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Transmission Lines and E.M. Waves Prof R.K. Shevgaonkar Department of Electrical Engineering Indian Institute of Technology Bombay Lecture-50 Welcome, up till now we discussed the Hertz Dipole the extension of the Hertz Dipole to a dipole of finite size we also saw general characteristics of antennas like directivity, effective aperture, radiation pattern and so on. In this lecture we investigate some practical antennas which are used at low frequency at u h f and v h f
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  Transmission Lines and E.M. Waves Prof R.K. Shevgaonkar Department of Electrical Engineering Indian Institute of Technology Bombay Lecture-50 Welcome, up till now we discussed the Hertz Dipole the extension of the Hertz Dipole to a dipole of finite size we also saw general characteristics of antennas like directivity, effective aperture, radiation pattern and so on. In this lecture we investigate some  practical antennas which are used at low frequency at u h f and v h f bands, essentially these antennas lie in the category of dipole, however, what we note when we go to the low frequencies since the dipole should have length comparable to the wavelength to have a substantial radiation the size of the dipole becomes excessively large when we go to low frequencies in that situation a monopole antenna is used or essentially it finds the application in medium wave broadcasting. (Refer Slide Time: 02:45 min) So if you go to the medium wave radio stations invariably the antenna which is used for  broadcasting is the monopole antenna. In fact the radiation principle behind monopole  antenna is identical to that of a dipole only thing is here we have only half of the dipole and we erect that half length of the dipole over a ideal ground plane. We have seen as we go to lower frequencies even if the conductivity of the medium is not very good the medium starts behaving more like a conductor. Precisely that’s how we make use of that when we go to low frequencies the earth which is not a very good conductor starts behaving more like a conductor and then if I have an antenna if I erect it over the ground surface it behaves as if it mounted over a ideal conducting surface and for investigating the characteristics of this monopole antenna which is erected over the conducting surface essentially we use the concept of images. So if you have a charge and if I put the charge over a conducting surface equivalently we have image of the charge so let us say if you are having a ground source like that and for understanding purpose let us say this ground is a ideal ground plane that means the conductivity of this is infinite so if I put a charge over this let us say this charge is  positive then you will have the electric field which will end on the conducting surface  perpendicular to it because the tangential component of the field has to go to zero so you will have the fields which will go like that so this field when they reach to the conducting surface they will be normal to the conducting surface and that is equivalent to saying that I have a negative charge which is located exactly at a same distance from here on the other side of the surface that also is going to give me the field distribution which will be identical to this field distribution.  (Refer Slide Time: 05:12 min) So the ground plane can be replaced by an image charge so this thing is essentially equivalent to putting a negative charge here and then replacing the ground plane, or we can say conversely if you have the positive and the negative charges we can say this is equivalent to having a charge and a ground plane, precisely that is the fact we make use of when we investigate the characteristics of a monopole antenna.  Now instead of having a charge if I have a current then let us say there is a current which is flowing like that so there is a positive charge here which is moving this way to get a negative charge we get a image of each of this charges on the other side so you get here this will be negative, this will be positive. So you will get a current which will be flowing in this direction but if I take a charge let us say this is positive here and negative here the current flows like that then the image of this positive charge will be negative and the image of this negative charge will be positive so this is positive, this is negative so the current flows in the same direction.  (Refer Slide Time: 06:46 min) So what we see from here is if you have a charge then the image has opposite polarity if you have a current which is parallel to the conducting surface in this case horizontal then the current will have a direction which is opposite to the srcinal element this image will  be opposite compared to this, however, if the current is vertical then the image has the direction same as the srcinal current there is no 180 degree phase shift this current and this current. So while investigating the antennas which have the current flows essentially depending upon the direction of the current of the orientation of the antenna you have to take appropriately the images and then we can replace the ground plane by its appropriate image. So essentially the problem is to have a charge or current located over the ground plane and replace the ground plane by their corresponding images or conversely if you are having the currents which are like that and that is equivalent to saying that we have only this current and there is a ground plane we have seen the current distribution on the dipole antenna and that is the current is sinusoidal distribution over the dipole so it is something like which is symmetric these two lines are equal and we also have seen that
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