161_1_Lecture_13

161_1_Lecture_13 - EE161 El EE161 Electromagnetic Waves...

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EE161 Electromagnetic Waves all 2010 Fall, 2010 Instructor: Dr. Shenheng Xu Electrical Engineering Dept., UCLA gg p , © Prof. Y. Ethan Wang
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ecture 3 Lecture 13 • Antennas • Short (Hertzian) Dipole Antennas • Radiation Characteristics ntenna Pattern • Antenna Pattern ntenna Directivity Antenna Directivity
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Antenna -Transducer “Antenna” – a transducer between a guided wave propagating in a transmission line and an electromagnetic wave propagating in an unbounded medium (usually free space), or vice versa.
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Various Types of Antennas
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ow Antennas Radiate How Antennas Radiate Radiation is a disturbance in the electromagnetic fields that propagates way from the source of the disturbance away from the source of the disturbance This disturbance is created by a time-varying current source exposed under an open space From one of the Maxwell’s equations, we know J E H  j Therefore, any time-varying current ill generate electromagnetic fields will generate electromagnetic fields, but they won’t radiate until it is exposed to an open space Closed region Open region
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Solving Radiation Problems STEP 1. Write down the expression of current density function J (r’) STEP 2. From current density J (r ) find vector potential A (r) using ' ' ) ( ~ ~ e jkR R J TEP 3 1 ind magnetic field based on the definition of the ' ' 4 ) ( dv R v i R A STEP 3. A H Find magnetic field , based on the definition of the vector potential STEP 4. H E  j 1 Find electric field Impedance relationship between E field and H field like in the plane waves no longer hold in general
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Short Dipole (1) (Ideal Dipole, Hertz electric dipole, infinitesimal dipole and doublet) 1. Write the expression of the current R’ P z p ˆ ~ 0 0 ) ( ~ cos ) ( I z I t I t i Short -> 50 l l R y z y x I ) ' ( ) ' ( 0 J 2. Construct vector potential A R’ means the distance between I x ' ' ' ' ) ' ( ~ 4 ) ( ~ dv R e r v jkR J R A
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161_1_Lecture_13 - EE161 El EE161 Electromagnetic Waves...

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