1 EE 543 Theory and Principles of Remote Sensing Topic 3 - Basic EM Theory and Plane Waves

O. Kilic EE543 2 Outline EM Theory Concepts Maxwell’s Equations –Notation –Differential Form –Integral Form –Phasor Form Wave Equation and Solution (lossless, unbounded, homogeneous medium) –Derivation of Wave Equation –Solution to the Wave Equation – Separation of Variables –Plane waves

O. Kilic EE543 3 EM Theory Concept The fundamental concept of em theory is that a current at a point in space is capable of inducing potential and hence currents at another point far away. J E, H

O. Kilic EE543 4 Introduction to EM Theory In remote sensing we are interested in the interactions of em waves with the medium and target of interest. The existence of propagating em waves can be predicted as a direct consequence of Maxwell’s equations. These equations satisfy the relationship between the vector electric field, E and vector magnetic field, H in time and space in a given medium. Both E and H are vector functions of space and time; i.e. E (x,y,z;t), H (x,y,z;t.)

O. Kilic EE543 5 What is an Electromagnetic Field? The electric and magnetic fields were originally introduced by means of the force equation. In Coulomb’s experiments forces acting between localized charges were observed. There, it is found useful to introduce E as the force per unit charge. Similarly, in Ampere’s experiments the mutual forces of current carrying loops were studied. B is defined as force per unit current.

O. Kilic EE543 6 Why not use just force? Although E and B appear as convenient replacements for forces produced by distributions of charge and current, they have other important aspects. First, their introduction decouples conceptually the sources from the test bodies experiencing em forces. If the fields E and B from two source distributions are the same at a given point in space, the force acting on a test charge will be the same regardless of how different the sources are. This gives E and B meaning in their own right. Also, em fields can exist in regions of space where there are no sources.

O. Kilic EE543 7 Maxwell’s Equations Maxwell's equations give expressions for electric and magnetic fields everywhere in space provided that all charge and current sources are defined. They represent one of the most elegant and concise ways to state the fundamentals of electricity and magnetism. These set of equations describe the relationship between the electric and magnetic fields and sources in the medium. Because of their concise statement, they embody a high level of mathematical sophistication.

O. Kilic EE543 8 Notation: (Time and Position Dependent Field Vectors) E ( x,y,z;t ) Electric field intensity (Volts/m) H ( x,y,z;t ) Magnetic field intensity (Amperes/m) D ( x,y,z;t ) Electric flux density (Coulombs/m 2 ) B ( x,y,z;t ) Magnetic flux density (Webers/m 2, Tesla)

O. Kilic EE543 9 Notation: Sources and Medium J ( x,y,z;t ) Electric current density (Amperes/m 2 ) J d ( x,y,z;t ) Displacement current density (Amperes/m 2 ) ee Electric charge density (Coulombs/m 3 )  Permittivity of the medium (Farad/m)  Permeability of the medium (Henry/m)  Conductivity of the medium (Siemens/m)

O. Kilic EE Two Forms of Maxwell’s Equations Differential form –This is the most widely used form. –They describe the relationship between the electric and magnetic fields and sources in the medium at a point in space. Integral form –Integral form of Maxwell’s equations can be derived from the differential form by using Stoke’s theorem and Divergence theorem. –These set of equations describe the field vector relations over an extended region in space. –They have limited use. Typically, they are applied to solve em boundary value problems with symmetry.

O. Kilic EE Maxwell’s Equations – Physical Laws Faraday’s Law  Changes in magnetic field induce voltage. Ampere’s Law  Allows us to write all the possible ways that electric currents can make magnetic field. Magnetic field in space around an electric current is proportional to the current source. Gauss’ Law for Electricity  The electric flux out of any closed surface is proportional to the total charge enclosed within the surface. Gauss’ Law for Magnetism  The net magnetic flux out of any closed surface is zero.

O. Kilic EE Differential Form Faraday’s Law: Ampere’s Law: Gauss’ Law: (1) (2) (3) (4)

O. Kilic EE Some Observations (1) How many scalar equations are there in Maxwell’s equations? Answer = 8 ?? Three scalar equations for each curl (3x2 = 6) 1 scalar equation for each divergence (1x2 = 2).

O. Kilic EE Some Observations (2) But, the divergence equations are related to the curl equations. This is known as the conservation of charge.

O. Kilic EE Conservation of Charge If we take the divergence of both sides of eqn. 2 and use the vector identity: (5) Using eqn. (3) in eqn. (5), we obtain the continuity law for current: (6) Flow of current out of a differential volume Rate of decrease of charge in the volume

O. Kilic EE Some Observations (3) Thus, the divergence equations (eqn. 3, 4) are dependent on the curl equations (eqn. 1, 2). So, Maxwell’s equations represent 6 independent equations.

O. Kilic EE Some Observations (4) How many variables are there in Maxwell’s equations? Answer:12, three for each component of E, H, D, and B vectors. Therefore, the set of Maxwell’s equations is not sufficient to solve for the unknowns. We need 12-6 = 6 more scalar equations. These are known as constitutive relations.

O. Kilic EE Constitutive Relations Constitutive relations provide information about the environment in which electromagnetic fields occur; e.g. free space, water, etc. Free space values. (7) (8) permittivity permeability

O. Kilic EE Duality Principle Note the symmetry in Maxwell’s equations

O. Kilic EE Integral Form of Maxwell’s Equations (9) Using Stoke’s theorem and Divergence theorem: HW #2.1 Prove by applying Stoke’s and Divergence theorems to Maxwell’s eqn in differential form

O. Kilic EE Time Harmonic Representation - Phasor Form In a source free ( ) and lossless ( ) medium characterized by permeability  and permittivity , Maxwell’s equations can be written as: (10)

O. Kilic EE Time Harmonic Fields We will now assume time harmonic fields; i.e. fields at a single frequency. We will assume that all field vectors vary sinusoidally with time, at an angular frequency w; i.e. In other words: (11) Note that the E and H vectors are now complex

O. Kilic EE Time Harmonic Fields (2) The time derivative in Maxwell’s equations becomes a factor of jw:

O. Kilic EE Phasor Form of Maxwell’s Equations Maxwell’s equations can then be written in phasor form as: Phasor form is dependent on position only. Time dependence is removed.

O. Kilic EE The Wave Equation (1) If we take the curl of Maxwell’s first equation: Using the vector identity: And assuming a source free, i.e. and lossless; medium:i.e.

O. Kilic EE The Wave Equation (2) Define k, which will be known as wave number:

O. Kilic EE Wave Equation in Cartesian Coordinates where

O. Kilic EE Scalar Form of Wave Equation For each component of the E vector, the wave equation is in the form of: Denotes different components of E in Cartesian coordinates

O. Kilic EE Solution to the Wave Equation – Separation of Variables Assume that a solution can be written such that

O. Kilic EE Separation of Variables This decomposition is arbitrarily defined at this point. Will depend on the medium and boundary conditions. Determines how the wave propagates along each direction.

O. Kilic EE Separation of Variables +

O. Kilic EE Possible Solutions to the Wave Equation Standing wave solutions are appropriate for bounded propagation such as wave guides. When waves travel in unbounded medium, traveling wave solution is more appropriate. Energy is transported from one point to the other HW 2.2: Show that the above are solutions to the wave equation by plugging the solution on the differential eqn on the previous page

O. Kilic EE The Traveling Wave The phasor form of the fields is a mathematical representation. The measurable fields are represented in the time domain. Then Let the solution to the  -component of the electric field be: Traveling in +x direction

O. Kilic EE Traveling Wave As time increases, the wave moves along +x direction

O. Kilic EE Standing Wave Then, in time domain:

O. Kilic EE Standing Wave Stationary nulls and peaks in space as time passes.

O. Kilic EE To summarize We have shown that Maxwell’s equations describe how em energy travels in a medium The E and H fields satisfy the “wave equation”. The solution to the wave equation can be in various forms, depending on the medium characteristics

O. Kilic EE The Plane Wave Concept Plane waves constitute a special set of E and H field components such that E and H are always perpendicular to each other and to the direction of propagation. A special case of plane waves is uniform plane waves where E and H have a constant magnitude in the plane that contains them.

O. Kilic EE Example 1 (1/5) Assume that the E field lies along the x-axis and is traveling along the z-direction. We derive the solution for the H field from the E field using Maxwell’s equation #1: Intrinsic impedance; 377  for free space wave number

O. Kilic EE Example 1 (2 of 5) Thus the wave equation (Page 26) simplifies to: Where as before

O. Kilic EE Example 1 (3 of 5) direction of propagation x y z E, H plane E and H fields are not functions of x and y, because they lie on x-y plane

O. Kilic EE Example 1 (4 of 5) phase term *** The constant phase term  is the angle of the complex number E o

O. Kilic EE Wavelength: period in space k = 2  Example 1 (5 of 5)

O. Kilic EE Velocity of Propagation (1/3) We observe that the fields progress with time. Imagine that we ride along with the wave. At what velocity shall we move in order to keep up with the wave???

O. Kilic EE Velocity of Propagation (2/3) Constant phase points E field as a function of different times kz

O. Kilic EE Velocity of Propagation (3/3) In free space: Note that the velocity is independent of the frequency of the wave, but a function of the medium properties.

O. Kilic EE Example 2 A uniform em wave is traveling at an angle  with respect to the z-axis. The E field is in the y-direction. What is the direction of the H field?

O. Kilic EE Solution: Example 2 x z y E  The E field is along the unit vector: The direction of propagation is along y. Because E, H and the direction of propagation are perpendicular to each other, H lies on x-z plane. It should be in the direction parallel to:

O. Kilic EE Plane Wave Characteristics amplitude frequency Wave number, depends on the medium characteristics phase Direction of propagation polarization

O. Kilic EE Example 3 Write the expression for an x-polarized electric field that propagates in +z direction at a frequency of 3 GHz in free space with unit amplitude and 60 o phase. =1 = 2  *3* o x + z-direction

O. Kilic EE Example 4 If the electric field intensity of a uniform plane wave in a dielectric medium where  =  o  r and  =  o is given by: Determine: The direction of propagation and frequency The velocity The dielectric constant (i.e. permittivity) The wavelength

O. Kilic EE Solution: Example 4 (1/2) 1.+y direction; w = 2  f = Velocity: 3.Permittivity:

O. Kilic EE Solution: Example 4 (2/2) 4. Wavelength: m

O. Kilic EE Example 5 Assume that a plane wave propagates along +z-direction in a boundless and a source free, dielectric medium. If the electric field is given by: Calculate the magnetic field, H.

O. Kilic EE Example 5 - observations Note that the phasor form is being used in the notation; i.e. time dependence is suppressed. We observe that the direction of propagation is along +z-axis.

O. Kilic EE Solution: Example 5 (1/4) From Maxwell’s equations in phasor form, we can write: Eqn. 1 Eqn. 2 where, in Cartesian coordinates the curl operator is given as:

O. Kilic EE Solution: Example 5 (2/4) From eqn. 1 and eqn. 2: Intrinsic impedance, I = V/R

O. Kilic EE Solution: Example 5 (3/4) E, H and the direction of propagation are orthogonal to each other. Amplitudes of E and H are related to each other through the intrinsic impedance of the medium. Note that the free space intrinsic impedance is 377 

O. Kilic EE Observation This answer can be generalized to the following (for plane waves): where o is the direction of propagation.

O. Kilic EE Homework 3.1 The magnetic field of a uniform plane wave traveling in free space is given by 1.What is the direction of propagation? 2.What is the wave number, k in terms of permittivity,  o and permeability,  o ? 3.Determine the electric field, E.

O. Kilic EE Plane Waves Along Arbitrary Directions E, H plane

O. Kilic EE Example 6 A z-polarized electric field propagating along direction in a dielectric medium where  = 9  o,  =  o. The frequency is 100 MHz. a)Write the electric field in phasor form and in time domain. Assume an arbitrary phase and unit amplitude. b)Calculate the magnetic field, H in phasor form and in time domain.

O. Kilic EE Solution: Example 6 (1/2) a) =1, unit amplitude

O. Kilic EE Solution: Example 6 (2/2) where b)

O. Kilic EE Polarization The alignment of the electric field vector of a plane wave relative to the direction of propagation defines the polarization. Three types: –Linear –Circular –Elliptical (most general form) Polarization is the locus of the tip of the electric field at a given point as a function of time.

O. Kilic EE Linear Polarization Electric field oscillates along a straight line as a function of time Example: wire antennas y x x y E E

O. Kilic EE Example 7 For z = 0 (any position value is fine) x t = 0 t =  - E o EoEo y Linear Polarization: The tip of the E field always stays on x- axis. It oscillates between ±E o

O. Kilic EE Example 8 Let z = 0 (any position is fine) x y t =  /2 t = Linear Polarization E xo =1 E yo =2

O. Kilic EE Circular Polarization Electric field traces a circle as a function of time. Generated by two linear components that are 90o out of phase. Most satellite antennas are circularly polarized. y x y x RHCP LHCP

O. Kilic EE Example 9 E xo =1 E yo =1 Let z= 0 x RHCP y t=0 t=  /2w t=  t=3  /2w

O. Kilic EE Elliptical Polarization This is the most general form Linear and circular cases are special forms of elliptical polarization Example: log spiral antennas y x LH y x RH

O. Kilic EE Example 10 ExEx EyEy Linear when Circular when Elliptical if no special condition is met.

O. Kilic EE Note: We have decomposed E field into two orthogonal components to identify polarization state. Examples were x and y components, assuming the wave travels in z-direction. For arbitrary propagation directions, the E field can still be decomposed into two components that lie on a plane perpendicular to the direction of propagation.

O. Kilic EE Example 11 Determine the polarization of this wave.

O. Kilic EE Solution: Example 11 (1/2) Note that the field is given in phasor form. We would like to see the trace of the tip of the E field as a function of time. Therefore we need to convert the phasor form to time domain.

O. Kilic EE Solution: Example 11 (2/2) Let z=0 Elliptical polarization

O. Kilic EE Example 12 Find the polarization of the following fields: a) b) c) d)

O. Kilic EE Solution: Example 12 (1/4) a) x y z Let kz=0 t=0 t=  /2w t=  t=3  /2w RHCP Observe that orthogonal components have same amplitude but 90 o phase difference. Circular Polarization

O. Kilic EE Solution: Example 12 (2/4) b) Observe that orthogonal components have same amplitude but 90 o phase difference. y z Let kx=0 t=-  /4w t=+  /4w t=3  t=5  /4w RHCP x Circular Polarization

O. Kilic EE Solution: Example 12 (3/4) c) Observe that orthogonal components have different amplitudes and are out of phase. Elliptical Polarization z Let ky=0 t=-  /w t=+  /w x y Left Hand

O. Kilic EE Solution: Example 12 (4/4) d) Observe that orthogonal components are in phase. Linear Polarization x y z

O. Kilic EE Example 13 Show that an elliptically polarized wave can be decomposed into two circularly polarized waves, one left-handed other right-handed.

O. Kilic EE Solution: Example 13 Elliptically polarized wave in general is of the following form: LHCP and RHCP waves can be written in the following form: If we let a b

O. Kilic EE Coherence and Polarization In the definition of linear, circular and elliptical polarization, we considered only completely polarized plane waves. Natural radiation received by an anatenna operating at a frequency w, with a narrow bandwidth,  w would be quasi-monochromatic plane wave. The received signal can be treated as a single frequency plane wave whose amplitude and phase are slowly varying functions of time.

O. Kilic EE Quasi-Monochromatic Waves amplitude and phase are slowly varying functions of time

O. Kilic EE Degree of Coherence where denotes the time average.

O. Kilic EE Degree of Coherence – Plane Waves

O. Kilic EE Unpolarized Waves An em wave can be unpolarized. For example sunlight or lamp light. Other terminology: randomly polarized, incoherent. A wave containing many linearly polarized waves with the polarization randomly oriented in space. A wave can also be partially polarized; such as sky light or light reflected from the surface of an object; i.e. glare.

O. Kilic EE Poynting Vector As we have seen, a uniform plane wave carries em power. The power density is obtained from the Poynting vector. The direction of the Poynting vector is in the direction of wave propagation.

O. Kilic EE Poynting Vector

O. Kilic EE Example 14 Calculate the time average power density for the em wave if the electric field is given by:

O. Kilic EE Solution: Example 14 (1/2)

O. Kilic EE Solution: Example 14 (2/2)

O. Kilic EE Plane Waves in Lossy Media Finite conductivity,  results in loss Ohm’s Law applies: Conductivity, Siemens/m Conduction current

O. Kilic EE Complex Permittivity From Ampere’s Law in phasor form:

O. Kilic EE Wave Equation for Lossy Media Attenuation constant Phase constant Wave number: Loss tangent, 

O. Kilic EE Example 15 (1/2) Plane wave propagation in lossy media: complex number

O. Kilic EE Example 15 (2/2) Plane wave is traveling along +z-direction and dissipating as it moves. attenuation propagation

O. Kilic EE Attenuation and Skin Depth Attenuation coefficient, , depends on the conductivity, permittivity and frequency. Skin depth,  is a measure of how far em wave can penetrate a lossy medium

O. Kilic EE Lossy Media

O. Kilic EE Example 16 Calculate the attenuation rate and skin depth of earth for a uniform plane wave of 10 MHz. Assume the following properties for earth:  =  o  = 4  o  = 10 -4

O. Kilic EE Solution: Example 16 First we check if we can use approximate relations. Slightly conducting

O. Kilic EE References HYS/Class/waves/u10l1b.htmlhttp:// HYS/Class/waves/u10l1b.html Applied Electromagnetism, Liang Chi Shen, Jin Au Kong, PWS