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Radio Wave Propagation Fundamentals

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Chapter 2: Radio Wave Propagation Fundamentals M.Sc. Sevda Abadpour Dr.-Ing. Marwan Younis INSTITUTE OF RADIO FREQUENCY ENGINEERING AND ELECTRONICS KIT – The Research University in the Helmholtz Association www.ihe.kit.edu Scope of the (Today‘s) Lecture D Effects during wireless transmission of signals: A . physical phenomena that influence the propagation analog source & of electromagnetic waves channel decoding . no statistical description of those effects in terms digital of modulated signals demodulation filtering, filtering, amplification amplification Noise Antennas Propagation Time and Frequency Phenomena Selective Radio Channel 2 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Propagation Phenomena refraction reflection zR zT path N diffraction transmitter receiver QTi path i QRi yT xR y Ti yRi xT path 1 yR scattering reflection: scattering: free space - plane wave reflection - rough surface scattering propagation: - Fresnel coefficients - volume scattering - line of sight - no multipath diffraction: refraction in the troposphere: - knife edge diffraction - not considered In general multipath propagation leads to fading at the receiver site 3 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics The Received Signal Signal fading Fading is a deviation of the attenuation that a signal experiences over certain propagation media. It may vary with time, position Frequency and/or frequency Time Classification of fading: . large-scale fading (gradual change in local average of signal level) . small-scale fading (rapid variations large-scale fading due to random multipath signals) small-scale fading 4 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Propagation Models Propagation models (PM) are being used to predict: . average signal strength at a given distance from the transmitter . variability of the signal strength in close spatial proximity to a particular location Severe multipath conditions in PM can be divided into: urban areas (small-scale fading) . large-scale models (mean signal strength for large transmitter receiver separation) . small-scale models (rapid fluctuations of the received signal over very short travel distances) 5 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Large-Scale Propagation Free Space Propagation 6 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Free Space Propagation T R x x Assumptions: Pt Pr . unobstructed line of sight (LOS) Gt Gr . no multipath propagation r no (influence of) ground Received power: Power density at Rx site: Antenna effective area: Friis free space equation: 7 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Received Power and Path Loss Using: i i Assumptions: . polarization matched receiving antenna . conjugate complex impedance matching of the receiver Path loss: Isotropic path loss (no antenna gains): 8 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Polarization Orientation of Field Vectors and Reference Planes 9 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Polarization of the EM Waves Every elliptically polarized EM wave can be decomposed into a horizontal and a vertical component. Elliptical Circular Linear 10 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Polarization: II, T , V or H? Plane of incidence: formed by the normal vector to the reflecting surface and Poynting vector of the incidence wave EII or EV E T or EH Polarization (E-field vector) with Polarization (E-field vector) with respect to the plane of incidence: respect to the earth coordinates: . parallel (II) . vertical (V) . perpendicular ( )T . horizontal (H) 11 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Reflection and Transmission Dielectric Boundary 12 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Snell’s Law of Reflection . surface large compared to the wave length θi θr 1 . smooth surface (otherwise scattering) . three angles: - incidence 2 - reflection - transmission / refraction θt . Relation between angles through Fermat’s principle (principle of least time): - “the rays of light (EM-waves) traverse the path of stationary optical length” . This results in* Snell’s laws: - “ratio of the sines of the angles of incidence and refraction is equivalent to the opposite ratio of the indices of refraction” - “the incidence and reflection angles are equal and they are in the same plane” sin(i ) n2 nx r,x r,x i r sin(t ) n1 *full derivation in Arthur Schuster: “An Introduction to the Theory of Optics” 13 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Which Part is Transmitted / Reflected? Derivation procedure: . Definition of the electric field strength of the incident wave . Reflected and transmitted field strengths . Faraday’s law of induction . Boundary conditions at the border between two dielectric media . Decomposition of the incident waves on parallel and normal components 14 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Fresnel Reflection & Transmission Coefficients parallel perpendicular where: Fresnel coefficients are frequency dependent and in general complex 15 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Brewster‘s Angle (I) Angle, where no reflection occurs is Brewster’s Angle: . exists only for parallel (II / V) polarization . calculation by comparing the reflection coefficient to zero . calculation by using “physical limitations” 16 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Brewster‘s Angle (II) Air (less dense) Glass (dense) 17 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Brewster‘s Angle (III) phase in degree 18 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Brewster‘s Angle (IV) Operation principle of Brewster window: . used for windows in optical or quasi optical systems . window with normal incidence reflection loses at window . window tilted at Brewster’s angle no reflection loses at window Microwave gyrotron Brewster window 19 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Total Internal Reflection (I) When does the total internal reflection appears? ni sini nt sint t 90 . a ray must strike the medium’s boundary at an angle larger than the critical angle nt c arcsin . calculation by comparing the ni transmission angle to 90 degree critical angle exists only for nt < ni Increasing the incidence angle Total reflection of red laser light in PMMA 20 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Total Internal Reflection (II) Operation principle of rain sensors: . IR-beam projected on the glass-air interface at a specific angle . total inner reflection in dry conditions . partial transmission to the second medium if windshield is wet . reduced receive power triggers the sensor Rain sensor in the rear view mirror 21 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Visualization Parallel Pol – E-Field Parallel Pol – Air to Glass Parallel Pol – Glass to Air 22 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Visualization Perpendicular Pol – E-Field Perpendicular Pol – Air to Glass Perpendicular Pol – Glass to Air 23 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Reflection and (no) Transmission Perfect Electric Conductor (PEC) 24 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics Orthogonal PEC Reflection Boundary conditions: Etan Etan,i Etan,r 0 Hnorm Hnorm,i Hnorm,r 0 reflected wave incident wave Ey Ey SR S SR Hx Hx H HxR E xR EyR yR y z x 25 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics PEC Reflection, Orthogonal Polarization H incident reflected wave Ey wave Hxi H i S S Plane of incidence Eyi ai Ey Hzi H Hzr Eyr PEC reflector PEC reflection: Hxr Hr x . RII = +1 y T z . R = -1 (to ensure E = 0) tan ai ar 26 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics PEC Reflection: Applications Reflection in the direction of incidence: Radar calibration with metallic: . dihedral . trihedral (corner reflector) Buoy with dihedral Radar image with corner reflectors Satellite radar calibration 27 12.11.2018 Chapter 2: Radio Wave Propagation Fundamentals Institute of Radio Frequency Engineering and Electronics
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