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
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
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
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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