161_1_Class15

161_1_Class15 - EE161 Electromagnetic Waves Spring, 2010...

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Unformatted text preview: EE161 Electromagnetic Waves Spring, 2010 Prof. Y. Ethan Wang Electrical Engineering Dept. UCLA Lesson 15 • Antennas • Short Dipole Antennas • Radiation Characteristics • Antenna Pattern • Antenna Directivity Antenna -Transducer “Antenna” – 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. Various Type of Antennas How Antennas Radiate Radiation is a disturbance in the electromagnetic fields that propagates away from the source of the disturbance This disturbance is created by a time-varying current source exposed under an open space J E H + = × ∇ ωε j From one of the Maxwell’s equations, we know Therefore, any time-varying current will generate electromagnetic fields, but they won’t radiate until it is exposed to an open space Closed region Open region Solving Radiation Problems STEP 2. STEP 3. STEP 4. From current density J (r ’ ) find vector potential A (r) using A H × ∇ = μ 1 Find magnetic field , based on the definition of the vector potential H E × ∇ = ωε j 1 Find electric field ' ' ' ' ) ( ~ 4 ) ( ~ dv R e v jkR i ∫ − = R J R A π μ Impedance relationship between E field and H field like in the plane waves no longer hold in general STEP 1. Write down the expression of current density function J (r’) Short Dipole (1) (Ideal Dipole, Hertz electric dipole, infinitesimal dipole and doublet) l R R’ P I z x y z y x I ˆ ) ' ( ) ' ( 1. Write the expression of the current ~ δ δ = J 2. Construct vector potential A , r' R − = R' ) ( ~ cos ) ( I z I t I t i = ⇒ = ω Short -> 50 λ < l ' ' ' ' ) ' ( ~ 4 ) ( ~ dv R e r v jkR ∫ − = J R A π μ R’ means the distance between the source point and...
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This note was uploaded on 06/06/2010 for the course EE 161 taught by Professor Huffaker during the Spring '08 term at UCLA.

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161_1_Class15 - EE161 Electromagnetic Waves Spring, 2010...

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