Electromagnetic waves -Review-

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Electromagnetic waves-Review-

• Sandra Cruz-Pol, Ph. D.
• ECE UPRM
• Mayagüez, PR

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Electromagnetic Spectrum
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Maxwell Equations in General Form
Differential form Integral Form
Gausss Law for E field.
Gausss Law for H field. Nonexistence of monopole
Faradays Law
Amperes Circuit Law
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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!

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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
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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!

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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.

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Advantages of phasors

• Time derivative is equivalent to multiplying its
  phasor by jw
• Time integral is equivalent to dividing by the
  same term.

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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.

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Maxwell Equations for Harmonic fields
Differential form
Gausss Law for E field.
Gausss Law for H field. No monopole
Faradays Law
Amperes Circuit Law
(substituting and
)
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A wave

• Start taking the curl of Faradays law
• Then apply the vectorial identity
• And youre left with

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A Wave

• Lets 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

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To change back to time domain

• From phasor
• to time domain

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Several Cases of Media

1. Free space
2. Lossless dielectric
3. Lossy dielectric
4. Good Conductor

Permitivity eo8.854 x 10-12 F/m Permeability
mo 4p x 10-7 H/m
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1. Free space

• There are no losses, e.g.
• Lets 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?

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3. Lossy Dielectrics(General Case)

• In general, we had
• From this we obtain
• So , for a known material and frequency, we can
  find gajb

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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
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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

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Loss Tangent

• If we divide the conduction current by the
  displacement current

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Relation between tanq and ec
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2. Lossless dielectric

• Substituting in the general equations

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Review 1. Free Space

• Substituting in the general equations

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4. Good Conductors

• Substituting in the general equations

Is water a good conductor???
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Skin depth, d

• Is defined as the depth at which the electric
  amplitude is decreased to 37

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Short Cut

• You can use Maxwells 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).

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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,

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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/
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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.

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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.
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Time Average Power

• The Poynting vector averaged in time is
• For the general case wave

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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)

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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 er9 is characterized by
  Determine the
  direction of wave travel and the average power
  density carried by the wave
• Answer

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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

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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
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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.

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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
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Several Cases

• Linear polarization ddy-dx 0o or 180on
• Circular polarization dy-dx 90o EoxEoy
• Elliptical polarization dy-dx90o Eox?Eoy, or
  d?0o or ?180on even if EoxEoy
• Unpolarized- natural radiation

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Linear polarization
Front View

• d 0
• _at_z0 in time domain

Back View
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Circular polarization

• Both components have same amplitude EoxEoy,
• d d y-d x -90o Right circular polarized (RCP)
• d 90o LCP

x
y
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Elliptical polarization

• X and Y components have different amplitudes
  Eox?Eoy, and d 90o
• Or d ?90o and EoxEoy,

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Polarization example
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Example

• Determine the polarization state of a plane wave
  with electric field
• a.
• b.
• c.
• d.

1. Elliptic
2. -90, RHEP
3. LPlt135
4. -90, RHCP

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Cell phone brain

• Computer model for Cell phone Radiation inside
  the Human Brain

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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.

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Attenuation rate, A

• Represents the rate of decrease of the magnitude
  of Pave(z) as a function of propagation distance

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Summary
Any medium Lossless medium (s0) Low-loss medium (e/elt.01) Good conductor (e/egt100) Units
a 0 Np/m
b rad/m
h ohm
uc l w/b 2p/bup/f m/s m
In free space eo 8.85 10-12 F/m mo4p 10-7 H/m In free space eo 8.85 10-12 F/m mo4p 10-7 H/m In free space eo 8.85 10-12 F/m mo4p 10-7 H/m In free space eo 8.85 10-12 F/m mo4p 10-7 H/m In free space eo 8.85 10-12 F/m mo4p 10-7 H/m In free space eo 8.85 10-12 F/m mo4p 10-7 H/m
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Exercise Lossy media propagation

• For each of the following determine if the
  material is low-loss dielectric, good conductor,
  etc.
• Glass with mr1, er5 and s10-12 S/m at 10 GHZ
• Animal tissue with mr1, er12 and s0.3 S/m at
  100 MHZ
• Wood with mr1, er3 and s10-4 S/m at 1 kHZ
• Answers
• low-loss, a 8.4x10-11 Np/m, b 468 r/m, l 1.34
  cm, up1.34x108, hc168 W
• general, a 9.75, b12, l52 cm, up0.5x108 m/s,
  hc39.5j31.7 W
• Good conductor, a 6.3x10-4, b 6.3x10-4, l
  10km, up0.1x108, hc6.28(1j) W

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Reflection and Transmission

• Wave incidence
• Wave arrives at an angle
• Snells Law and Critical angle
• Parallel or Perpendicular
• Brewster angle

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EM Waves

• Normal , an
• Plane of incidence
• Angle of incidence

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Property Normal Incidence Perpendicular Parallel
Reflection coefficient
Transmission coefficient
Relation
Power Reflectivity
Power Transmissivity
Snells Law Snells Law Snells Law Snells Law
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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 gt qc, the incident wave is totally
  reflected.

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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).

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• Optical fibers have cylindrical fiber core with
  index of refraction nf, surrounded by another
  cylinder of lower, nc lt nf , called a cladding.
• For total reflection

Figure from Ulaby, 1999
Acceptance angle
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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
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Reflection vs. Incidence angle.
Reflection vs. incidence angle for different
types of soil and parallel or perpendicular
polarization.
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Antennas

• Now lets review antenna theory