Welcome again, welcome at this introduction web lecture on antennas and this web lecture is in fact the first in a series on antennas, theory and applications. So today we're going to look into the following topics. So first, what does this source of electromagnetic radiation? Then we going to look in some history of antennas, we're going to discuss a few examples of antennas, and we also going to introduce the coordinate system that we're going to use throughout all the web lectures on the task. At the end we going to define also the field regions around an antenna. Now let's start with a well known configuration. So suppose we have a static charge, not moving with the charge density Q. If we look at the electric field at a distance r from this charge, then we can apply Coulomb's law if you might remember from your Bachelor course on electromagnetics. And Coulomb's law states that the electric field is given by this equation. So it's directed in the radial direction, which is indicated by this unit vector and more importantly, it shows a 1 over r squared dependence. So when distance increases, is as this 1 over r squared dependence with distance. Now, if this same charge is moving so in fact is accelerating, so it's in this case we show an oscillating charge, oscillating between two points. Then if we again look at the distance r from this charge, then as we will prove in the upcoming web lectures. The electric field far away from this accelerating charge has this type of form. So in this case it looks quite similar to Coulomb's law, but in fact the main difference is that it has one over r dependence far away from the antenna. So instead of a 1 over r squared, we now have a 1 over r dependence. And in addition, this accelerating charge also generates a magnetic field, so we have an electric and a magnetic field, both with the 1 over r dependence, and as a result we have radiation of energy. So energy is propagating away from the accelerating charge. So keep in mind acceleration of charges is the source of electromagnetic radiation. Now, some history of antennas. Well, the first kind of antenna experiments were done by the in Italian scientist Galvani. He did it in the 18th century he has a lab where he conducted experiments with wires and frog legs. And the frog legs were in fact a microwave receiver. So if there was electromagnetic energy for example from lighting received by the wire antenna, then there would be a current flowing on this antenna. And the frog leg would act as a receiver and if there was any current detected then the legs would move, so that would be a kind of indication that there are signals received by the antenna. Well, in about 100 years later Maxwell formulated his Maxwell's equations and that were the kind of theoretical foundation of the antenna theory. And a couple of years later in 1887, the German Hertz he did this experiments and he made a real transmitter and receiver system, so he showed over a distance of five kilometers at a frequency 430 megahertz. That indeed you could transmit electromagnetic energy from a transmitter to receiver. Well, it was about 14 years later that Marconi, he was more like in an engineer, Italian engineer and he commercialized this invention of Hertz in fact. And he performed the first transatlantic widest link in 1901. Well, several types of antennas and in this picture you see four commonly used antenna types, you probably know them from practical applications. So the first antenna Type shown here is the reflector antenna which is used in many applications. So for example, this application is for ready astronomy to investigate the universe. And reflector antennas have typically a large receiving aperture and in that sense they can work with fairly weak signals. Another antenna type very commonly used is the wire antenna, for example to receive FM radio signals. Why antennas are used in your car on radios as I indicated. Another antenna type is the horn antenna, which is also an example of an aperture antenna similar to the reflector antenna. We also call it an aperture antenna, and the horn antenna is often used as a feed in a reflector antenna. So to illuminate the reflector or to receive the receiving signals from another reflector antenna. Now these three types are very conventional you could say. There have been used throughout the past 100 or 100 years. More modern antenna types are often printed on PCB technology and an example is the microchip antenna, which is in fact a copper patch which can be printed on, as I said, the PCB material. Or can even be integrated if we go to much higher frequencies in a semiconductor technology. So it could be integrated on chip. Now, throughout the course we will discuss many more examples of antennas, of course. Nowadays we also see more complex antenna systems, so not only the passive antennas but also active more complex antenna systems. And a nice example of that is the phased-array antenna, an example is shown here. This is a prototype which was built about 20 years ago for the square kilometer array. Also for radio astronomy and what you see here is that it consists of many small antenna elements and each antenna element is connected to electronics at the backside. And by this combination of passive antennas and electronics, it's possible to do beam steering so the antenna can electronically steer the beam without any mechanical movement. Now, traditionally, phased-arrays are expensive, power hungry, and are used in professional applications like for radar or radio astronomy. But we see now trends towards the application of phased-arrays for wireless communication and with introduction of 5G, especially 5G operating at higher frequencies, at millimeter wave frequencies at Ka bands. That's 30, about 30 Giga Hertz. We see that phased-array technologies coming into the commercial applications and in this picture you see a prototype that we developed with our PhD students. Of a base station for 5G millimeter wave. And it's a phased array and which can electronically do perform beam steering as we have seen also in the professional applications. Now we're going to talk more about applications, including phased-arrays throughout the upcoming web pledge. Now let's go back to the starting point. The theory and we're going to introduce the theoretical framework for antennas, and of course we have to use a coordinate system to do that. Now we're going to use spherical coordinates, as shown here. So in this XYZ coordinate system, we have spherical coordinates indicated by r, Theta and Phi. And at a point P we can identify three unit vectors and these unit vectors are indicated by u r, u Theta and u Phi. So mind that the notation in several books of unit vectors might be different from this notation, but in these web lectures here we're going to use this notation. Now, antennas are a kind of connection between electronics and the free space. So apparently there is a transformation from guided waves into radiated waves and as indicated here in this picture, we have some kind of generator that generates a signal and the signal can be transported over transmission line. For example, with a guided TEM wave. And the antenna could then converts this guided wave into an unguided or radiating wave, a spherical wave that propagates away from the antenna. Now in this picture, you see The orientation of the field, so this is a kind of situation at a particular time moment t is t naught. And throughout the web lectures on tennis, we will assume again that we have time mnemonic fields similar to what we did in it transmission line theory. If we consider an antenna in this case, a dipole antenna with a length L and fed with some voltage at the input, we can distinguish several regions around the antenna. As you can imagine, if you're very close to the antenna in the near field, if you put your hand for example fairly close to such a wire antenna and then probably the impedance will change of the antenna. So it will really affect the impedance of the antenna. If you're very far away, then of course this will not happen again. Now in between being far away, an being in the near field we have defined Fresnel region which is kind of intermediate region. Where we do not have the impact on the impedance anymore, but we also do not have the one over our field dependence that we talked about earlier. Far away from the antenna, we talk about the far field region and these three regions are also indicated by numbers, so you can calculate it as indicated here. So the transition between the near field and the fresnel regions indicated by this equation. And the far field starts at a distance 2L squared divided by lambda naught, and L is the length of the antenna in this case, and lambda naught is the wavelength in free space. Now, as a summary of this web lecture, we have shown that the acceleration of charges is the source of electromagnetic radiation. We introduce several antenna types ranging from wire antennas to aperture antennas like horn and reflectors, and even phased-array so more complex beam steering antennas. We talked about fields regions and we introduced to coordinate system. Hope to see you back in the next web lecture.
