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Electromagnetic waves - Review -

Electromagnetic waves - Review -. Sandra Cruz-Pol, Ph. D. ECE UPRM Mayag ü ez, PR. Electromagnetic Spectrum. Maxwell Equations in General Form . Would magnetism would produce electricity?.

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Electromagnetic waves - Review -

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  1. Electromagnetic waves-Review- Sandra Cruz-Pol, Ph. D. ECE UPRM Mayagüez, PR

  2. Electromagnetic Spectrum Cruz-Pol, Electromagnetics UPRM

  3. Maxwell Equations in General Form Cruz-Pol, Electromagnetics UPRM

  4. Would magnetism would produce electricity? • Eleven years later, and at the same time, Mike Faraday in London and Joe Henry in New York discovered that a time-varying magnetic field would produce an electric current! Cruz-Pol, Electromagnetics UPRM

  5. Electromagnetics was born! • This is the principle of motors, hydro-electric generators and transformers operation. This is what Oersted discovered accidentally: *Mention some examples of em waves Cruz-Pol, Electromagnetics UPRM

  6. Special case • Consider the case of a lossless medium • with no charges, i.e. . The wave equation can be derived from Maxwell equations as What is the solution for this differential equation? • The equation of a wave! Cruz-Pol, Electromagnetics UPRM

  7. Phasors for harmonic fields Working with harmonic fields is easier, but requires knowledge of phasor. • The phasor is multiplied by the time factor, ejwt, and taken the real part. Cruz-Pol, Electromagnetics UPRM

  8. Advantages of phasors • Timederivativeis equivalent to multiplying its phasor by jw • Timeintegralis equivalent to dividing by the same term. Cruz-Pol, Electromagnetics UPRM

  9. Time-Harmonic fields (sines and cosines) • The wave equation can be derived from Maxwell equations, indicating that the changes in the fields behave as a wave, called an electromagnetic field. • Since any periodic wave can be represented as a sum of sines and cosines (using Fourier), then we can deal only with harmonic fields to simplify the equations. Cruz-Pol, Electromagnetics UPRM

  10. Maxwell Equations for Harmonic fields Cruz-Pol, Electromagnetics UPRM * (substituting and )

  11. A wave • Start taking the curl of Faraday’s law • Then apply the vectorial identity • And you’re left with Cruz-Pol, Electromagnetics UPRM

  12. A Wave • Let’s look at a special case for simplicity • without loosing generality: • The electric field has only an x-component • The field travels in z direction • Then we have Cruz-Pol, Electromagnetics UPRM

  13. To change back to time domain • From phasor • …to time domain Cruz-Pol, Electromagnetics UPRM

  14. Several Cases of Media • Free space • Lossless dielectric • Lossy dielectric • Good Conductor Permitivity:eo=8.854 x 10-12[ F/m] Permeability:mo= 4px 10-7 [H/m] Cruz-Pol, Electromagnetics UPRM

  15. 1. Free space There are no losses, e.g. Let’s define • The phase of the wave • The angular frequency • Phase constant • The phase velocity of the wave • The period and wavelength • How does it moves? Cruz-Pol, Electromagnetics UPRM

  16. 3. Lossy Dielectrics(General Case) • In general, we had • From this we obtain • So , for a known material and frequency, we can find g=a+jb Cruz-Pol, Electromagnetics UPRM

  17. Intrinsic Impedance, h • If we divide E by H, we get units of ohms and the definition of the intrinsic impedance of a medium at a given frequency. *Not in-phase for a lossy medium Cruz-Pol, Electromagnetics UPRM

  18. Note… • E and H areperpendicular to one another • Travel is perpendicular to the direction of propagation • The amplitude is related to the impedance • And so is the phase Cruz-Pol, Electromagnetics UPRM

  19. Loss Tangent • If we divide the conduction current by the displacement current Cruz-Pol, Electromagnetics UPRM

  20. Relation between tanq and ec Cruz-Pol, Electromagnetics UPRM

  21. 2. Lossless dielectric • Substituting in the general equations: Cruz-Pol, Electromagnetics UPRM

  22. Review: 1. Free Space • Substituting in the general equations: Cruz-Pol, Electromagnetics UPRM

  23. 4. Good Conductors • Substituting in the general equations: Is water a good conductor??? Cruz-Pol, Electromagnetics UPRM

  24. Skin depth, d • Is defined as the depth at which the electric amplitude is decreased to 37% Cruz-Pol, Electromagnetics UPRM

  25. Short Cut … • You can use Maxwell’s or use where k is the direction of propagation of the wave, i.e., the direction in which the EM wave is traveling (a unitary vector). Cruz-Pol, Electromagnetics UPRM

  26. Exercises: Wave Propagation in Lossless materials • A wave in a nonmagnetic material is given by Find: • direction of wave propagation, • wavelength in the material • phase velocity • Relative permittivity of material • Electric field phasor Answer: +y, up= 2x108 m/s, 1.26m, 2.25, Cruz-Pol, Electromagnetics UPRM

  27. Power in a wave • A wave carries power and transmits it wherever it goes The power density per area carried by a wave is given by the Poynting vector. See Applet by Daniel Roth at http://www.netzmedien.de/software/download/java/oszillator/ Cruz-Pol, Electromagnetics UPRM

  28. Rate of change of stored energy in E or H Ohmic losses due to conduction current Total power across surface of volume Poynting Vector Derivation… • Which means that the total power coming out of a volume is either due to the electric or magnetic field energy variations or is lost as ohmic losses. Cruz-Pol, Electromagnetics UPRM

  29. Power: Poynting Vector • Waves carry energy and information • Poynting says that the net power flowing out of a given volume is = to the decrease in time in energy stored minus the conduction losses. Represents the instantaneous power vector associated to the electromagnetic wave. Cruz-Pol, Electromagnetics UPRM

  30. Time Average Power • The Poynting vector averaged in time is • For the general case wave: Cruz-Pol, Electromagnetics UPRM

  31. Total Power in W The total power through a surface S is • Note that the units now are in Watts • Note that power nomenclature, P is not cursive. • Note that the dot product indicates that the surface area needs to be perpendicular to the Poynting vector so that all the power will go thru. (give example of receiver antenna) Cruz-Pol, Electromagnetics UPRM

  32. Exercises: Power 1. At microwave frequencies, the power density considered safe for human exposure is 1 mW/cm2. A radar radiates a wave with an electric field amplitude E that decays with distance as E(R)=3000/R [V/m], where R is the distance in meters. What is the radius of the unsafe region? • Answer: 34.6 m 2. A 5GHz wave traveling in a nonmagnetic medium with er=9 is characterized by Determine the direction of wave travel and the average power density carried by the wave • Answer: Cruz-Pol, Electromagnetics UPRM

  33. x x z z y TEM wave Transverse ElectroMagnetic = plane wave • There are no fields parallel to the direction of propagation, • only perpendicular (transverse). • If have an electric field Ex(z) • …then must have a corresponding magnetic field Hx(z) • The direction of propagation is Cruz-Pol, Electromagnetics UPRM

  34. x z y x z y Polarization of a wave IEEE Definition: The trace of the tip of the E-field vector as a function of time seen from behind. Simple cases • Vertical, Ex • Horizontal, Ey x y x y Cruz-Pol, Electromagnetics UPRM

  35. Polarization: Why do we care?? • Antenna applications – • Antenna can only TX or RX a polarization it is designed to support. Straightwires, square waveguides, and similar rectangular systems support linear waves (polarized in one direction, often) Circular waveguides, helical or flat spiral antennas produce circular or elliptical waves. • Remote Sensing and Radar Applications – • Many targets will reflect or absorb EM waves differently for different polarizations. Using multiple polarizations can give different information and improve results. Rain attenuation effect. • Absorption applications – • Human body, for instance, will absorb waves with E oriented from head to toe better than side-to-side, esp. in grounded cases. Also, the frequency at which maximum absorption occurs is different for these two polarizations. This has ramifications in safety guidelines and studies. Cruz-Pol, Electromagnetics UPRM

  36. x Ex x y Ey y Polarization • In general, plane wave has 2 components; in x & y • And y-component might be out of phase wrt to x-component, d is the phase difference between x and y. Front View Cruz-Pol, Electromagnetics UPRM

  37. Several Cases • Linear polarization: d=dy-dx =0oor ±180on • Circular polarization: dy-dx=±90o & Eox=Eoy • Elliptical polarization: dy-dx=±90o & Eox≠Eoy, or d=≠0o or ≠180on even if Eox=Eoy • Unpolarized- natural radiation Cruz-Pol, Electromagnetics UPRM

  38. x Ex x y Ey y Linear polarization Front View • d =0 • @z=0 in time domain Back View: Cruz-Pol, Electromagnetics UPRM

  39. Circular polarization • Both components have same amplitude Eox=Eoy, • d =dy-dx= -90o = Right circular polarized (RCP) • d =+90o = LCP x y Cruz-Pol, Electromagnetics UPRM

  40. Elliptical polarization • X and Y components have different amplitudes Eox≠Eoy, andd =±90o • Or d≠±90o and Eox=Eoy, Cruz-Pol, Electromagnetics UPRM

  41. All light comes out Unpolarized radiation enters Nothing comes out this time. Polarizing glasses Polarization example Cruz-Pol, Electromagnetics UPRM

  42. Example • Determine the polarization state of a plane wave with electric field: a. b. c. d. • Elliptic • -90, RHEP • LP<135 • -90, RHCP Cruz-Pol, Electromagnetics UPRM

  43. Cell phone & brain • Computer model for Cell phone Radiation inside the Human Brain Cruz-Pol, Electromagnetics UPRM

  44. Decibel Scale • In many applications need comparison of two powers, a power ratio, e.g. reflected power, attenuated power, gain,… • The decibel (dB) scale is logarithmic • Note that for voltages, the log is multiplied by 20 instead of 10. Cruz-Pol, Electromagnetics UPRM

  45. Attenuation rate, A • Represents the rate of decrease of the magnitude of Pave(z) as a function of propagation distance Cruz-Pol, Electromagnetics UPRM

  46. Summary Cruz-Pol, Electromagnetics UPRM

  47. Exercise: Lossy media propagation For each of the following determine if the material is low-loss dielectric, good conductor, etc. • Glass with mr=1, er=5 and s=10-12 S/m at 10 GHZ • Animal tissue with mr=1, er=12 and s=0.3 S/m at 100 MHZ • Wood with mr=1, er=3 and s=10-4 S/m at 1 kHZ Answers: • low-loss, a= 8.4x10-11 Np/m, b=468 r/m, l= 1.34 cm, up=1.34x108, hc=168 W • general, a=9.75, b=12, l=52 cm, up=0.5x108 m/s, hc=39.5+j31.7 W • Good conductor, a= 6.3x10-4, b= 6.3x10-4, l= 10km,up=0.1x108, hc=6.28(1+j) W Cruz-Pol, Electromagnetics UPRM

  48. Reflection and Transmission Wave incidence • Wave arrives at an angle • Snell’s Law and Critical angle • Parallel or Perpendicular • Brewster angle Cruz-Pol, Electromagnetics UPRM

  49. Medium 2 : e2,m2 Medium 1 : e1, m1 kiz ki kix qi z y qt qr kt kr z=0 EM Waves • Normal , an • Plane of incidence • Angle of incidence Cruz-Pol, Electromagnetics UPRM

  50. Cruz-Pol, Electromagnetics UPRM

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