiver. 0:06:57.560000 --> 0:06:59.500000 That would certainly be a bad situation. 0:06:59.500000 --> 0:07:03.380000 So that metal could cause that reflection to happen. 0:07:03.380000 --> 0:07:08.700000 Another thing that could happen to an RF signal is what's called refraction. 0:07:08.700000 --> 0:07:16.020000 This is where the signal is bent as it passes through objects of one density 0:07:16.020000 --> 0:07:20.740000 into another. So for example, a radio frequency that's going through air 0:07:20.740000 --> 0:07:22.800000 that suddenly hits glass. 0:07:22.800000 --> 0:07:27.280000 Well, the glass is not going to stop it, it's not like metal that will 0:07:27.280000 --> 0:07:29.960000 stop it and completely reflect it back in a different direction. 0:07:29.960000 --> 0:07:33.720000 It will propagate into and through that glass, but because the density 0:07:33.720000 --> 0:07:40.280000 is different between air and glass, the RF energy will actually be refracted 0:07:40.280000 --> 0:07:43.980000 or change its direction. 0:07:43.980000 --> 0:07:47.100000 And different materials have different refractive indexes. 0:07:47.100000 --> 0:07:51.860000 Once again, to draw an example of that, here's my original RF energy going 0:07:51.860000 --> 0:07:53.100000 in this direction. 0:07:53.100000 --> 0:07:56.320000 Maybe right now it's going through the air. 0:07:56.320000 --> 0:08:03.000000 All of a sudden here is a window in the side of a building. 0:08:03.000000 --> 0:08:06.040000 And over here, right there is my receiver. 0:08:06.040000 --> 0:08:10.620000 That's my receiver, my antenna. 0:08:10.620000 --> 0:08:14.080000 Well, as it hits this glass and propagates through it, refraction means 0:08:14.080000 --> 0:08:18.020000 as it comes out the other side, it's bent. 0:08:18.020000 --> 0:08:20.460000 It's going in a slightly different direction now. 0:08:20.460000 --> 0:08:25.380000 Maybe that receiver won't get it at all or maybe he'll just get a tiny 0:08:25.380000 --> 0:08:27.540000 portion of that signal. 0:08:27.540000 --> 0:08:29.480000 That's called refraction. 0:08:29.480000 --> 0:08:38.820000 Other types of problems we could have are diffraction and scattering. 0:08:38.820000 --> 0:08:42.720000 So diffraction is a weird physics property. 0:08:42.720000 --> 0:08:48.600000 It's the bending and spreading of an RF signal when it encounters an obstruction. 0:08:48.600000 --> 0:08:52.940000 Hills or buildings can cause this for Wi-Fi, which creates an RF shadow 0:08:52.940000 --> 0:08:56.780000 on the other side of the object, which causes dead coverage or degraded 0:08:56.780000 --> 0:09:00.740000 signals. An example of this. 0:09:00.740000 --> 0:09:09.580000 Here is my RF signal going in this direction. 0:09:09.580000 --> 0:09:14.040000 All of a sudden it encounters a building. 0:09:14.040000 --> 0:09:23.020000 And maybe here is another building, a little bit less tall. 0:09:23.020000 --> 0:09:28.340000 And my receiver is right here. 0:09:28.340000 --> 0:09:31.260000 Here's my antenna. 0:09:31.260000 --> 0:09:40.280000 Well, as it hits this building, it might go through, but my receiver is 0:09:40.280000 --> 0:09:41.920000 not quite enough to hit it. 0:09:41.920000 --> 0:09:45.920000 And right here is what we would consider a dead zone. 0:09:45.920000 --> 0:09:49.680000 There's nothing in this particular section. 0:09:49.680000 --> 0:09:55.800000 Or if you've ever seen like water, imagine this is water here going through 0:09:55.800000 --> 0:09:58.980000 a pond. It's a wave of water going through a pond. 0:09:58.980000 --> 0:10:04.440000 And we hit a rock or a stone in that pond. 0:10:04.440000 --> 0:10:07.600000 It's kind of interesting how the wave looks is that you'll actually see 0:10:07.600000 --> 0:10:12.620000 that the wave will go through it in some cases. 0:10:12.620000 --> 0:10:16.680000 And the water will sort of bend around the rock and then keep going like 0:10:16.680000 --> 0:10:23.100000 this. Well if we catch this, this is not quite as strong as if we caught 0:10:23.100000 --> 0:10:28.180000 these. So if we're talking about Wi -Fi here, this RF energy for catching 0:10:28.180000 --> 0:10:33.120000 it right here at this point has been diffracted and it's not quite as 0:10:33.120000 --> 0:10:36.480000 strong as if we had placed our antenna in a slightly different location 0:10:36.480000 --> 0:10:39.500000 where the rock did not affect us. 0:10:39.500000 --> 0:10:41.700000 That is diffraction. 0:10:41.700000 --> 0:10:48.260000 And they will also have scattering. 0:10:48.260000 --> 0:10:51.740000 Now scattering is the same as refraction but on a larger scale. 0:10:51.740000 --> 0:10:56.200000 It's unpredictable, causes signal degradation. 0:10:56.200000 --> 0:11:00.880000 This is typically caused when a lot of very, very small things are going 0:11:00.880000 --> 0:11:02.660000 through the atmosphere or through the air. 0:11:02.660000 --> 0:11:08.380000 For example, particles of dust, tree foliage, even tiny little water droplets 0:11:08.380000 --> 0:11:12.440000 and humidity. All of that will affect your radio frequency as it's going 0:11:12.440000 --> 0:11:16.500000 through the air and cause that radio frequency to scatter and go in different 0:11:16.500000 --> 0:11:24.900000 directions. So it's refraction but on a much larger scale. 0:11:24.900000 --> 0:11:28.520000 We also have the term of free space path loss. 0:11:28.520000 --> 0:11:33.100000 Believe it or not, RF energy is lost simply as it travels through the 0:11:33.100000 --> 0:11:37.640000 air. From the point where the transmit or transmit that RF energy as it 0:11:37.640000 --> 0:11:40.420000 goes further and further through the air, maybe there's nothing physically 0:11:40.420000 --> 0:11:45.140000 to stop it. There's no rain droplets, no buildings, no glass. 0:11:45.140000 --> 0:11:48.860000 Just the fact is that it's going through the air, it will get weaker and 0:11:48.860000 --> 0:11:49.760000 weaker and weaker. 0:11:49.760000 --> 0:11:52.840000 This is called free space path loss. 0:11:52.840000 --> 0:11:57.620000 Now if you are seriously into physics and you love math, this can actually 0:11:57.620000 --> 0:12:01.160000 be calculated. And here is your calculation for this. 0:12:01.160000 --> 0:12:03.780000 Holy mackerel. Here's the great thing. 0:12:03.780000 --> 0:12:07.820000 If you're just learning about Wi-Fi, either because you're curious or 0:12:07.820000 --> 0:12:12.000000 you need to pass like a low level certification, you don't have to memorize 0:12:12.000000 --> 0:12:14.900000 this formula. You don't have to understand this formula. 0:12:14.900000 --> 0:12:18.480000 Just know it is something that can be calculated by people interested 0:12:18.480000 --> 0:12:21.800000 in becoming like a physics major or something like that. 0:12:21.800000 --> 0:12:26.820000 The main takeaway here is that just because you say, oh, here's my transmitter, 0:12:26.820000 --> 0:12:30.160000 here's my receiver, there's absolutely nothing in the middle of them. 0:12:30.160000 --> 0:12:32.440000 This guy should get the full strength. 0:12:32.440000 --> 0:12:37.820000 No he's not. Free space path loss means free space path loss means the 0:12:37.820000 --> 0:12:45.620000 farther you get away from the will weaken. 0:12:45.620000 --> 0:12:48.560000 And loss is relative to frequency and distance. 0:12:48.560000 --> 0:12:55.840000 We can see actually here in this drawing here that so the higher we go 0:12:55.840000 --> 0:13:01.480000 up, the more loss, the less powerful the radio frequency is. 0:13:01.480000 --> 0:13:07.240000 And so we can see here, for example, if we look at this point, a 2.4 gigahertz 0:13:07.240000 --> 0:13:12.240000 signal, yes, it will experience some free space path loss, but not as 0:13:12.240000 --> 0:13:15.880000 much as a 5 gigahertz signal. 0:13:15.880000 --> 0:13:21.020000 So the faster the frequency, the higher the frequency, the greater the 0:13:21.020000 --> 0:13:24.480000 free space path loss will be. 0:13:24.480000 --> 0:13:30.660000 And then the last topic I want to talk about that can also negatively 0:13:30.660000 --> 0:13:36.320000 impact your RF signal is something called multipath. 0:13:36.320000 --> 0:13:41.340000 So we know just like a wave of water, if you drop a rock into a standing 0:13:41.340000 --> 0:13:45.440000 pool of water, that wave will get bigger and bigger and bigger as it leaves, 0:13:45.440000 --> 0:13:49.940000 as it as the distance away from where you drop that rock will occur. 0:13:49.940000 --> 0:13:53.580000 Same thing here is true of radio frequency, radio frequency waves. 0:13:53.580000 --> 0:13:56.960000 They will expand from the point at which they left. 0:13:56.960000 --> 0:14:00.740000 So your access point has an antenna, maybe the antenna is only that tiny, 0:14:00.740000 --> 0:14:06.220000 maybe, you know, some access points, the antenna is only on a little chip. 0:14:06.220000 --> 0:14:09.560000 That might be half of an inch long, you don't even see it. 0:14:09.560000 --> 0:14:12.900000 So the original waveform might be that big, but it gets farther and farther 0:14:12.900000 --> 0:14:17.100000 away from that transmitting antenna, it expands and gets larger and larger 0:14:17.100000 --> 0:14:20.620000 and larger, like a wave does in a body of water. 0:14:20.620000 --> 0:14:26.180000 Well as that radio frequency wave encounters things, some of those things 0:14:26.180000 --> 0:14:31.440000 will absorb it, some of those things will refract it, and some of those 0:14:31.440000 --> 0:14:33.820000 things will reflect it. 0:14:33.820000 --> 0:14:39.400000 And so we can see here, as it encounters multiple objects and it's reflected, 0:14:39.400000 --> 0:14:42.860000 multiple waveforms are created. 0:14:42.860000 --> 0:14:48.440000 So now we've got multiple duplicate waveforms are created that are going 0:14:48.440000 --> 0:14:53.800000 in different directions, taking a different path, and copies of all those 0:14:53.800000 --> 0:14:58.380000 original signals will eventually reach the receiver, but not at the same 0:14:58.380000 --> 0:15:03.080000 time. They will be delayed depending on how often they've bounced around 0:15:03.080000 --> 0:15:07.240000 before they finally get to the receiving antenna. 0:15:07.240000 --> 0:15:09.800000 Metal is a typical culprit of this. 0:15:09.800000 --> 0:15:16.260000 The more metal you have it in your environment, metal nails in your walls, 0:15:16.260000 --> 0:15:20.220000 metal dividing, you know, one wall from another metal filing cabinets, 0:15:20.220000 --> 0:15:24.540000 the more multipath you'll experience. 0:15:24.540000 --> 0:15:28.860000 So the effective multipath, depending on your Wi-Fi standard, as you could 0:15:28.860000 --> 0:15:33.820000 have data corruption, the signal could be completely nulled out. 0:15:33.820000 --> 0:15:38.140000 In some cases, the signal could be increased due to increased signal amplitude 0:15:38.140000 --> 0:15:46.000000 or decreased. Using multiple antennas on the receiver can reduce the effects 0:15:46.000000 --> 0:15:48.420000 of this. For example, take a look at this animation here. 0:15:48.420000 --> 0:15:53.540000 The transmitter on the left is going to transmit a Wi-Fi signal, but as 0:15:53.540000 --> 0:15:57.440000 it encounters various things like the filing cabinet and even the ceiling, 0:15:57.440000 --> 0:16:01.220000 we're going to see how that creates multiple signals that all reach the 0:16:01.220000 --> 0:16:03.380000 receiver at different times. 0:16:03.380000 --> 0:16:10.520000 So there we go, that receiver just received multiple copies of the same 0:16:10.520000 --> 0:16:14.080000 original signal, and this is what's called multipath. 0:16:14.080000 --> 0:16:18.040000 And depending on how many antennas we have in the receiver, depending 0:16:18.040000 --> 0:16:22.020000 on the logic and the code of the receiver, this could cause some problems 0:16:22.020000 --> 0:16:24.800000 to where he can't decode it correctly. 0:16:24.800000 --> 0:16:26.680000 This is a problem with multipath. 0:16:26.680000 --> 0:16:33.080000 So all these things we've looked at, absorption, reflection, refraction, 0:16:33.080000 --> 0:16:37.020000 diffraction, scattering and multipath, these are all things that could 0:16:37.020000 --> 0:16:41.180000 affect your radio frequency, that could cause the receiver to either not 0:16:41.180000 --> 0:16:46.100000 get it at all, get it a lot less, or just be completely confused by multiple 0:16:46.100000 --> 0:16:50.160000 frequencies all coming in in different directions all at once. 0:16:50.160000 --> 0:16:52.740000 So that concludes this video. 0:16:52.740000 --> 0:16:54.040000 I hope you have found it useful.