 Myself, Piyusha Shedgar, Assistant Professor, Valchan Institute of Technology, Solapur. In today's session, we will discuss the BABINETS principle. These are the learning outcomes for today's session. At the end of this session, students will be able to illustrate the BABINETS principle. They will be able to discuss the various cases of the BABINETS principle. These are the contents for today's session. So, before going to start the BABINETS principle, you can pause the video here for a second and recall that what is the meaning of polarization of an antenna. Yes, the polarization is the direction in which the electric field of a radio wave oscillates while it propagates through a medium. When an electromagnetic wave passing through any medium, then in which direction how much amount of the signal is radiates is decided by this polarization. So, let us start with the today's topic BABINETS principle. It states that when the field behind a screen with an opening is added to the field of a complementary structure, the sum is equal to the field when there is no screen. In another way, the BABINETS principle states that the sum of the field at a point behind a plane having screen and the field at the same point when a complementary screen is substituted, it becomes equal to the field at the point when no screen is present. In detail, the BABINETS principle we will discuss in the next slide. So, Jacob's BABINET which is French Physicist and the Mathematicians formulated the theorem that similar diffraction patterns are produced by two complementary screens. Whenever the two screens are considered and these two screens are complement to each other, then there will be the similar diffraction pattern will obtain. This is nothing but the BABINETS principle. BABINETS principle relates the radiated fields and the impedance of an aperture or it is also known as a slot antenna to that of the field of a dipole antenna. So, using BABINETS principle how the dipole and the slot antenna relate to each other we will discuss in BABINETS principle. So, the in BABINETS principle the polarization of a slot antenna gives the linear polarization. The fields of the slot antenna almost the same as that of the dipole antenna and the field components are interchanged. Although the fields of the slot antenna almost same as that of the dipole antenna, but the field components are interchanged. That means whenever the antenna having the vertical slot which gives the horizontal electric field whereas, in case of the vertical dipole antenna has got the vertical electrical field component. Now, the different features of the BABINETS principle in BABINETS principle does not consider the polarization of the antennas in establishing the field relations. It dealing with the absorbing screens. It is useful to reduce the slot antenna problems wherever it used the slot antenna problems will be reduced by using the BABINETS principle. So, suppose this is the figure which having the figure one having one of the screen and the figure two having one of the screen. Suppose in figure one this is the metal sheet is considered at the center of this metal sheet the slot is provided. Whereas in second figure the slot is replaced by the complimentary screen that is this slot is having only the metal sheet and around of this metal sheet there is nothing. So, these two screens are complement to each other. This is nothing but the BABINETS principle. BABINETS principle discuss the three cases, three different cases. It depends upon the source placed at one of the point, plane of the screen which is denoted with the S1 and the observation plane which is denoted with the S2. So, the figure A shows the presence of the screen between the observation plane and the source whereas, the second figure complimentary screen is placed between the source and observation plane and the absence of the screen is considered as the third case. So, let us discuss these three cases in detail. So, in above figure you can observe that the S is nothing but the point source, S1 is the plane of the screen, S2 is the observation plane, E1 is nothing but the electric field at any point p, x, y, z behind the screen whereas, E2 is the electric field at point x, y, z behind the screen when S1 is replaced by its complimentary screen and E3 is nothing but the field at the same point when there is no any screen is present. So, according to the Babinette's principle the sum of the field at a point behind the screen and the field at a point behind the screen when the S1 that is the plane of the screen is replaced by its compliment. So, addition of these two screens, addition of these two fields is nothing but the field at the same point when the no screen is present that is E3 becomes equal to E1 plus E2 this is nothing but the Babinette's principle. Now, let us discuss the case one whenever the point source is placed here and the electromagnetic signal is radiates from this point source it radiates the signal to the screen in this manner. So, whenever the plane of the screen is placed between the source and the observation plane you are getting this dashed portion is nothing but the shadow of the signal. This shadow is observed within this signal why because the plane of the screen is placed between the source and the observation plane. So, here the light source and the conducting plane with an aperture before the screen is provided. So, here you can observe that whenever the screen is placed between the source and the observation plane then it passes through this screen and you getting the shadow portion between these two points. Now, consider the second case, second case is nothing but in the previous case S1 is the screen that S1 screen is complemented in the second case that is the complementary screen is provided between the source and the observation plane. Now, consider this is the light source and the conducting plane of the size of the aperture in the previous case being held against the screen. So, here you are getting the shadow in this portion instead of this portion because the complementary screen is placed between the source and the observation plane. So, the light does not pass through the plane but through the remaining portion. So, this is nothing but the complementary screen when placed between source and the observation plane. In the third case when the no screen is there that is the absence of the screen, there is no any screen placed between the source and the observation plane. In that case the resultant combination becomes equal to the addition of the first two cases. So, that means the electric field or the energy generated without screen is becomes equal to the addition of the energy generated due to the screen placed between source and observation plane and this energy generated due to the complementary screen. This is nothing but actually the BABINETS principle. So, by using BABINETS principle the slot antenna difficulties are minimized. But BABINETS principle at radio frequencies is it not possible to have the perfectly absorbing screen which is the requirement of the BABINETS principle. Hence, it cannot be extended to the electromagnetic problems. So, an extension of this BABINETS principle which induces the polarization and more practical conducting screens was introduced by the booker. So, in case of the slot antenna this BABINETS principle is used to get the proper characteristics of the slot antenna. Slot antenna is nothing but one of the type of the aperture antenna. In slot antenna BABINETS principle is used so that you can relate the slot antenna and the dipole antenna to each other. Not exactly the practically it is possible to relate it between the polarization, but the slot antenna and the dipole antenna is relate to each other by using this BABINETS principle. These are the references for today's session, thank you.