4.7 - Department of Electrical and Computer Engineering...

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Department of Electrical and Computer Engineering ECSE 352 Electromagnetic Waves and Optics 7 Introduction to Antennas 4.7 Introduction to Antennas References: Section 14.8 @Andrew Kirk 4.7-1 Antennas ECSE 352 4.7-2 Overview In this first class we will discuss the concept and operation of antennas for electromagnetic radiation. We will then introduce the elemental dipole antenna. This is a short length of wire that supports an oscillating current. We will see that it radiates electromagnetic waves, which in the far-field look ke plane waves We will finally calculate how much like plane waves. We will finally calculate how much power this antenna can radiate and introduce the idea of radiation resistance. ECSE 352 4.7-3 Learning outcomes After taking this class you should be able to: • Describe the elemental (Hertzian) dipole • Sketch its radiation pattern (E and H) l l t E H nd int nsit in th f i ld Calculate E, H and intensity in the far-field • Calculate the power radiated from an elemental dipole antenna p • Calculate the radiation resistance of an elemental dipole antenna ECSE 352 4.7-4
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Contents • Introduction to antennas • The elemental dipole antenna • Radiation resistance and radiated power ECSE 352 4.7-5 What does an antenna do ? • Transform electrical signals into electromagnetic waves, and ice versa vice versa • Converts oscillating electrons into photons • Power is transferred from the antenna to a distant object E.g. parallel plate transmission line with flared end ECSE 352 4.7-6 Radiation Resistance • What is the load impedance of the flared transmission line ? • Not infinite (open circuit), because no power would e broadcast from it be broadcast from it • An antenna has a radiation resistance • This is the equivalent resistance that would dissipate the same amount of power as the antenna is broadcasting fficient antennas have a high radiation resistance Efficient antennas have a high radiation resistance ECSE 352 4.7-7 Antenna pattern and directivity • It’s important that antennas send energy where e want it to go we want it to go • E.g. consider a radio transmission antenna: Which is the most efficient pattern ? ECSE 352 4.7-8 isotropic two-lobed
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Types of antennas • Linear – Hertzian dipole –Ha lf -wa ve Simple to model Not very directional Array antennas Better directionality Steerable ! Horn antennas Microwave broadcast ECSE 352 4.7-9 Elemental dipole: summary • The elemental dipole antenna is the basic element of all near antennas linear antennas • It’s just a short length of wire ! • Changing the current in it changes the external E and H i lds fields • There is a lag between the current changing and the external fields changing •The information about the changing current travels as a wave into space • In the far-field, the waves look like plane waves ECSE 352 4.7-10 Elemental (Hertzian) dipole • Electric dipole with oscillating charges • Current flows from one pole to the other H a R i t   I cos t   I exp j t 
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4.7 - Department of Electrical and Computer Engineering...

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