 We've already given some examples of guided or wired media Co-actual cable twisted pair optical fiber. We want to talk about wireless media In fact, we've already mentioned examples of wireless media. You know some like Wi-Fi TV broadcasts satellite communications. So there are many examples. I think you know your mobile phone but to to really understand them we need to look at some of the theory or concepts of wireless transmission and and especially we'll look at antennas and The other aspect is how much signal do we lose when we transmit wirelessly many different examples of wireless communication systems receiving TV either via satellite or via by the the normal antennas on your TV Wi-Fi mobile phones infrared home communications many examples that I think you're aware of a Simple model of wireless transmissions is that we have a transmitter It generates an electrical signal Then we have an antenna the antenna takes that Electrical current and converts it to an electromagnetic wave some wave that will propagate through space at a particular range of frequencies Usually the radio and microwave range of frequencies that signals propagates through the air or through some other sometimes Maybe through water if necessary like underwater communications the signal propagates through air and is received by another antenna and That receive antenna does the opposite to the transmit antenna. It receives an electromagnetic wave converts it back to electricity and that electrical current received is the signal received and That carries our data. We need to understand. What's the role of the antennas in this? What do they do? What are their characteristics? We'll introduce that today. The other thing we need to understand is when we transmit a signal We'd like to know how far we can send it wirelessly often, we'd like to have set up the two Points and I'd like to know well how far apart can I put them such that they can still communicate and One thing that that depends upon is how much signal is lost when we transmit through the air So we'll spend some time on that as well a more mathematical view of those components We start with some signal with some power level PT the transmit power We will see that the antenna effectively acts like an amplifier So we'll try and explain that concept such that it has some gain When we try to analyze how far we can transmit we treat the antenna like an amp Amplifier and it has some gain. So we transmit a signal It goes into the antenna. What comes out will be bigger have a higher power as That signal propagates through the air. It loses power. It gets weaker due to attenuation Any signal that travels some distance gets weaker By how much we'll denote as L the loss Sometimes we'll call that the path loss from the path to transmitter to receiver. How much do we lose? We receive a signal and the receive antenna acts as an amplifier and we have some gain and The resulting receive power is the combination of we start with PT We Amplify by GT. We divide by the loss We multiply by the gain of the receiver and we get the receive power In the same way as our audio system. We started with a transmit power For the components that introduced the loss we divided the components that introduce a gain We multiplied and we ends it up with the receive power Or here we simplify we start with a transmit power the antenna we can think introduces a gain The signal loses strength across distance So we have a loss and then the receive antenna has a gain and we get the receive power We will do some calculations and see Under what conditions can we successfully receive in this topic? So we need to know What's the role of the antenna? What does it do in this? Where does GT and GR come from the antenna gain? What does that mean and we'll do that today? We'll try and introduce that The other thing would do in the in the next lecture is look at How does signal propagate through different obstacles? We'll come back to does the signal go through a wall or not under what frequencies and why and Maybe the last thing will be to say well by how much do we lose power when the signal attenuates? If I transmit my signal from here to rung sit the other campus about 12 kilometers 15 kilometers Will my signal be received at the receiver with enough strength for the receiver to understand? How much power do I lose across that 12 or 15 kilometers? I would like to know that So today let's focus on antennas Can anyone see an antenna? Maybe look at your mobile phone your tablet or laptop. Can you see your antenna? They all have wireless transmitters. They all have antennas Get your phone out another chance to use your phone find your antenna Maybe a tablet if you had one of the old phones quite old now you could pull up an antenna Okay extend out But now they embedded in the back of the device usually use the antennas in laptops phones Think of some cables or some wiring which sort of wrapped around usually in the back of the device But there is an antenna in there We cannot see it same in the laptop usually the antenna is some wiring in the back of the screen and That acts as our transmit and receive antenna Our wireless access point in the back of the room Can you see the antennas? two of those stick antennas Dipole antennas, they're called so different shape usually What other types of antennas do we see I just grabbed some from Wikipedia The dish shaped antennas you've seen maybe if you have satellite TV reception you have a small dish about 50 centimeter diameter and That receives a signal from a satellite up in space a parabolic dish or These ones are for I think receiving signals from space for for detecting space signals or sending I think receiving in this case to learn about the space so large dishes So many antennas will have this parabolic dish shape, so we'll talk about them as we go through But there are many different shapes and sizes of antennas For TV reception sometimes we have these UHF and VHF antennas, so you may have seen the shapes there the design of the antenna is is done to Handle the particular frequencies they want to transmit and receive and The impacts upon the gain that they have so how far that they can transmit and receive and and Your mobile phone although you cannot see the antenna of your phone. You can probably see the base stations or cell phone towers around and if you look at the top of those towers you'll see some antennas and They may look like this they so in this picture. It's hard to see but these rectangles here are antennas The sector type antennas in that they're designed such that the signal goes out in a particular sector So you can see they wrote multiple antennas around Some pointing in this area some pointing there and around there So we split the area up into multiple sectors and those antennas pick up your mobile phone signal and send back to your mobile phone Many types of antennas What do they do? They take at the transmitter and electrical current as input and generate electromagnetic waves that go through air and at the receiver do the opposite convert it back to electricity The waves which are transmitted are usually in the radio and microwave range of frequencies About three kilohertz up to about 300 gigahertz when we analyze antennas transmit and receive antenna we Look at the characteristics of those antennas. We treat them the same. We'll see that they have the same specs usually usually the antenna Whether it's transmit or receive doesn't matter they are effectively the same so we have Electricity coming into the antenna and the signal comes out in what direction does the signal come out and How far does it go? Depends upon the shape of that antenna the design of the antenna so the examples we saw the the parabolic dish those UHF and VHF TV antennas or the sector antennas they are designed in different ways to impact upon What direction the signal will propagate and how far it will propagate? So we need to understand a little bit about that. So let's consider some different types of antennas and Consider how the the signal propagates out of them The first one is what we call an isotropic antenna And it's very simple. Let's say this is our isotropic antenna When we transmit a signal through this isotropic antenna the signal comes out of it and In every direction the signal propagates with equal power equal strength that is going up and Going forward back down every direction around this transmitting or this antenna the signal is spreading with equal strength That means if You measure the signal one meter away from that antenna If you measure one meter in front and let's say you measure the signal strength to be one watt You measure one meter behind it will also be one watt You measure one meter to the left one meter above At the same distance from that transmitting antenna the power will be the same at all those points This is what we define as an isotropic antenna So if you try and draw those measurements of power strength say some distance away You can think of it as a sphere a ball One meter above this the power is let's say one watt One meter to the left to the behind and every Point one meter away which makes up a sphere Has the same power level two meters away A sphere centered on the transmit antenna It'll be the same power level not necessarily one watt, but the same as two meters away from every point The signal goes in all directions equally We use an isotropic antenna to compare real antennas No antennas that we use are Actually isotropic they may be closed, but in in practice that's not quite isotropic And we usually design antennas such that they will concentrate the energy in one particular direction So we talk about direct directional antennas Instead of sending equally in all directions. I would like the power to go in that direction So the antenna will be designed to do that So let's try and illustrate the difference between isotropic and directional antennas and then introduce antenna gain I cannot draw in 3d very well Okay, so we're going to go through an example in two dimensions It's easy to draw, but you could expand in your mind to be 3d Let's first consider an isotropic antenna and put some numbers or values to it The black dot in the middle that's the location of our isotropic antenna When it transmits a signal that signal goes in all directions with equal strength So If I was one meter away at this point and I have a device to measure the receive power We transmit with power Pt And I measure one meter away and I measure the receive power to be pr some value And let's denote it as pr By definition of an isotropic antenna if I measure at a different point also one meter away The power will also be pr so at this point One meter away The receive power will be the same as this point And the same as all of those points on the circle The receive power will be pr So that's what this circle is showing At Points equal distance away from the transmitter from the antenna the power will be the same and I just denote as pr Wake up for this easy question Is pr greater than equal to or less than pt Greater than hands up. So is pr greater than pt Is pr equal to pt Is pr less than pt It's less than We transmit with power pt Attenuation says that as that signal propagates across some distance it will get weaker Okay, just considering attenuation We start with pt As it covers that one meter it will get weaker So pr will be less than pt I don't know what the value is but we just know that it will be less If I measured two meters away and I measured the power to be p a for example That will be less than pr And three meters away will be even lower and lower the power gets less and less the further we go away from the transmitter Half a meter away the power will be greater than pr But less than pt Somewhere in between So here's our isotropic antenna At some point Equidistant from the transmitter the power will be the same But in practice we have directional antennas. So let's try and explain How they differ So the black circle still shows the power received from the isotropic antenna But let's assume we have a different antenna a directional antenna the blue one And it's located at the same point as the isotropic antenna We have two antennas at the exact same location the black one and the blue one And what we do is we measure the received signal at different points So when we're using the blue directional antenna it transmits some signal And this blue shape which I've drawn it may not be Perfectly to scale but the blue shape tries to illustrate that If I measure the signal at any point on that blue shape I measure it to be pr Exactly the same as any point on the black circle that is At this point with an isotropic antenna I measure the received signal and it's pr And every point on that circle it becomes pr now with our different antenna the directional antenna I measure it here, which is of course further away from the transmitter than the circle And the received power is also pr And I measure it here Maybe this is 2. something meters away from the transmitter the received power is pr Here maybe 1.3 meters away from the transmitter the received power is also pr At this point Maybe half a meter away from the transmitter The received power is pr That's what the blue shape shows at all points. The received power is the same Given that Let's consider you're at the red dot If we're using the isotropic antenna and you measure the signal strength at that red dot What is the signal strength? What's the received power? at the red dot what what so Use the black isotropic antenna we transmit with pt we measure the signal here at the red dot and the value is pr By definition every point on the circle the value is pr now We use the blue isotropic antenna to transmit And at that same red point I measure the power from the blue isotropic antenna I measure it to be px Now the question is Compare px to pr Is px greater than pr Equal to pr or less than pr Hands up for greater than Hands up for equal to Less than some people tried to put their hands up. That's all right Let's consider the blue one we transmit with pt At this point, I don't know two and a half meters away the signal strength is pr So this red point which is between the transmitter And the value that we measure as pr the signal strength must be Less than pt, but it should be greater than pr We know the signal gets weaker across distance So if it's pr here At a closer point it should be greater than pr And let's say we measure and denote it as px So what we're saying now is that in this particular direction If we use our black isotropic antenna at this red point the signal strength is pr If we use our blue directional antenna at the same point The signal strength is px And px is larger than pr So we can say that this blue antenna has some gain relative to our isotropic antenna The gain of the blue antenna in this direction Is by how much larger px is than pr And of course we could also express that gain in the logarithmic scale take the logarithm times by 10 Let's consider And see if people understand consider in the opposite direction You stand at This point here, okay one meter away from the antennas And you measure the signal strength to be py All right This point is py For the blue antenna for the black one. It's pr For the blue one. It's py is py Greater than pr less than or equal to That is behind the antenna in the opposite direction here Well, we know we transmit with pt We said this point with the blue antenna would be pr Therefore if we measure further away, it's going to be less than pr That is this py if we measure here will be less than pr so in effect The received power is less than what we would get if we use the isotropic antenna We also have also have a gain, but it's going to be Uh smaller than one Or a loss in fact So we use this this concept to determine the characteristics of an antenna Usually we focus on the direction where it's strongest in our blue one the direction where our signal is strongest is this direction And we can say the gain of my blue antenna Can be calculated is if I measure the point If I measure the power at the red dot I measure it to be px The gain is px divided by pr where pr would be the power if I used an isotropic antenna And that's a key characteristic of antennas in practice when you go buy one You're trying to compare two antennas. Usually in the spec it will say the gain of the antenna And a larger gain Generally means you can transmit further We'll see that come up when we look at the path loss So the gain of the antenna is actually measured relative to an isotropic antenna Relative to this perfect all directions antenna We can take the logarithm of this gain of this px divided by pr and times by 10 to express it in db And we don't just write db. We'll see we'll talk about antenna gain as decibels relative to an isotropic antenna the reference point is not One watt one milli watt the reference point is what we would get if we used an isotropic eye for isotropic antenna Let's see if we can put some numbers to that to to finish this introduction to antenna gain Let's bring up the picture this one Let's instead of talk about pr and pt Let's just make up some numbers to put with those values So it may be a little bit easier to to understand And the numbers that I'll write I'll just make them up. They may not be to scale, but let's say we transmit with a power level pt of Let's say 100 milliwatts So the transmit power is 100 milliwatts And one meter away with our isotropic antenna we measure at the black points on that circle pr to be 10 milliwatts So at every black point when we're using an isotropic antenna, we measure to be 10 milliwatts one meter away And by definition with our blue directional antenna pr at every point on the blue shape is also 10 milliwatts That is the same value It's pr Let's say when i'm using my blue directional antenna, I measure the signal strength at this red point It's one meter away Here the power Transmitted is 100 milliwatts Here it's 10 milliwatts So the red point is going to be between 110 It's going to be less than 100 Larger than 10 What value well, let's make up a value Let's say it's 70 milliwatts What's the gain of our blue antenna in this direction? well px is 70 milliwatts pr The reference point when we use the isotropic antenna at that same point Would be 10 milliwatts So the gain is seven or We use our calculator and convert it to db 10 times log of seven 8.45 And here's the the notation we use that which is equivalent to 8.45 db Relative to an isotropic antenna, so we write dbi lowercase i there That's our reference point My blue directional antenna in that particular direction has a gain of 8.45 dbi Seven times larger than if we use the perfect isotropic antenna Let's try a different point We measure at some other point We measure here with our blue antenna Well, no with our black isotropic antenna if we measure here, we get pr of 10 milliwatts Every point on the black circle 10 milliwatts If I use my blue directional antenna The power at py Is going to be less than 100 milliwatts And in fact also less than 10 milliwatts Because it's 10 milliwatts here further away is going to be less than Let's make up a number and say it's two milliwatts This is 100 this point is 10. Let's say this point is two Then the gain in this direction we can calculate The gain at point y Is our two milliwatts divided by 10 milliwatts is what we would have got with an isotropic antenna 0.2 Or with our calculator 10 times log Of 0.2 Minus about seven dbi It's in fact a loss Our signal strength is less than what we would have got if we used an isotropic antenna So less than one zero point two means it's really a loss a factor of five or minus seven dbi I mean it's a loss of seven dbi So a directional antenna may be strong in one direction But weak in other directions Why? Because the energy is conserved It's the same as that comes out of our original antenna But it's concentrated in one direction Usually when we look at the spec of an antenna it talks about the The gain in the strongest direction But to be more precise you will see some plots or pictures that try to draw the gain in different directions In our case, we've only done it in two dimensions, but we'd also need to consider in the third dimension as well so Isotropic antennas are theoretical antennas that we compare our real antennas against And we can arrive at an antenna gain The gain of using this real antenna compared to is if we use the theoretical isotropic antenna And we'll use that when we compare and do analysis of how far we can transmit