RADIOWAVE PROPAGATION: PHYSICS AND APPLICATIONS. Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

Transcription

1 RADIOWAVE PROPAGATION: PHYSICS AND APPLICATIONS Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

2 I. Introduction 1 EM waves and propagation 2 Influence of frequency 3 Propagation mechanisms Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

3 II. EM Waves and propagation All of us are familiar with use of EM waves for information transmission All systems have three generalized components: Transmitter: information carrying circuit level signal generated and converted to EM wave Propagation: EM wave travels from transmitter to receiver through external environment Receiver: EM wave converted to circuit signals and information extracted Environmental effects can have large influence on propagation component Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

4 Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

5 Use of EM waves eliminates need for wires: first predicted in 1865 by Maxwell Maxwell s predictions verified experimentally by Hertz in 1880 s Demonstrated by Marconi in 1901 with wireless communications across the Atlantic Since then many studies of propagation have been conducted so there is a vast amount of experience and information available The complexity of most propagation environments makes exact predictions impossible, so usually only simple physical models combined with empirical data are available Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

6 III. Influence of frequency Current technology exists to transmit and receive electromagnetic waves over a large range of frequencies: from 10 khz to Hz! A basic rule of thumb: EM waves tend to be affected most by structures comparable to or larger than one wavelength From this, we can see that since the Earth environment has structures on a wide range of scales, we should expect propagation phenomena to vary with frequency Example Earth scales: atmospheric particles, vegetation and forests, mountains, Earth curvature! Because of this, frequency plays a large role in propagation effects, and names have been provided to specific frequency bands to illustrate this Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

7 Band name Abbreviation Frequencies Very-low frequency VLF 3 to 30 khz Low frequency LF 30 to 300 khz Medium frequency MF 300 khz to 3 MHz High frequency HF 3 MHz to 30 MHz Very-high frequency VHF 30 MHz to 300 MHz Ultra-high frequency UHF 300 MHz to 3 GHz Super-high frequency SHF 3 GHz to 30 GHz Extremely-high frequency EHF 30 GHz to 300 GHz Table: IEEE Frequency Band Designations Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

8 Band name Frequencies (GHz) Wavelengths (cm) L S C X K u K K a V W Table: Microwave Frequency Band Designations Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

9 III. Propagation mechanisms Direct transmission - Ch 5 Atmospheric refraction - Ch 6 Ducting - Ch 6 Direct plus ground reflections - Ch 7 Terrain diffraction - Ch 7 Empirical path loss and fading models - Ch 8 Groundwave - Ch 9 Ionospheric reflections - Ch Others - Ch 12 Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

10 Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

11 Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

12 Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

13 Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

14 Propagation Mechanisms and Applications

15 Propagation Mechanisms by Frequency

16 Characterization of Propagation Media 1 Information resources 2 Maxwell s equations 3 Constitutive relations 4 Types of dielectric media Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

17 II. Information resources International Telecommunication Union (ITU): Regulates communications internationally. Holds an international meeting every 4 years, issues reports and recommendations Institute for Telecommunications Sciences (ITS): Branch of U.S. Dept. of Commerce, National Telecommunications and Information Administration (NTIA) Naval Command, Control, and Ocean Surveillance Center (NCCOSC) Federal Communications Commission World Wide Web Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

18 III. Maxwell s Equations H = D t + J (1) E = B (2) t D = ρ (3) B = 0 (4) Eight scalar equations for twelve scalar unknown functions, not all independent J = ρ t J S = ρ S t (5) (6) Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

19 III. Maxwell s Equations (cont d) ˆn ( ) H 2 H 1 = JS (7) ˆn ( ) E 2 E 1 = 0 (8) ˆn (D ) 2 D 1 = ρs (9) ˆn (B ) 2 B 1 = 0 (10) Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

20 n Region 2 Region 1 n Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

21 IV. Constitutive relations A relationship between D and E and between B and H is required to make Maxwell s equations solvable; these are the constitutive relations The simplest possible consitutive relations: but these are not always applicable! D = ɛe (11) B = µh (12) Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

22 IV. Constitutive relations (cont d) Model: fields in a material are those in free space plus fields produced by induced dipole moments in material D(r, t) = ɛ 0 E(r, t) + P(r, t) (13) where P is the induced dipole moment per unit volume in the material. Note we still need a relationship between P and E! Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

23 V. Types of dielectric media Relationship between applied field E and induced dipole moment density P will depend of the properties of the medium. There are many possible medium properties: Nonlinearity - P proportional to powers of E other than first Anisotropy - P depends on direction of E Dispersion - P depends on the frequency of E Inhomogeneity - P depends on r Time dependence - P depends on t Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

24 V. Types of dielectric media (cont d) The simplest medium has none of the aforementioned properties, so P(r, t) = χɛ 0 E(r, t) (14) and D(r, t) = ɛ 0 (1 + χ) E(r, t) = ɛ 0 ɛ r E(r, t) = ɛe(r, t) (15) Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

25 Inhomogeneous and time-varying media can be handled through D(r, t) = ɛ(r, t)e(r, t) (16) Anisotropic media can be handled through a tensor-valued permittivity ɛ xx ɛ xy ɛ xz ɛ yx ɛ yy ɛ yz ɛ zx ɛ zy ɛ zz (17) Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

26 Dispersion results due to time response of medium or P(r, t) = ɛ 0 t P(r, t) = Re [ɛ 0 E 0 (r)e jωt E(r, t )f (t t )dt (18) 0 ] e jωu f (u)du (19) for the sinusoidal steady state. Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

27 A relationship between phasors P 0 and E 0 is thus for dispersive media, where P 0 (r) = χ(ω)ɛ 0 E 0 (r) (20) χ(ω) = 0 e jωu f (u)du (21) The last equation defines the complex susceptibility, χ(ω), and D 0 (r) = ɛ(ω)e 0 (r) (22) Imaginary part of ɛ indicates dielectric losses. Power lost in frictional motion of particles. Note that D(r, t) and E(r, t) may not be in the same direction! Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

28 Dielectric constant Interfacial Molecular Orientational Real Imaginary Atomic Rotations/ Vibrations (Molecular Spectra) Electronic Transitions (Atomic Spectra) Frequency (Hz) Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

29 (a) Distilled water, 25 C Dielectric Constant Real Imaginary Frequency (GHz) (b) Carbon Tetrachloride, 25 C 4 Dielectric Constant Frequency (GHz) Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

30 Conductive media can be handled through Ohm s law, J(r, t) = σe(r, t) (23) and the phasor form of Ampere s law, H 0 (r) = J 0 (r) + jωd 0 (r) (24) to get H 0 (r) = (σ + jωɛ) E 0 (r) (25) = jω (ɛ jσ/ω) E 0 (r) (26) Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

31 If one separates out the real and imaginary parts of the dielectric constant and rearranges the terms slightly, the result is ɛ = ɛ R jɛ I (27) H 0 (r) = jω [ɛ R j (ɛ I + σ/ω)] E 0 (r) (28) showing that conduction and dielectric losses effects can be combined into a single imaginary part of the permittivity or into an effective conductivity. Be careful when using tables! We will define ɛ e = ɛ R j (ɛ I + σ/ω) (29) Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31

32 Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31 D01-sc

33 Levis, Johnson, Teixeira (ESL/OSU) Radiowave Propagation August 17, / 31