Electromagnetic waves -Review- Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Electromagnetic waves -Review- Sandra Cruz-Pol, Ph. D. ECE UPRM Mayagüez, PR Electrical Engineering, UPRM

Electromagnetic Spectrum Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Electromagnetic Spectrum Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Cruz-Pol, Electromagnetics UPRM Uniform plane em wave approximation Cruz-Pol, Electromagnetics UPRM

Maxwell Equations in General Form Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Maxwell Equations in General Form Differential form Integral Form Gauss’s Law for E field. Gauss’s Law for H field. Nonexistence of monopole Faraday’s Law Ampere’s Circuit Law Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Would magnetism would produce electricity? Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

Electromagnetics was born! Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

Phasors for harmonic fields Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Advantages of phasors Time derivative is equivalent to multiplying its phasor by jw Time integral is equivalent to dividing by the same term. Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Time-Harmonic fields (sines and cosines) Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

Maxwell Equations for Harmonic fields Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Maxwell Equations for Harmonic fields Differential form* Gauss’s Law for E field. Gauss’s Law for H field. No monopole Faraday’s Law Ampere’s Circuit Law * (substituting and ) Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics A wave Start taking the curl of Faraday’s law Then apply the vectorial identity And you’re left with Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

To change back to time domain Dr. S. Cruz-Pol, INEL 4152-Electromagnetics To change back to time domain From phasor …to time domain Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Several Cases of Media Free space Lossless dielectric Low-loss Lossy dielectric Good Conductor Permitivity: eo=8.854 x 10-12[ F/m] Permeability: mo= 4p x 10-7 [H/m] Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

3. Lossy Dielectrics (General Case) Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Summary Any medium Lossless medium (s=0) Low-loss medium (e”/e’<.01) Good conductor (e”/e’>100) Units a [Np/m] b [rad/m] h [ohm] uc l w/b 2p/b=up/f [m/s] [m] **In free space; eo =8.85 10-12 F/m mo=4p 10-7 H/m Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Cruz-Pol, Electromagnetics UPRM (Relative) Complex Permittivity For lossless media Cruz-Pol, Electromagnetics UPRM

Cruz-Pol, Electromagnetics UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Note… E and H are perpendicular to one another Travel is perpendicular to the direction of propagation The amplitude is related to the impedance And so is the phase H lags E Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Loss Tangent If we divide the conduction current by the displacement current Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Relation between tanq and ec Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Relation between tanq and ec Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 2. Lossless dielectric Substituting in the general equations: Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Review: 1. Free Space Substituting in the general equations: Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 4. Good Conductors Substituting in the general equations: Is water a good conductor??? Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Skin depth, d Is defined as the depth at which the electric amplitude is decreased to 37% Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

Exercises: Wave Propagation in Lossless materials Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

Poynting Vector Derivation… Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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. Total power across surface of volume Rate of change of stored energy in E or H Ohmic losses due to conduction current Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Power: Poynting Vector Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Time Average Power The Poynting vector averaged in time is For the general case wave: For general lossy media Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics TEM wave z x y 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 Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 z x y x y Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Polarization: Why do we care?? Antenna applications – Antenna can only TX or RX a polarization it is designed to support. Straight wires, 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 Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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. x y Ey Ex y x Front View Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Several Cases Linear polarization: d=dy-dx =0o or ±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 Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Linear polarization Front View d =0 @z=0 in time domain x y Ey Ex Back View: y x Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Circular polarization Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Circular polarization Both components have same amplitude Eox=Eoy, d =d y-d x= -90o = Right circular polarized (RCP) d =+90o = LCP x y Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Elliptical polarization Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Elliptical polarization X and Y components have different amplitudes Eox≠Eoy, and d =±90o Or d ≠±90o and Eox=Eoy, Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Polarization example Polarizing glasses Unpolarized radiation enters Nothing comes out this time. All light comes out Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Polarization Parameters Ellipticy angle, Rotation angle, Axial ratio Cruz-Pol, Electromagnetics UPRM

Cruz-Pol, Electromagnetics UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Cell phone & brain Computer model for Cell phone Radiation inside the Human Brain Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

Cruz-Pol, Electromagnetics UPRM Power Ratios G G [dB] 10x 10x dB 100 20 dB 4 6 dB 2 3 dB 1 0 dB .5 -3 dB .25 -6 dB .1 -10 dB .001 -30 dB Cruz-Pol, Electromagnetics UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Attenuation rate, A Represents the rate of decrease of the magnitude of Pave(z) as a function of propagation distance Assigned problems ch 2 1-4,7-9, 11-13, 15-22, 28-30,32-34, 36-42 Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Summary Any medium Lossless medium (s=0) Low-loss medium (e”/e’<.01) Good conductor (e”/e’>100) Units a [Np/m] b [rad/m] h [ohm] uc l w/b 2p/b=up/f [m/s] [m] **In free space; eo =8.85 10-12 F/m mo=4p 10-7 H/m Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Exercise: Lossy media propagation Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

Reflection and Transmission Dr. S. Cruz-Pol, INEL 4152-Electromagnetics 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 Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics EM Waves z y z=0 Medium 1 : e1 , m1 Medium 2 : e2, m2 qi qr qt ki kr kt kix kiz Normal , an Plane of incidence Angle of incidence Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Property Normal Incidence Perpendicular Parallel Reflection coefficient Transmission coefficient Relation Power Reflectivity Power Transmissivity Snell’s Law: Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Cruz-Pol, Electromagnetics UPRM

Critical angle, qc …All is reflected Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Critical angle, qc …All is reflected When qt =90o, the refracted wave flows along the surface and no energy is transmitted into medium 2. The value of the angle of incidence corresponding to this is called critical angle, qc. If qi > qc, the incident wave is totally reflected. Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Fiber optics Light can be guided with total reflections through thin dielectric rods made of glass or transparent plastic, known as optical fibers. The only power lost is due to reflections at the input and output ends and absorption by the fiber material (not perfect dielectric). Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Optical fibers have cylindrical fiber core with index of refraction nf, surrounded by another cylinder of lower, nc < nf , called a cladding. For total reflection: [Figure from Ulaby, 1999] Acceptance angle Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Brewster angle, qB Is defined as the incidence angle at which the reflection coefficient is 0 (total transmission). The Brewster angle does not exist for perpendicular polarization for nonmagnetic materials. * qB is known as the polarizing angle http://www.amanogawa.com/archive/Oblique/Oblique-2.html Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Reflection vs. Incidence angle. Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Reflection vs. incidence angle for different types of soil and parallel or perpendicular polarization. Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM

Dr. S. Cruz-Pol, INEL 4152-Electromagnetics Antennas Now let’s review antenna theory Cruz-Pol, Electromagnetics UPRM Electrical Engineering, UPRM