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Transmission Lines & E. M. Waves Prof. R. K. Shevgaonkar Electrical Engineering Department IIT Bombay Lecture – 43 Up till now, we discussed propagation of electromagnetic wave along a guiding structure. That means we had a structure which can guide electromagnetic waves along it. For example, we had a coaxial line where energy was confined between the two conductors and the energy was moving along the length of that coaxial cable. We had a parallel wire line where t
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  Transmission Lines & E. M. Waves Prof. R. K. Shevgaonkar Electrical Engineering Department IIT Bombay   Lecture – 43 Up till now, we discussed propagation of electromagnetic wave along a guiding structure. That means we had a structure which can guide electromagnetic waves along it. For example, we had a coaxial line where energy was confined between the two conductors and the energy was moving along the length of that coaxial cable. We had a parallel wire line where there were two conductors which were parallel to each other and electromagnetic energy was guided along the length of these conductors. Then we discussed propagation of electromagnetic waves along the media interfaces. Again the media interface was the guiding mechanism. So, electromagnetic wave used to be guided  by this interface. Then we went to structures like waveguides where again the conducting boundaries were guiding the electromagnetic energy along with them. If you recall, we also have said when we started discussing the topic of electromagnetic waves that let us not worry at that moment how the electromagnetic waves are generated. Somehow, we ask the question, if the time varying electric and magnetic fields existed, in what forms they would exist and we concluded that they would exist in the form of electromagnetic waves. So, without asking the question how these fields were generated, we investigated the characteristics of the electromagnetic waves. Now we ask the question however, how the electromagnetic waves are generated and that is the topic which is covered under the title what is called radiation. A device which generates electromagnetic radiation is called antenna. 1  (Refer Slide Time: 03:26) So, this topic essentially deals first with the principles of generation of electromagnetic waves and then we will talk about the practical devices which can generate electromagnetic waves from currents and voltages and which can convert the electromagnetic energy electromagnetic waves to currents and voltages when these waves impinge on the structure, what is called antenna. So in this topic, essentially we investigate the characteristics of this device, what is called antenna, which essentially is the transducer which converts the electrical quantity, like current and voltage, into the electromagnetic quantities like electric and magnetic fields and vice versa. So, when we use this device like a transmitting device, we excite this with the voltage and current, and antenna can generate electromagnetic wave, the same structure is capable of receiving electromagnetic wave. That means when the electromagnetic wave is incident on the structure, you get currents and voltage at the terminals of the structure, what is called antenna. So, first we will develop the foundation of radiation and then we will go to the practical structures like antennas. Now, the basic for radiation is the accelerated charges. If you recall, in the electrostatic case, we had a charge; and then the charge essentially produced 2  what is called electric field. The effect of this charge could be felt by electric field. If the charges are kept in motion, uniform motion, then they constitute current and then we know there is a constant uniform current; gives you the magnetic field. So, if you have a charge which gives you electric field, if the charges are kept in motion, then they give you the magnetic field. Question one can ask is, if the charges are accelerated, and that is what will happen if you are having time varying current; so, if you take a current which is varying as a function of time, the charges are no more moving with the uniform field. So, they get accelerated and decelerated. Then, what kind of field should be generated? Thus the question essentially is asked under this topic - what is called radiation. So, as we know from our basic physics laws, that when the charges are accelerated, there is a  possibility of radiation; they throw energy in their surroundings. So behind radiation, there has to be acceleration of charges because accelerated charges are capable of throwing the energy in their surrounding environment. However, every accelerated charge may not give you the radiation. So, when we say that you require acceleration of charges or the time varying current for generation of radiation, we say that if we have a time varying current, there is a possibility of radiation and the reason for this is very simple. When we talked about the coaxial cable or parallel wire transmission line, there also we are having time varying currents. Electrons are moving on the conductors and we have seen that the electrons get accumulated; again they move; they get accumulated somewhere else. So, we have a movement of electrons back and forth along these conductors. So again, there was acceleration and deceleration of charges. But this was not giving you radiation. The energy was guiding along these conductors and there was no energy loss from this structure. 3  What do I mean by that is that if you are having a guiding structure, if the energy starts moving perpendicular to that, that is a loss of energy because that energy now cannot be collected at the other end of this guiding structure. So, this structure which is guiding structure, can guide the electromagnetic energy; but it will not give you an energy which will be going transverse through the length of this structure and in that case we say there is no radiation from the structure. So, we have acceleration, deceleration of charges. But we do not have radiation from this structure. So, though we have acceleration and deceleration of charges, since we do not have radiation, I cannot make a general statement that every accelerated and decelerated charges would give you radiation. But we can say that there is a possibility because accelerated and decelerated charges are capable of giving radiation. So then, why these things do not give radiation? Because, you have two currents which are flowing equal and opposite in the two conductors of a transmission line. (Refer Slide Time: 11:01) 4

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Jul 23, 2017

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