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다음은 PACRIM Training Course (Workshop) 의 강의 Note 중에서 1.1 Wave Properties 만을 발췌한 것으로, 전파의 성질을 이해하는데 도움이 될 것입니다. Wave Polarization, Polarimetric SAR, and Polarimetric Scattering Models Yisok Oh Dept. of Radio Engineering, Hong-Ik University Seoul National University, February 16-19, 2000 1 Contents 1. Wave Polarization 1.1 Wave Properties 1.2 Polarization Synthesis 2. Polarimetric Radar System 2.1 A Scatterometer System 2.2 NASA/JPL POLSAR System 3. Polarimetric Scattering Models 3.1 Surface Scattering 3.2 Volume Scattering 2 1.1 Wave Properties -. What is the “Field”? -. Waves : Electromagnetic Waves by Maxwell -. Planewave Propagation in free space -. Polarization : Basic concepts -. Microwave Generation : DC to AC -. Microwave Guidance by Waveguides/ Trans. lines -. Microwave Radiation by Antennas -. EM Wave Reflection from infinite planes -. Microwave Scattering from -. Point Targets -. Distributed Targets 3 Electromagnetic Fields Fields: Spatial distribution of a physical quantity. Electromagnetic Fields Static Fields : DC No time-variation Separation of Electric Field and Magnetic Field Dynamic Fields : AC Time-varying Fields Co-existence of Electric and Magnetic fields 4 Electrostatic Fields E : Electric Field (showing flux lines) V + - E d Conducting Plates Assuming infinite plates, V E d V / m Direction of E : From + charges to - charges 5 Magnetostatic Fields I I : Current H H : Magnetic Field (showing flux line) IEEE Emblem voltage Electric Fields current Magnetic Fields Assuming an infinite current line, H Direction of H : Right-hand rule, A / m I 2 Hˆ Iˆ Rˆ 6 Dynamic (Time-Varying) Fields B E t H J D t B c E dl S t ds D c H dl SJ ds S t ds Electromotive Force (Voltage Source) Time-varying Electric Field E(r,t) Conduction Current Maxwell’s Equations Displacement Current Time-varying Magnetic Field H(r,t) 7 Waves Consider Water wave in a pond. Cut water surface at once (t=t0) with Kwan-Woo’s Sword (청룡언월도) and look Wave Height Log the height of Fishing Buoy (x=x0) as a time function Wave Height X (Spatial Displacement) t (Time) Even though the wave comes toward me, the water doesn’t ! 8 Electromagnetic (EM) Waves An Example of an EM wave: E (r , t ) E0 cost kz 0 xˆ Magnitude (source, distance, etc.) Ex Sinusoidal Wave Time Variation 2 T Z-directed propagation 2 k Vector (Polarization) T Time Ex z, distance 9 Phase Velocity Assume these circles are surfing boards. t kz 0 = constant Same Phase Phase velocity = velocity of the equi-phase point t 0 const z k vp t t k Poynting Vector: S r , t E r , t H r , t : Real Power Flow (Magnitude and Direction) 10 Time-Harmonic Fields Time-harmonic Assumption: E ( r ) E0 e jkz e j t xˆ Time variation (Phasor form) E jH (Maxwell Equation) E0 jkz H (r ) e yˆ xˆ yˆ zˆ 377 in vacuum z : wave prop. direction 11 Planewave Propagation Planewave: wavefront is plane Approximate Planewave Spherical wave near an antenna in the Far-zone x 2D 2 R E (r ) E0 e jkz xˆ Ex z Hy y E H zˆ Z-directed propagating Linear polarized (x-direction) Wave 12 Polarization : shape of the locus of the E vector tip at a given point in space as a function of time. E z ax xˆ a y yˆ e Polarization Linear Circular Elliptical Conditions 0, j e jkz Examples E ( z ) E0 e jkz xˆ ax a y , E z xˆ j yˆ e jkz 2 Other Cases E z xˆ j 2 yˆ e jkz 13 Exercise (determination of polarization) Find polarization of the wave, E z 2 xˆ yˆ e jkz E z, t Re E z exp jt 2 xˆ yˆ cost kz E 0, t 2 xˆ yˆ cos t Find instantaneous electric field: Ey Plot electric field: t 0 1 2 Determine polarization t Linear pol. with tan 1 2 Ex 1 14 Polarization Ellipse Lin. Pol. 0 0 Circular Pol. 90 0 450 tan 2 tan 2 cos tan 2 sin 2 sin = Rotation Angle = Ellipticity Angle 15 Various Polarization States Left Circular pol. Wave direction Electric Field Thumb Other fingers of left hand 16 Microwave Generation Oscillators Tubes Solid State Klystron TWT Magnetron A MESFET Oscillator: High Power Gunn Diode MESFET HEMT, etc. Instability Microwave (A.C.) Resonator Amplifying Light, Cheap D.C. Power 17 Microwave Guidance Waveguides Two Conductors Coaxial Cable Two-wire Microstrip TEM wave Single Conductor No Conductor Rectangular, Circular Waveguides Dielectric Waveguide (Optical fibers) TE, TM waves (Transverse ElectroMagnetic) A Coaxial Cable : E H direction 18 Microwave Radiation Dipole Antenna : Transmission Line (Wave guider) Radiator (Discontinuity) * Current : temporal Variation of charges 19 Antennas Wire Antenna Aperture Antenna Reflector Antenna Printed Antenna Electric Field Lines Coaxial Cable Microstrip Antenna 20 EM Wave Reflection Ei Hi z H x Perpendicular Polarization Electric field is perpendicular to the incidence plane Electric field is horizontal to Earth surface z Horizontal Polarization i E Infinite plane i x Parallel Polarization Electric field is parallel to the incidence plane Vertical Polarization Magnetic field is horizontal to Earth surface 21 Microwave Scattering Radar System Radar System Point Target Pr Pt Gt Gr 2 4 3 R4 : Radar Cross Section m2 0 Distributed Target : Scattering Coefficient Pr Pt Gt 0 Gr 0 2 43 where Aill illum . area Aill 0 gt g r , R , 4 ds 22