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Basics of Antennas.docx

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La Trobe University
Eddie Custovic

BASICS OF ANTENNAS – Lecture Note 1 INTRODUCTION Antennas are devices that are capable of launching RF (radio frequency) energy into space and detect it as well. How well an antenna is able to launch RF energy and how well it can detect, depends on some important parameters that will be introduced here. Many people find it hard sometimes to understand some or all of these parameters, but I have found out that explanations with examples can help. I have tried my best to choose the examples close to the concepts. This Lecture note does not contain any mathematical formulae, instead I have concentrated on only the concepts of antennas. Mathematical considerations will be dealt in the following lecture notes. So first, how do antennas radiate RF energy? To understand this, check out the picture of the leaf below. The leaf can be thought equivalent to an antenna, the ripples are the radiation & the circular nature of ripples is the radiation pattern. If the ripples were to be seen, somehow hovering from the top of the water surface, you would see concentric circles forming from the point where the stick was inserted in water and moving away from it. As ripples (radiation) travel away from this point, they die out. In antenna theory, this is equivalent to RF energy eventually fading out as it moves away from antenna. According to physics, ripples do not travel, it is the disturbance that caused the ripples that travels. This travel of disturbance is what makes the ripples. In antenna theory, this is equivalent to the radiation travelling. The Omnidirectional antenna: The word ‘Omnidirectional’ means ‘in all directions’. So an omnidirectional antenna is the one that radiates equal RF energy in all directions. Of course, such an antenna does not exist! But it is very useful to understand the many complicated parameters of a real world antenna. Omnidirectional antennas used in real world are not perfect i.e. they do not radiate the same energy in all directions. An omnidirectional antenna is not a very good antenna to use in real world scenarios because it spreads out the RF energy in all directions. Mostly directional antennas which direct RF energy in the direction of interest (say in the direction where a TV receiver is location) are used. An example of such an antenna is the one shown in Figure 3. Some applications of omnidirectional antennas are in data transmission base stations for secure WIFI communications. More about this is not in the scope of this lecture note and can be made available on request. Figure 3 : Omnidirectional Antenna IMPORTANT PARAMETERS OF AN ANTENNA : Frequency : The very word frequency means the number of times any event occurs. That is precisely what it means even in electromagnetic and antenna theory. The best part about our world is that any type of energy (there are lots of different types out there) can simply be divided into sine or cosine waves. Any sine or cosine wave has an amplitude (defines power), frequency (number of complete cycles/sec or Hertz (named after the scientist who discovered it)), wavelength (distance between two heights). The following figure 1 will help get these terms right. Figure 2 helps in understand low frequency and high frequency sine waves. Figure 1 : Frequency, Amplitude & Wavelength of a Sine Wave Figure 2 : Low and High Frequency Sine waves Frequency Bands : Just like every country has a government, there are also governing bodies (e.g. ITU – International Telecommunications Union) which are responsible for dividing all the frequencies available into what we call as ‘frequency bands’. By dividing all frequency or ‘spectrum’ into bands, the governing bodies then auction these bands to communications companies to use. If this regulation was not in place, then we would receive TV signals on our phones and calls would come to our TVs! If an antenna operates in the frequency range of 400Mhz to 500Mhz then its bandwidth is 100Mhz! So if someone asks you to design an antenna, ask them which frequencies they want it to cover. You will then get the bandwidth of the antenna! Antenna Gain : Antenna gain in dB is a number which defines how much the antenna amplifies the RF signal before launching it in space. For a receiving antenna, the gain defines how much the receiving antenna amplifies the detected RF energy before future processing. Good gain in the direction of interest helps in better transmission and reception of RF signals. Good gain during transmission is more important than during reception simply because the transmitted signal needs to be strong enough to travel the distance and reach the receiv
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