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Transmission Lines and E.M. Waves Prof R.K. Shevgaonkar Department of Electrical Engineering Indian Institute of Technology Bombay Lecture-52 Welcome, up till now we studied the radiation characteristics of Hertz Dipole and following that the linear dipoles we saw that the Hertz Dipole has a very broad radiation pattern and consequently it has a very low directivity. For making the directivity higher that means for making the radiation pattern narrower we invest
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  Transmission Lines and E.M. Waves Prof R.K. Shevgaonkar Department of Electrical Engineering Indian Institute of Technology Bombay Lecture-52 Welcome, up till now we studied the radiation characteristics of Hertz Dipole and following that the linear dipoles we saw that the Hertz Dipole has a very broad radiation  pattern and consequently it has a very low directivity. For making the directivity higher that means for making the radiation pattern narrower we investigated dipole antennas which are of finite length however while doing this we found that a terminal impedance of antenna gets modified so as we increase the length of the dipole two things happen the directivity of the antenna increases and beam width becomes narrower but the same time we start getting multiple beams that means the radiation starts going in some undesired directions. (Refer Slide Time: 02:41 min) We also develop some nulls that means is the directions in which there is no radiation and at the same time the terminal impedance of the antenna also gets modified. So what we  find is that as we try to manipulate the radiation characteristics like radiation pattern the terminal impedance automatically gets modified we do not have a independent control over the terminal impedance against the radiation pattern. In many applications we would like to modify the radiation pattern depending upon the need but the same time we want that the impedance characteristics of the antenna should not get modified. We also saw that for a dipole since the current distribution was sinusoidal we knew the current distribution we could find out the radiation pattern, however, if we take some arbitrary antenna then it is very difficult to find out the current distribution on that but once we get the current distribution on that antenna then finding out the radiation pattern is a straight forward problem but the reverse problem we mentioned earlier that somebody gives you the radiation pattern and says tell us the physical structure which will give me this radiation pattern or giving just the physical structure can we just say half hand what kind of current distribution will be existing or for a given current distribution what should be the physical structure these problems are extremely difficult or sometimes impossible  problems. So what we want is we want to manipulate the current distribution because from our Fourier transform relationship between the current distribution and the radiation pattern we know that if you manipulate the current distribution we can get the desired radiation  pattern. So now our requirement is that we must have a mechanism of modifying the current distribution without affecting the terminal characteristics of the antenna and that kind of flexibility is provided by the antenna arrays. So as the name suggests the antenna array is collection of basic antenna elements.  (Refer Slide Time: 05:04 min)  Now what we do is we essentially have a large number of antennas whose terminal characteristics are decided by the pre designing this antenna and by placing this antenna in the vicinity of each other and exciting them simultaneously we essentially get the superposition of the fields due to each of the antennas and because of the phase space which will be there because of the placement of these antennas the total radiation pattern gets modified. So by the use of the antenna array essentially we decouple the terminal characteristics of the antenna and the radiation pattern, of course when two antennas are  brought in the vicinity of each other the terminal character gets modified but this modification is marginal if the spacing between two antennas which are kept in the vicinity of each other are more than λ/2. S o that means if the antenna elements are separated by a distance more than λ/2  the terminal characteristics of the antenna  practically remain unchanged, however, the superposition of the field due to different antennas can give you modification in he radiation pattern. So in fact by using proper distribution and the excitation of the different antenna elements one can achieve any current distribution and consequently we can realize any arbitrary radiation patterns. So an antenna array essentially provides flexibility in designing the  radiation patterns without affecting the terminal or the impedance characteristics. The idea is as follows, what ever frequency we want to work on we first find out a suitable antenna with proper impedance bandwidth characteristics let us say we can take a dipole antenna and match it to what ever data we want to make it to over the bandwidth so this is now the basic element which is the radiating element. Now by reproducing the same element at different locations in space essentially we create an antenna arrays so each antenna now is a best behaved radiating element at that frequency for that bandwidth and superposition of all the radiations from different antennas of the similar type we will get a radiation pattern which will be the desired radiation pattern. So when we talk about antenna arrays principally there is no necessity of having different antenna elements which are identical you may have different elements one may be dipole other may be parabolic dish third may be something else, however, it does not really give an advantage of using different antenna elements in the array in fact it is more advantageous to use identical antenna elements in the antenna array so that your analysis  becomes simpler and the final radiation pattern which we get for the antenna array is essentially decided by the array characteristics rather than by the individual antenna characteristics. So once you have a basic antenna element we worry only about the terminal characteristics and the radiation characteristics are decided by array and the basic radiation pattern of the antenna elements does not play any role in the final radiation  pattern. So with that understanding then one can say we are essentially going to put identical antenna elements in some special configurations excite them with some proper  pattern and this is going to create a radiation pattern which will be the desired radiation  pattern. So let us say antenna array as we say is collection of antennas so let us say we have dipole antennas which could be like that they are located in different locations and each of them might be excited with different currents which are complex currents so this one may be having a current I 1 , this may have a current I 2 , this may have current I 3 , and this is I 4  I 5  and so on.

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Jul 23, 2017
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