Uniform plane wave. Radiation from a current filament. Spherical waves from a source of finite dimension. Transmitted waves from a finite sized source behave like spherical waves. Concept of plane waves. - PowerPoint PPT Presentation
1Uniform plane waveEMLAB2Radiation from a current filament
Spherical waves from a source of finite dimension
Transmitted waves from a finite sized source behave like spherical waves.EMLAB4
Concept of plane waves
EM waves transmitted from a finite sized source spread spherically in the space. At great distances from the source, EM waves behave as a plane wave locally.EMLAB5E & H in source free region
In source free region, E and H can be obtained easily.With J and equal to zero, a source free wave equation is obtained.
If the source current away from the field point has the direction of x-axis, the electric field can have only x component. If the extent of the source is infinite along x and y directions, variations of the field along those directions become zero.
The first term represent the wave propagating toward +z direction, while the second stands for the wave propagating to the opposite direction. Considering only the wave propagating in the positive direction,
wave impedance of free spaceIf the propagating direction is in +z-axis, the Helmholtz equation becomes a simple expression.
7Plane wave propagating general direction
If the propagating direction is other than x, y or z direction, the phase term in the exponential function can be obtained by the inner product of the k-vector and the position vector.the k-vector can be compared to the normal vector of equi-phase plane.
The magnetic field can be obtained.
Magnetic field of a plane wave
Vector identityEMLAB9Wave propagation in dielectricsIf the wave is propagating into a medium other than free space, the molecules in that medium vibrate under the action of electric field. Meanwhile, the energy of the wave is dissipated and the medium become heated.The dissipation can be modeled by a complex permittivity.
3. The ratio of the real part of r to the imaginary part is called the loss tangent of that media.
EMLAB11From the equations on the left, it can be seen that the phase of displacement current leads that of conduction current by 90 degree.That is, electric field propagates first, then charges move under the action of that electric field.Helmholtz equation in a lossy medium becomes,
Propagation constant in a lossy dielectricEMLAB12
To obtain the approximate expression of and , we consider the two extreme cases of
Good dielectric : Good conductor :
Plane wave in good dielectrics
Plane wave in good conductors
is called as the skin depth of the medium at the given frequency. If the electric field penetrate into a lossy medium as much as one skin depth, its strength decreases by 1/e. That is its strength becomes 36.7% of the original value.conductor
EMLAB17ExampleFind the skin depth of sea water at the frequency of 1MHz. In sea water
Calculate the resistance of a round copper of 1mm radius and 1km length at DC and 1MHz.
The Helmholtz equation has three components (x, y, z).But the divergenceless condition imposes that E = 0, which is a constraint among the vector components of E-field. That is, all the component of Ex, Ey, Ez are not independent.
From the above condition, only two components among the Ex, Ey, Ez are independent. That is, two kinds of independent polarizations comprise an arbitrary E-field.a changing direction of electric field observed at a position.Among the components of an electric field vector, only two of them is independent.EMLAB20
Example of a electric polarization of a wave propagating in +z direction.
Linear polarizationCircular polarizationEMLAB21
Polarization diversity antennaEMLAB