 So, we briefly spent some time yesterday on guided media where we, actually before we got to guided media we spoke about the electromagnetic spectrum that we have available for communication signals and some characteristics and some examples of communication systems and what range of frequencies they use. And then we got to a very quick overview of three examples of guided media. Today we'll just recap on those three, we're not going to spend much time on them. We're going to move on to wireless. Just use these three as examples. So the first two examples were twisted pair and coaxial cable and both of them involved sending electrical signals across some conducting material. So in generally for such guided media the wiring is designed in such a way to reduce the effect of interference. With twisted pair the pairs of wires are twisted about each other. That's one way to reduce the impact of interference. With coaxial cable there's two conductors, there's an inner conductor and an outer conductor and again the fact that they're on the same axis, they're one inside the other, the signals we send across them can reduce the interference on other transmissions. So the design of the layout of the wiring is to limit that interference and to limit the pickup of interference from others. Why do we want to limit interference? Who tells us that more interference gives us a lower data rate? Who told us that? Not me, before me, Shannon's equation. If you remember the Shannon capacity equation it's got bandwidth in it, of course more bandwidth, more data rate, but it's also got signal to noise ratio. The more noise the lower the, so if noise is larger SNR is lower and therefore capacity is lower. More noise, lower capacity. Well the interference from the perspective of our signal is noise. So the more interference the more noise the lower the capacity. So that's why we want to keep the interference low so that we can get a higher capacity. In practice twisted pair is the most common of the two. Twisted pair in telephone networks, LANs, very, very cheap, very easy to install. If you need to put some cables through the ducts and through the cavities in the walls it's very easy just to feed the cables and they'll bend as necessary. Some other cabling, in particular for twisted pair what's called unshielded twisted pair is what we see. It doesn't have any special shielding on it. Shielded twisted pair adds an extra protective layer to stop the interference. So with shielded twisted pair there's less interference, less noise, therefore a higher possible data rate. But the problem with shielded twisted pair is it doesn't bend so well and it's a little bit more expensive. So it's not so common. That is if you want to feed it through the cavities in the wall it's very hard to make it bend at the right location. So shielding improves the speed at which we can send our data but sometimes it has some other costs, some financial cost or some cost of ease of use or convenience. There are many different types of the cabling depending upon the usually the thickness and the quality, the thickness of the wires and the quality of the coating. Different categories. We'll see some numbers, some typical data rates that we achieve in a moment. Coaxial cable mainly in audio systems or hi-fi systems to connect components together nowadays. Cable TV, so the cable coming from the wall to your TV if you've got a subscription to cable TV, TV is usually coaxial cable. And in the past some long distance communications like between cities, but mainly that's been replaced with optical fiber. So with optical fiber we send not electrical signals but we send light. So we have a light source that creates some light that reflects or refracts, is it right? It reflects across the some thin fiber of usually glass or plastic and the lights received at the other end point which is used to indicate the data being sent. In terms of practical use, optical fiber is much more expensive compared to twisted pair and even coaxial cable. It's much harder to deal with. You have very thin glass or plastic fibers. Yesterday I showed you a LAN cable, twisted pair and I showed you cut up into pieces. It would be quite easy for you to put it back together because you just get the copper wires and attach them to the right end point. You could just wrap them around each other and you could still send a signal if you connect them back together. Not much fun but you could do it. With optical fiber, if you cut the fiber you essentially break the glass or plastic fibers and you need special devices to join them together, like special devices which are quite expensive. You can't just go and buy one. So in fact the idea of installing optical fiber is usually not something that anyone can do by themselves in a small location you could but to install across the, across a building for example or across a campus usually is much more expensive because of the equipment needed. Optical fiber is used in long distance communications so mainly if you think across cities, between cities, between countries but also used inside some local area networks, inside some buildings when a very high data rate is needed. Maybe in a data center for say Google or Dropbox that has to connect many servers together and they need to transfer data at a high data rate then optical fiber makes sense there. When we transmit a signal we know that the signal power gets weaker across distance. How much weaker, one thing depends upon the material being used that the signal is being transmitted across and also depends upon the frequencies used. Generally the amount of signal that we lose across distance is much smaller with optical fiber compared to the electrical alternatives, twisted pair. So we can send further with optical fiber. The bandwidth available and it's captured although not so well to see but on this one the bandwidth available for optical fiber even though this line is smaller it's approximately 10 to the power of 15 which is what around 100 terahertz is the bandwidth 100 or 1000 terahertz whereas twisted pair is only 100 megahertz. So it's about a million times larger bandwidth available and depends upon the particular systems. Much larger bandwidth with optical fiber, coaxial cable a little bit larger than twisted pair well about 10 times larger. So twisted pair up to about 100 megahertz, the bandwidth available, coaxial cable up to about 1 gigahertz and optical fiber what 1000 or 100 terahertz. Larger bandwidth both the Shannon and Nyquist capacity equations tell us larger bandwidth larger capacity, larger data rate. Much higher bandwidth available so usually a single fiber we can carry the same amount of data per second as tens or hundreds of electrical cables. So you want to connect or carry data from one city to another a single fiber can be used to replace many old electrical cables. As a result small size, lightweight and that becomes important when you need many connections there's a lot of data to transfer say between cities and across countries. So when you have a large amount of data to send it becomes lower or lower cost to install. To install just inside say but from my computer to another computer using optical fiber will be quite expensive because the amount of data that we need to send is quite small. But when we have got the amount of data that say Bangkok sends to Singapore a large amount of data to send using optical fiber will be cheaper to install than using hundreds of electrical cables. The electrical cables get interference from other electrical sources. So my LAN cable can be interfered by the power cable on the computer and other sources that's not the case with optical fiber. So the interference from other sources is not so significant which is a good thing. It's isolated from other sources. So that's about all we want to say about these three examples of guided media. The last few are them will summarize and compare some numbers. Electrical cables in the order of gigabits per second data rates. It's not fixed at one gigabit per second but that's the ranging from lower to maybe several gigabits per second, tens of maybe. Twisted pair the distance is usually limited to a maximum of several kilometers. So if you want to connect our campus here at Bunker D to Rungset, distance of about 15 kilometers then you can't just run one long twisted pair cable under the ground if we even had a duct to run it through because we would not be able to transmit that required distance. What we would need is to have repeaters along the way. Transmit across the first two kilometers from one device to another maybe two kilometers from here and then have a special device that receives like a computer receives and then transmits again. These repeating devices are costly whereas if we used optical fiber maximum distance is in the order of tens of kilometers, say 40 kilometers it differs in some cases. Which we could connect via just a single cable between the two campuses. It makes significant impact when you want to cover a large distance. For example across an ocean, across the Pacific Ocean from Asia, Tokyo for example to the west coast of the US where thousands of kilometers then having to have a repeater every two kilometers becomes very expensive. Having one every 40 kilometers is possible or better. So electrical cables if you only have a small amount of data or your data rates required in the order of gigabits per second then they're the cheapest. But when you want to start to send the data of many users like a whole city of users that's when optical fiber becomes more cost effective. The last few slides we will not look at but just I'll pick out a couple of values just for reference no need to even understand them but this summarizes the range of frequencies we have available. Most of the pair differs depending upon what type is used. Co-actual cable up to about half a gigahertz. Optical fiber 100 plus terahertz so the bandwidth is much larger. The other things you see and we may make sense of them later are repeater spacing so how far can we send with a single cable before we need a device to take the signal and send it again the longer the better the larger the better. Typical delay is like this the propagation delay. We've normally assumed that the speed to send a signal is the speed of light 300 million meters per second but it depends upon the actual material and you can check the calculations but 5 microseconds per kilometer is I think about 200 million meters per second. Speed of light is 300 million meters per second 5 microseconds per kilometer is about 200 million meters per second saying that with most of these technologies we cannot send our signal at the speed of light it's slightly less about two-thirds it varies sometimes. The question is how much signal strength do we lose across distance. We transmit a signal at some power level as it travels distance it gets weaker by how much. This gives some typical values for example co-actual cable 7 dB per kilometer. Remember dB indicates a ratio 7 dB is what? A factor of 7 dB equals 10 log base 10 of our factor our ratio therefore it's a factor of what 5 about 5 which means that we transmit our signal after one kilometer the signal will be 5 times weaker that's what it means there. After the next kilometer it will reduce by another factor of 5 and keep reducing. So the larger the attenuation the more it reduces the signal reduces so co-actual cable is very high attenuation here twisted pair is much lower and optical fiber lower again. A larger attenuation means that the signal gets weaker faster okay we started a signal with some strength say a strength of 100 and over a distance of one kilometer with a attenuation of 7 dB if it starts at 100 after one kilometer it will be five times smaller if it starts at 100 it will go down to 20 the signal strength five times smaller over the next kilometer it will go from 20 down to what four five times weaker five because 7 dB convert it back to a ratio is just a factor of 5 so if you're what point point 2 dB what is point 2 dB as a ratio calculator someone who has a calculator 0.2 dB 10 to the power of point 002 10 to the power of point 002 let's try it on the calculator sorry one zero the attenuation is 0.2 dB which is equivalent to 10 to the power of what 0.02 remember it's 10 log so we divide by 10 first point 2 divided by 10 is point 02 is 1.05 so it's just larger than one that is every kilometer if we started a signal of 100 after one kilometer it will be 1.05 times smaller which is about so if we start at a signal power of 100 after one kilometer it will be down to about 95 but at attenuation of 7 dB if we start at 100 after one kilometer it will be down to 20 the signal strength gets much much weaker faster right the larger this attenuation the weaker the signal will be across a fixed distance that is the attenuation through optical fiber is very very small which means it doesn't get weak across distance as much as the others we will when we look at wireless we will show you some ways to do calculations of the attenuation how much how to get these numbers with wireless it's a little bit simpler than the different materials because with wireless the material is what air but with optical fiber the material that our signal travels through is different than coaxial cable so we need to know about the materials but in air we can have a good approximation of how much signal do we lose again the point is not to remember or know these values just just be aware that when we choose wired technologies we care about the bandwidth available the range of frequencies the larger the bandwidth the larger the data rate we can achieve we care about attenuation because attenuation will impact upon distance usually and other factors we want to be able to send further in many cases especially if we want to build a large network across a country or between countries and delay well usually they're about the same it's a little bit less than the speed of light let's not look at those plots questions on guided media before we look at unguided so let's look at unguided and wireless transmission really and look at the some of the characteristics of transmitting signals through the air and we spend a bit more time on this we'll talk about the concepts first because there are a number of theoretical concepts we need to explain and then we'll talk about just quickly some examples of wireless media there are many different types of wireless communication systems you use at least two probably on a daily basis you use Wi-Fi and your mobile phone so two different wireless communication systems but there are others some you you know about infrared for your remote control Bluetooth but there are many other systems satellite access for satellite internet TV terrestrial TV and a few others will mention for all the wireless systems that we'll talk about we can look at them in a general perspective and this model tries to capture what a wireless system looks like it's common for TV transmission for satellite internet access for Wi-Fi and many other systems we have a transmitter and a receiver and with wireless transmission we send some radio signal some radio waves between the transmitter and receiver and the way that we do that is that so instead of sending an electrical signal through a copper conductor or sending light through some fibres we send some radio waves and to do that we use antennas and the simplest view of an antenna is that it takes some electrical input and produces some radio wave as an output and that wave propagates through the air and the receive antenna receives that and converts it back to an electrical output which is out contains our data we're trying to communicate so we need to look a bit about well what what are the characteristics of antennas we're not going to study how antennas work but we want to at least get to well what are the main characteristics when you buy an antenna what do you look for and then we'll look at in this topic how far can we separate the receive and transmit antennas such that they can still communicate like our wireless access point on the wall if you walk away with your laptop or phone at what distance away from the access point will you stop being able to communicate with it what's the range of communications so we'll look at some ways to calculate that under some ideal conditions so today let's start with antennas an antenna takes some electrical current is input the transmit antenna and produces some electromagnetic waves as output and opposite at the receiver antenna where those waves range from around 3 kilohertz up to about 300 gigahertz other frequencies larger than 300 gigahertz we normally cannot transmit and it's the range this range of frequencies is often called the RF or the radio frequency so sometimes I'll say radio waves it's just the name of that range of frequencies a transmit antenna and a receive antenna if they're the same same characteristics that is if they're the same shape and size same design then they operate in exactly the same manner so often we do not distinguish between how a transmit antenna and a receive antenna operates we just talk generally about any antenna so normally you buy an antenna you can use it for both transmit or receive you don't buy a transmit antenna and then buy a separate device which is receive they can do both now when we send our signal out from the transmit antenna it needs to go we want it to go to the receive antenna so it can receive the signal so we're going to care about the direction at which the signal propagates which direction does it go from the transmitter and how far it propagates the distance of the two main things we care about and they depend upon the shape of the antenna and other things but the design of the antenna impact on that so we'll look at some examples and talk about some typical designs of antennas examples that you have seen you see the Wi-Fi access point these antennas are the called dipoles the most common you'll see around it's called a dipole it comes off here it unscrews very simple just a straight up antenna okay there's just some material in there so you think of the antenna is just a what a 10 or 15 centimeter stick we have two on this device but we could do it with just one okay what other types of antennas have you seen what antennas look like see if we have some pictures I'm sure you've seen many many different types of antennas okay maybe on TV to pick up terrestrial home terrestrial ground wave TV not satellite TV you can have these antennas that just sit on the top also dipole antennas the same as these but just bigger okay usually there's two sometimes if you look these towers for wireless transmission I think this one is for example for transmitting over a long distance say between two towns or to create a link between cities across a large distance so there's big towers and these antennas on those towers this one has others other towers I don't think I have a picture if you see you'll see many around the mobile phone towers you'll see them around and on top of either antennas like this or sometimes more often there are rectangular long rectangular shape if you look closely just different types and shapes of antennas here's one for communicating I think this was to some deep deep space objects Mars observatory or the Mars rover and so on what's the shape of this antenna what do we call it dish good one and the dish the shape of the dish not a donut we'll see a donut later a bowl or parabola parabola parabolic antenna sometimes we we talk about think it's a circle well it's not quite a circle it's got that dish shape a parabolic antenna we'll talk about many antennas have that shape not necessarily that big here's one of the biggest I think the biggest antenna in the world the biggest parabolic antenna it's 300 meters across okay so it's built into the ground to collect signals from space see this thing hanging at the top what happens and we'll see a picture of it later that this transmits the signal down to here and the shape of this parabolic antenna the dish is such that it transmits down and it reflects off and a single small signal comes down here and one large signal reflects off that's how it's shaped that way to achieve this effect of really amplifying the signal the larger the dish will see the larger it will amplify the signal we'll talk about the gain of the antenna a dish or parabolic antenna but use for home access so if you get one of the popular ones is IP star some of the the schools and government offices some homes have them for internet access here it's about half a meter but the same dish shaped antenna do I have any others this is I think this one was a UHF antenna for UHF TV remember we mentioned ultra high frequency yesterday it's just for different channels for TV we usually use different shaped antennas that's all I have whereas the doughnut we don't normally see doughnut antennas we talk about and we'll see a picture of a doughnut later is the shape of the signal that comes out of the antenna that's what we want to explain so in general we can talk about different types of antennas and the first one we'll introduce is called the isotropic antenna let's say my my hand is an isotropic antenna this is the isotropic antenna it transmits a signal which direction does the signal go with an isotropic antenna the signal goes in all directions with equal strength that's what an isotropic antenna is we have the source the transmitter the antenna and it goes forward at the same strength as backward and the same as up and down remember we have three dimensions so an isotropic antenna propagates power so transmits and the power of the signal transmitted is equal in all directions from the source so if you can think of the shape of the power spreading out from the signal spreading out from the antenna you can draw it as some sphere some spherical pattern it gets the signal gets weaker across distance but with an isotropic antenna it gets weaker at the same rate in all directions our donor antenna is next omnidirectional is another general type of antenna and these dipoles are generally considered omnidirectional if you look at the antenna the signal propagates in one plane about the antenna in the same strength so when we transmit with an omnidirectional antenna if we hold it this way the signal going forward going back to the left and to the right goes at the same strength that is one meter in this direction from the antenna if we measure the signal it will be the same strength as one meter to the left one meter to the behind a one meter to the right but if we measure one meter up the signal will be much weaker than one meter on this plane so the signal power is directed to go on one plane so think of the horizontal plane in the same strength in each direction but on the other plane on the vertical plane going up and down it's much weaker so if you could draw a picture of that that's where it looks like a donor it's round around the horizontal plane but up and down it doesn't go the same shape as a sphere so if you think of a sphere and you squeeze the top and the bottom you'll get a donor we'll see some different pictures of that as we go through so that's another common type of antenna the power is propagated in all directions on just one of the planes we have two planes in 3d we have the horizontal plane and the vertical plane what azimuth and elevation other names we'll see with isotropic in all directions it goes equally and we talk about more generally a directional antenna is one that concentrates power in a particular direction more so than the others likely and I've like the pictures I showed of the parabolic antenna don't worry about the details of this but remember the parabolic antenna the picture I showed we have some source device it transmits a signal to the dish and the dish propagates the signal back all in this one direction the signal does not go behind the dish or at least it's much weaker in that direction so a directional antenna concentrates the signal in one particular direction and we'll talk about how they compare and how much they concentrate the signal and we'll use gain to do that it's hard for me to draw them in 3d but try and imagine isotropic is a sphere that is take your isotropic antenna let's say it transmits with a power of 10 watts that's the transmit power what the antenna does is the signal propagates it starts at 10 watts as it goes in this direction it gets weaker and let's say we measure the strength 1 meter away and it's 5 watts if it's reduced from 10 down to 5 across 1 meter then with an isotropic antenna if we measure 1 meter above 1 meter below 1 meter in any direction away from the antenna it would be 5 watts that is the signal will reduce at the same amount in all directions whereas with an omnidirectional antenna if we transmit say at 10 watts we measure on the horizontal plane this way 1 meter away and let's say it's 7 watts and 1 meter here is 7 watts so all around it will be the same signal strength 1 meter away but if you measure 1 meter above it it may be just 2 watts much weaker the signal is weaker 1 meter above and below the signal is propagated in one plane only and generally a directional antenna think is the antenna concentrates the signal in one particular direction so 1 meter away from a directional antenna say in front of me if I transmit at 10 watts maybe it's 8 watts 1 meter away but behind me it's 0.5 watts it's very weak behind me but very strong in the direction at which I transmit depending on the shape of the antenna the design of the antenna will impact upon how much directionality it has how much is the power concentrated into a particular direction and when you buy antennas one of the characteristics that you look for is how much is the signal concentrated in one direction and that's measured by antenna gain so we need to try and look at antenna gain yes yes these this router the antennas those dipole antennas are an omnidirectional antennas so in fact it does matter as to the the orientation of the antenna so generally you think if it's straight up like this and like the one on the wall then think if in a coming out across the ceiling the signal will be the same strength in that direction as this direction but going down it will be weaker across the same distance and going up will be weaker that's ignoring interference ignoring interference and and obstructions like the ceiling and the wall now how much weaker it depends upon the antenna design we'll see some plots or some information about that let's try and first explain antenna gain and then we'll come back to some examples of different types of antennas and see how do we characterize how much stronger or weaker the signal is in a particular direction if you go forward a few pages you'll see there's another handout I included at this time very simple picture if you in your lecture notes you have a page with four four pictures this is one of them just a quick example you go to this one and I'll try and draw it on the screen it won't be as nice but you'll follow along just so we can add a few other notes to it so this you've got the handout with just four pictures on it we'll go through those four but I'll try and create them as we go and explain what they show so we start let's say the idea is we have some antenna this is the location of the antenna and these pictures we only have two dimensions I cannot draw 3d on a piece of paper so you can imagine that we're you need to try and imagine in 3d but we can only draw in 2d but I think you'll be able to make sense of it as we go with an isotropic antenna the signal propagates in all directions with the same strength so if we measure at a point let's say this is a distance of 1 meter just as an example 1 meter away from the source we transmit and I'm just going to switch back to the one you see we transmit at a power denoted as PT we know that as the signal propagates it will get weaker it attenuates so PR the receive power is always going to be less than PT but by how much will it depend upon the shape of the antenna so I'm just going to try and draw that picture and to explain what it looks like why it looks like that so we have PT the transmit power let's say I measure one meter away the signal strength and I measure it to be some power level PR let's give them some values so in the slide the picture you have we I think we don't have values but let's just make up some values so you can also make sense of that what can we use and the values that I put may not be realistic but they're just simple numbers let's say PT is 10 watts we transmit it with a power of 10 watts and then one meter away we measure the the receive power and let's say we measure it to be say two watts if this is an isotropic antenna doesn't matter what direction from the source that we measure any point one meter away if we measure it will be also two watts okay one meter in each direction anywhere from the the source if they're all one meter away you measure and you'll always get it to be two watts because the signal disperses in all directions equally and that's why in the picture you see we have a circle to illustrate that okay I cannot draw a circle as good as on the computer but say we have a circle that at any point on that circle one meter away from the transmitter we receive some signal and we measure that it will be two watts in our example that's if we use an isotropic antenna but in fact we should also draw the the other plane so coming out in 3d now in general with a directional antenna the signal is concentrated in a particular direction think you take this circle and squeeze it then one part may come out here and it'll get smaller at this point and that's what this tries to draw the blue one here tries to draw the fact or the the receive power at particular points with a directional antenna so now we use a different type of antenna same transmit power but change the antenna some directional antenna let's try and draw it so let's say on top of this we have another antenna ours is blue is it same location blue antenna but it's directional so and say the direction is in this direction it's going to be stronger than the opposite direction and that's what the picture that you have in front of you shows what this blue line shows is that okay with our first example the isotropic antenna we transmit at 10 watts it shows the points at which we receive at 2 watts what this blue line shows is that if we use a direction directional antenna at what points at what locations would we receive with a power which is also 2 watts that is it shows that at this point if we measure the receive power from our directional antenna it will be 2 watts and same at this point and all of these points along the blue line it's indicating if we measure the power it will be 2 watts using our example so the shape is it's smaller back here and it may be not the same but yours is better on the picture the blue one says that at this point if we measure the power it will be 2 watts and at this point it will be 2 watts and at all points along that blue line assume it's 2 watts whereas the black line it's 2 watts if we use an isotropic antenna why 2 watts I just made up that number but the point is it's the same power along the line why is that well directional antenna concentrates the energy in one direction that means if our directional antenna is pointing this direction the energy goes in this direction and it will go further using the same amount of power let's say the distance was it went twice as far not so relevant but let's say now we measure the distance between the antenna and this further is point away and let's say that distance is 2 meters so with the isotropic antenna 1 meter away we got 2 watts but with the directional antennas the best case is that 2 meters away we get the same amount of power 2 watts so we've concentrated the power in a particular direction allows allowing us to send further and receive the same amount of power now in the opposite direction how far can we transmit a signal such that we receive it 2 watts well it's less than 1 meter maybe 40 centimeters or whatever the value is here so with a directional antenna the energy goes in one direction but the opposite direction or the other direction it's much weaker the next point focus right at this point that is 1 meter away with isotropic we got 2 watts we transmit with 10 watts using an isotropic antenna 1 meter away we measure 2 watts if we measure using our directional antenna at this red point what will the power be some power level at point x not the exact value but relating to the transmit power and the other numbers what do you think the the red point power level would be we know we transmit at 10 watts we know that the signal gets weaker as it travels some distance so any received powers always going to be less than the transmit power so the red points going to be less than 10 watts but we know at this point 2 meters away the power is 2 watts so the red point well it's going to be greater than 2 watts and less than 10 watts we don't know the value yet but it's going to be less than the transmit power it must be for some point away from the transmitter it's going to be weaker but we know it's going to be greater than 2 watts because we already know if we keep going that same direction a little bit further we get to 2 watts so it's somewhere in between 10 and 2 watts now let's put a number to it so we don't know the number but let's say it was 6 watts just to make up a value let's say we did measure at this point 1 meter away from the transmit antenna and it's 6 watts why 6 because I made it up it's a number less than 10 and greater than 2 okay we would need to measure it so imagine we have a real device we have a transmitter and we have device to measure this receive power and it turns out to be 6 this example is not about the specific values we'll see what it's about right now it's about the ratio between the values so now if I use my isotropic antenna my original isotropic antenna and I measure at this red point what's the receive power with the isotropic antenna transmit at 10 watts receive at 2 watts at this red point but now if I change antennas and use my directional antenna I transmit at 10 watts and at that same point let's say I receive at 6 watts how much better is my directional antenna we can say that let's make some space with the isotropic antenna 1 meter away we measured it to be 2 watts the receive power with my directional antenna 1 meter away let's say we measure it to be 6 watts in the best direction that is that's the strongest direction we can say the directional antenna has a power level 3 times stronger than the isotropic antenna that is the gain of our directional antenna is 6 watts divided by 2 watts a factor of 3 focusing just on that direction that particular direction measure with isotropic antenna we get a power level of 2 watts measure with our directional antenna we get 6 watts at the exact same location therefore we can say the gain of my directional antenna relative to an isotropic antenna is a factor of 3 it's 3 times stronger someone can convert that to db for me 3 equals what 10 times log base 10 of 3 people are busy drawing maybe 3 is our gain so 10 times log of 3 4.77 so I can say my gain of the directional antenna equals 4.77 db relative to an isotropic antenna and here we use a new notation my our directional antenna is 4.77 db greater than the isotropic antenna and in fact when we talk about antennas we normally measure the strength relative to this isotropic and therefore we write instead of saying 4.77 db relative to an isotropic we write 4.77 db I with the I refers to an isotropic antenna and this is an important characteristic of antennas when you look at real antennas you go and buy one the main property that you'll see advertised is the gain of that antenna measured in dbi and it tells you in in the best direction how much stronger the signal will be compared to an isotropic antenna the reason we compare it to an isotropic antenna is because it's our well it's the reference point it's that perfect antenna that just sends the signal in all directions the same any questions so the values are not so important six or two watts because the gain is a ratio if it was 12 and 4 watts it'd be the same game it's still a ratio of 3 if it was 600 milliwatts and 200 milliwatts we still get the same ratio why was it six watts why did I choose six well I just chose a number between 2 and 10 to give an example gain in when you buy an antenna it usually specifies what is the value of the game so antennas are measured relative to this isotropic antenna and in fact there is no such thing as an isotropic antenna you can't build one they're just a theoretic that is in practice you cannot build a perfect isotropic antenna you can get close but the gain the signal spreads a little bit in different directions but real antennas are measured relative to an isotropic antenna let's add a little bit more explanation what about in the opposite direction for our directional antenna we transmitted at 10 watts what is the value here it's 2 watts remember the idea is that the blue blue line at every point on the blue line the received signal would be 2 watts so this would be 2 what is the value at this point for our blue directional antenna if we transmitted 10 watts at this point it's 2 watts so as we go further it's going to be less than 2 watts we don't know the exact value but it's we know it's going to be less than 2 watts let's say we measure it and it's half a watt so for our isotropic antenna 1 meter away we measure we get 2 watts with our directional antenna 1 meter away we measure and we get half a watt calculate the gain of the directional antenna in this direction in a different direction we measure the isotropic to be 2 watts and the directional to be half a watt find the gain the gain relative to isotropic half a watt divided by 2 watts is a quarter 0.25 dbi that is we're saying that if I use my directional antenna and we look at the opposite direction the bad case the signal we one quarter the strength of as if I used an isotropic antenna it's smaller than what we would get if we use isotropic so the gain is less than one it's actually a loss in that case minus 6 db when we log we'll get a negative minus 6 dbi relative to an isotropic antenna my gain is minus 6 db it's actually a loss of 6 db is 6 db weaker than the isotropic and if we look at other directions we'd get different values that is the blue one you can see in this direction it's going to be strong with a high gain but going the opposite direction there's a weak or a low gain a loss in this case and in other directions the gain is going to be different so it depends upon the shape and the design of the antenna as to how the gain or what is the gain relative to isotropic at different locations from that antenna and you see plots like this that show the antenna pattern where it's strongest and weakest normally when you buy an antenna you care about the strongest point that is the first one we don't normally care about this because if we point it in the right direction if we orient the antenna correctly then we will use the strongest our receiver will be in the strongest direction so we care about the strongest point so when you buy an antenna it's usually this value which is listed the the gain relative to isotropic in the strongest direction I said for a directional antenna I mean it's for any antenna any real antenna omnidirectional is also directional compared to isotropic think of isotropic as a reference antenna the perfect antenna that sends the signal equally in all directions every other antenna can be compared to that isotropic antenna and we can compare it in terms of the gain questions correct the gain in different directions will be different it depends upon the shape of the antenna right the same as when I talk to you if I didn't have the microphone it would be easier to hear me if you're in front of me compared to behind me because my signal is going in this direction it's not propagating at the same strength or it doesn't come out in the same strength in the reverse direction or there because of different reasons again yes in this case if we want to transmit further distance we need an antenna with a higher gain because with the same amount of input power we'll be able to go further if the gain was in the our example we had a factor of three it was a factor of four it would mean that it would be four times larger than the isotropic antenna one meter away which means we could get further away to get to two watts so for the same transmit power and at the same receive power the larger the gain of the further the distance between the transmit and receiver the further we can send with other conditions being the same the look at our example same transmit power both antennas both we transmitted at 10 watts with a particular receive power in this example two watts if we want to get a receive power which is the same then the higher the gain the further away from the antenna we would be able to receive at two watts with our isotropic antenna we have to we are one meter away when we receive at two watts with my directional antenna I was two meters away when I receive at two watts if I had a higher gain then maybe I could be five meters away and still receive at two watts the higher gain we will effectively lose less power over the same distance in the last five minutes I'll just show you some examples of different antennas and and the gain values not this one so I think we've covered that the gain in the absolute value is the the signals received at some point when using our directional antenna versus what would be the power received if we use an isotropic antenna so Px is the power received using our blue antenna at that same point what power would we receive if we use our isotropic antenna PR the gain is the ratio of them and of course we can convert to db just look at a couple of websites that lists I'll go direct to some pictures Cisco is a company of the builds networking equipment and they sell some Wi-Fi antennas they may not be the best or the most recent but they have nice specs so this is the antenna that is the website has some good information what's happened on maybe my browser has crashed there it goes something crashed let's bring this one back up try and zoom in this is just the specs of a particular antenna one of these dipole antennas you can buy one and we scroll through one of the characteristics is the gain this dipole antenna has a gain of 2.2 dbi which means in the strongest direction and with a dipole the strongest direction is should be the same in the one plane in the strongest direction it's 2.2 dbi larger than if we used isotropic and you see these plots here try to capture like we drew but in two two planes to the azimuth and elevation the horizontal and vertical plane that is in the same direction around it the power will be the same but up and down the power will be strong at this level it will be weaker up and weaker down that's what this second blue diagram tries to capture you don't need to understand them but there are plots that show in which direction is the signal strongest strongest and where is it the weakest scroll down to right this is just a different dipole antenna let's find some others this is an antenna that you can say placed on the wall on the ceiling different shape it's no longer just a stick it has a gain of five dbi it's called a sector antenna look at the this plot here this focuses that the signal in a particular direction so it's strong in this direction but weak in this direction and it's different going up and down vertical plane and a gain of five dbi which means if you orient it in the right right way such that your receiver is in this direction the signal risk recede will be stronger than if you use the other antennas you have others some ceiling mount and omnidirectional antenna five dbi so just a bigger antenna than these stick antennas a masked mount so one you can again bigger that you can stick on a stick outside a different wall mount antenna again one of these sort of square antennas you can stick on the wall and it concentrates slightly different than the others so you can buy an antenna depending upon what area you want to cover you choose an antenna to fit the others that just get stronger I think here's 10 dbi okay concentrates a lot in one direction and it spreads out a little bit in other directions 12 dbi you see these start to get strange shapes because the way the antennas design so you think of going up and down the signal strong all around but up and down can be quite weak and just keep giving different shapes it's like trying to concentrate the signal in one direction it goes in one direction but also goes a little bit in other directions I think we just get different shapes and different now we get to a dish antenna parabolic dish it's highly directional a gain of 21 dbi you can think the energy is really squeezed into one particular direction much higher gain but if your receiver is not in the right direction you will not pick up the signal and I think that's all the useful examples they have there next week we'll we'll look at the attenuation and how much power we lose with signal so we talked about antennas then we'll look about their power loss in wireless