Notes3-1

Notes3-1 - Notes #3, ECE594I, Fall 2009, E.R. Brown A Quick...

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54 A Quick Look at THz Attenuation Mechanisms and Propagation (1) Absorption - Conversion of electromagnetic radiation into heat (a) Conduction current (Joule) heating: P diss = J·E (e.g., losses in undepleted semiconductors) (overbar denotes time average) (b) Polarization current heating P diss = (d P /dt)· E (e.g., polar liquids, such as water in a microwave oven) (2) Scattering (a) Non-resonant internal scattering from inhomogeneous dielectrics (e.g., composite materials such as RT-Duroid) (b) Resonant scattering from particles of dimension ~ λ ( e.g., raindrops) (c) Mixture of specular scattering from smooth surfaces and diffuse scattering from rough surfaces (in infrared, more surfaces tend to be diffuse; in RF bands more surfaces tend to be specular) Two types of attenuation, both more common than in lower RF bands: Notes #3, ECE594I, Fall 2009, E.R. Brown
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55 Radiative Transfer r Β S Β 0 4 dB J B dr ρ α π += B(r)= B S exp(- τ ) + ( ρ J/4 πα )[1 – exp(- τ )] Homogeneous Solution Particular Solution α attenuation constant ; ρ→ density ; J emission coefficient 0 () L rd r τα = “Optical Depth” L 1D Radiative Transport Equation Special Cases: (1) j = 0 B = B S exp(- τ ) Beer-Lambert Law (2) B = uniform dB = 0 B = ( ρ J/4 πα ) Kirchoff’s Law Good Introductory Reference: J.D. Kraus, “Radio Astronomy” (McGraw-Hill, New York, 1966). Notes #3, ECE594I, Fall 2009, E.R. Brown
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56 exp( ) t A i I Tz I α ≡= −⋅ Describes attenuation effects for many THz point sensors (active and passive), and for remote active systems. Especially useful when intervening material is a linear, isotropic, and homogeneous (LIH) dielectric, or an array of independent scatterers: Τ → transmission , α A attenuation coefficient For absorption, α Α = α ABS , the absorption coefficient (a characteristic of the material, and generally a function of frequency) For scattering α Α = ρσ S , ρ being the density and σ S the scattering cross section I i incident intensity, I t transmitted intensity Beer-Lambert Law Notes #3, ECE594I, Fall 2009, E.R. Brown
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57 Atmospheric Attenuation Simulation Simulation Tool: Windows ® -based atmospheric propagation tool called PcLnWin (Ontar, Corp.) FASCODE radiative transfer engine developed by U.S. Air Force Geophysics Lab in 1970s. HITRAN96 database (> 1 million molecular lines) developed by molecular chemists and maintained by U.S. Air Force (Hanscom AFB) Atmospheric Conditions Seven “standard” molecules: H 2 0, CO 2 , O 3 , N 2 O, CO, CH 4 , O 2 1976 U.S. standard atmospheric model @ sea level - Temperature = 288 K, Pressure = 1 atm, Humidity = 46% Between 300 GHz and 2.0 THz, water vapor dominates the attenuation by way of molecular dipole transitions Notes #3, ECE594I, Fall 2009, E.R. Brown
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58 1E-30 1E-28 1E-26 1E-24 1E-22 1E-20 100 200 300 400 500 600 700 800 900 1000 Line Strength [cm -1 /(molecule-cm -2 )] Frequency [GHz] 183.4 325.4 380.5 448.3 548.0 475.0 621.1 752.6 916.8 971.0 988.6 557.3 Stick Diagram of Atmospheric Molecular Lines Notes #3, ECE594I, Fall 2009, E.R. Brown
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59 Why is Water Vapor Absorption So Strong and So Complicated ?
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This note was uploaded on 12/02/2009 for the course ECE 000 taught by Professor O during the Spring '09 term at UCSB.

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Notes3-1 - Notes #3, ECE594I, Fall 2009, E.R. Brown A Quick...

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