 Hi, I'm Zor. Welcome to Unisor Education. Well, let's continue talking about radio. The last lecture was about how we receive radio signal. Today I would like to talk about how we transmit it so somebody can receive. This lecture is part of the course called Physics for Teens, presented on Unisor.com. The website contains the prerequisite course, Mass for Teens. Mass is a mandatory piece of knowledge which you have to really study before going into physics relatively seriously. Now, the video recording is on YouTube and that's where you probably might find this lecture, but I do suggest you to go to Unisor.com website because it contains basically the whole course. There's a menu divided into certain parts, topics, lectures, etc. They're all logically connected to each other and I'm referencing from one to another. And the site is completely free. There are no advertisements, no strings attached. You don't even have to sign in if you don't want to. Just if you would like to participate in the educational process which involves exams or maybe your teacher or parent will supervise you, etc., then you might need to go into signing in, etc., etc. But nevertheless, all is free, no ads. Okay, so let's talk about transmission. The first thing, let me remind you how we received information. Now, there are many different ways, basically, many different designs. And my idea is just to introduce you the principles which are probably lying in the foundation of any design. So you're just getting acquainted with idea rather than with a concrete design. The real design is much more complex, but the idea is very important. Unless you are a professional, you probably don't need all these small details about implementation of this idea. But that's how it started. And whenever it started, that's the very big jump forward. Then the idea is gradually developing into better quality, etc. But the big jump is what I will just draw you right now. So here is something which is called LSE circuit, if you remember. It contains capacitor and inductor. And we have actually talked about if I will have a switch here. So this is the battery. And this is a switch which I can connect either to position A. And in the position A, my capacitor can be charged from the battery. So this is plus, this is minus. So electrons will accumulate here. And there will be deficiency of electrons here. And then as soon as we switch from A to B, we will observe oscillations. Why? Because electrons will start moving from excess of electrons to deficiency of electrons from minus to plus. And the inductor will assure that whenever these electrons will completely exhaust their excess, the magnetic energy accumulated in the inductor will support the movement and electrons will go even further making deficiency on this side and excess of electrons on that side. So that's all related to the Faraday's law of induction. So whenever my current diminishing, my magnetic flux is changing. It accumulates certain energy and the electromotive force is developed by the Faraday's law. And that electromotive force supports the electricity, the electric current to continue moving from this to this. And that would create excess of electrons on this side up until a certain limit and then back. So it will oscillate all the time. Now, what's interesting is this is just an LC circuit. Now let's forget about this for a while and attach antenna to this. So this is antenna and I am attaching this antenna, this is grounding, and I am attaching this antenna to a circuit using some kind of transformer or something. You can have the same core and two different wiring on the same core. There are many different ways to transform one magnetic field to another. Again, that's induction. So if there is a common core between these two wires, the magnetic field will be created and it will be a variable magnetic field because it's oscillating, right? And since it's a variable magnetic field, it will create the variable magnetic flux actually, it will create the similar thing here. So from one inductor, the oscillations will always go to another. So if this is an antenna and antenna is again piece of wire basically where electric current occurs, if there is an oscillation of electromagnetic field around it, so if somebody transmits this onto this antenna, in the antenna we will have variable current, it will create variable magnetic flux here and this magnetic flux will cause the induction, inducting current in this particular inductor and the oscillation will be here. But the problem is that if this is as this, antenna accepts many different transmitting signals with different frequencies and all of them are going into this particular circuit. But if you remember, the circuit has its own frequency of oscillation, which has omega is equal to 1 over square root of L times C and only this particular frequency will resonate, everything else will be significantly smaller. So the resonate frequency will be actually the major oscillations which are coming here. So that's how we accept signals, but now the question is how we transmit to this antenna. But let's look at this from a different perspective, from a reverse perspective. What if you already have oscillations here and relatively strong oscillations with this particular frequency? Well, it will cause the oscillation of electromagnetic field and the electric current here, which will again be variable, so it will actually create an electromagnetic field around it and if this is significantly powerful device, then the electromagnetic field will actually spread all around it. So transmission in theory can be achieved using a very similar device as a receiver. So if receiver accepts everything and then the only one particular frequency gets resonated and we can use this to extract, to receive that particular frequency, on the transmitting side, if you already have a specific frequency, it will go to antenna, antenna will create the electromagnetic waves of that same frequency and that's if it's powerful enough, it will spread all over and that's how this particular frequency will go out. Now some other installation might have another frequency, so that's how different radio stations are transmitting their own signals on their own frequencies. Now, I did not talk yet about how to send some kind of a signal, like a speech for instance or image on a TV etc. I'm talking only about main frequency. We will use this main frequency using certain modulators, we can transmit information but that would be a subject of a different lecture. Right now we are talking about how to transmit certain electromagnetic waves of certain frequency. So if we transmit certain frequencies and somebody else can receive that particular frequency by, if this is a receiver, this is usually a variable capacitor which can be tuned to a specific frequency. If it's a transmitter then this is a fixed thing and it's fixed on certain, I mean it also can be variable so it will be able to transmit on different frequencies but it will transmit the frequency which is exactly equal to this in this particular case. So this is a general idea of transmitting information. Now what's the problem? It will obviously not work like that. Too simple, right? Okay, so we have one major problem. The oscillations do not occur by themselves. I mean even if we will attach some kind of a battery to this and charge our capacitor it will start oscillating but it will finish very soon because any circuit has certain resistance and whenever you have a resistance involved that changes everything because if you remember the oscillations in case we have a resistor are damped and damped oscillations will exponentially go down in amplitude and that will be it. So we have to somehow maintain oscillations which are sustainable. So we have to somehow connect the source of energy to this not just on a one-time basis and then let it go but we have to constantly push it. Now there is an obvious analogy. The swing for instance on the children's playground if you just push it once well it will start oscillating back and forth and it will be damped oscillations because of the friction. So the oscillations will gradually go down exponentially by the way. Okay, so we really have to do something which parents on the children's playground do on the swing which means they push on every swing. So just a little bit but we need a little push every time and that's how we maintain stable oscillations. So we have to somehow connect the source of energy, the battery here but not always, it should always help which means that if this plate is negative we can add additional negative charge to this but if it's positive and we will add negative that would be the opposite effect. We will just slow down the oscillation. Not slow down but we will reduce the amplitude. So how can we do it in such a way that something like this switch would be only in case for instance if this is again something like positive negative. So if this is positive now we have to really open the switch to this direction when this is positive and this positive will add a little bit more positive and this negative will add a little bit more negative and then as soon as this is done as soon as one particular period of oscillation is finished we have to flip the switch into this to let it continue oscillating. It's kind of complex to arrange obviously but that's actually the whole thing that's exactly what we have to do. So the question is how can we make such a circuit which would be sustainable which will continue oscillating using some kind of a source of energy to compensate for damping effect of the resistance. Okay so let's not talk about antenna. We know what to do with antenna. What we have to do is we have to somehow devise the circuit which would be oscillating using certain source of energy but it will be actually synchronized. The source of energy will be synchronized with oscillations. So how can we do that? Now I will suggest some very very basic idea and probably it's similar to what the first developers inventors of these type of schemas these type of circuits were actually going through. Again let me re-emphasize that what's important is the initial idea. Then it can be developed into a better one but initial idea somebody just came up with this or similar, I mean there are many similar ideas actually which achieve exactly the same thing. So I will just explain one of those ideas and then just basically let's just think about this beginning of a very very long way of developing and perfecting these or similar ideas. Okay how can we do it? Now first of all the switch. Now we cannot obviously have a mechanical switch here. We have to have some kind of a switch which is electronic which can be made in sync which can be synchronized with oscillations. So we will open it whenever you know pluses here and minuses there and we will close it otherwise something like this. Now how can we do it? Well what is a switch in electronics? Well in electronics we have a very simple device which we were discussing before which can be used as a switch. Now long time ago it used to be called triodes and later on the transistors were developed and using semiconductors the functionality was achieved exactly the same. I will explain it on triodes because in the triodes you just basically see better what exactly is happening but then obviously it was changed to transistors it was lower voltage probably different other things but in any case contemporary switches are obviously on transistors but with triodes it is easier to explain. Triodes has cathode which is kind of emitter it's hot and when it's hot it emits electrons it's like an electron clouds above it and again all this was explained in a separate chapter of this course which is devoted to electromagnetism and there is an anode now there is a grid in between now this if it's heated it emits electrons but usually there is a small heater here with a small source of energy so let's just not talk about this whatever it is it's hot now whenever it's hot it emits electrons surrounding this cathode like a cloud now if this grid is positive then electrons will go this way through the grid and reach anode so there will be some kind of a current if we will close the circuit if this grid is negative it will repulse electrons and there will be no current so basically depending on this charge of this grid we can open and close the circuit okay great so let's make this circuit with some kind of a source of energy and close it here okay now in this particular case now all we have to do is to change plus to minus or minus to plus in sync with these oscillations how can we do this well here is the way how I can do it I'll have another inductor here connected one end here and another end to the grid so oscillations are here which means the electric current goes this or this and electrons are actually going in one or another direction so an analogous oscillations will be in this particular inductor it will this changing electric current which is pulsating from plus to minus and it will induce electric current here and electrons either go this way or that way so whenever electrons are going this way this would be negative and that will repulse electrons from the heated cathode and it will not reach this as soon as it's positive electrons will go here so basically this is exactly what we need in sync with oscillations of this oscillations in this particular inductor will cause opening or closing this circuit and as soon as it's open it will go here so it will charge what we have to do I mean we will not do it but what really designers do they have to really adjust parameters all these capacitors inductors L1 and L2 the voltage here and parameters of the triode or transistor in a later time they have to be adjusted properly in such a way that we really are feeding our oscillations in sync so again analogous with swings on the children's playground you really have to push whenever the swing goes this way and you push it this way if you will do it whenever the swing goes this way and you push it this way obviously the amplitude will diminish immediately so there are certain calculations which people who designed these types of circuits they had to go through and experiments the problem is it's really complicated because if you will obviously add a resistor here which basically is the source of the necessity for this you really have a very complex forces which are actually acting on the current because on one hand you have the oscillations within this circuit by itself and then you have oscillations in this circuit where the source of energy is coming through you see before whenever we were explaining the LC circuit I mechanically changed the switch so it's either this or that either I fed the capacitor or capacitor and inductor start oscillating now everything seems to be exactly in the same circuit all the different parameters and a lot of experiments actually were made before people came up with certain numerical characteristics necessary for all these components to be in sync in such a way that the whole thing is really working it's really maintaining the oscillations of certain frequency by the way the frequency will change because of the resistor and all other devices not significantly but slightly changed all these have to be really brought into perspective and into calculations which is not easy it's completely beyond the scope of this particular course it's basically for professionals whenever they're going into real things about this my idea was just to introduce you to something which can actually be transformed into some kind of device with proper calculations, experiments, etc and there are other ways of feeding our oscillations with energy now we are feeding through inductor there are certain other ways to feed it through a capacitor or a second capacitor I mean if you will take a look on internet for instance how the transmitter is basically created you will see many different circuits this is probably one of the simplest ones much more complex with not one but other capacitors other inductors sorry were introduced into this circuit to do certain things to filter something, to stabilize something to improve certain characteristics and because it's so complex you can see now that there are many different manufacturers and many different radio devices and they all have different qualities we have more expensive devices which are capable of doing significantly better job like hi-fi for instance and there are some older maybe circuits which are not capable of doing something like this so it's all very much involved and again my purpose was to introduce you to idea that we need some kind of a circuit which maintains certain oscillations with certain frequency and if these oscillations are strong enough then we can connect an antenna which probably is another inductor somewhere here which is connected to antenna and the strong one actually in this particular case if we are talking about transmitting we need a very strong signal so the whole thing is very much involved and again this is just how it started if you wish how the first thoughts about radio transmission came to certain people who really invented the radio okay basically that's how we can maintain certain oscillations of certain frequency and transmit it now the question is how to use this to transmit some signal so one thing is to just transmit free oscillations with certain frequency but it doesn't carry any information so how can we put information into these oscillations so we have learned how we can transmit the oscillations now we can learn how to use these oscillations of electromagnetic field to transmit certain information and receive it so we have to now not only be able to transmit the main as they are saying carrying frequency but use this carrying frequency and modulate it in some way to transmit some useful signal and that would be the next lecture thank you very much and good luck