A CASE STUDY ON CERAMIC INDUSTRY OF BANGLADESH.pptx
Fundamentals of EM Waves
1.
2. Electromagnetic Waves
Electromagnetic wave consists of oscillating electric
and magnetic fields in certain directions with
propagate .
Propagate through free space at the velocity of light
5. TEM Propagation
Radio waves in space are transverse
electromagnetic waves (TEM)
Electric field, magnetic field and direction
of travel of the wave are mutually
perpendicular
Waves will propagate through free space
and dielectrics
Conductors have high losses due to induced
current
6.
7. Propagation Velocity
Speed of light in free space: 3 108 m/s
In dielectric and plasma the velocity of propagation is
lower:
r
c
v
10. Electric and Magnetic Fields
For waves we use the following units:
Electric field strength E (V/m) Magnetic field
strength H (A/m) Power density PD (W/m2)
Ohm’s law holds if characteristic impedance Z of
medium is used
For free space, Z = 377 Ohm
12. Plane and Spherical Waves
Waves from a point in space are spherical
Plane waves are easier to analyze
At a reasonable distance from the source,
spherical waves look like plane waves, as long
as only a small area is observed
14. Free-space Propagation
Assume an isotropic radiator at the center of a
sphere
Let receiving antenna be on surface of sphere
As we move farther from transmitter the amount
of power going through the surface remains the
same but surface area increases
16. Geometrical loss
2
4πr
PPD
Because of the power P on the spherical surface is constant
for every spherical surface (4π r2 ) we consider, the power
flux density at the distance r from the isotropic antenna must
decrease as 1/4πr2.
If an isotropic antenna radiates 10 W of power at the
distance of 1 km the power flux density (PD)is about 0.796
microW/m2
17. Attenuation of Free Space
Power density is reduced with increasing
distance r
Power density is total power divided by
surface area of sphere
Unit: watts/meter
2
4 r
P
P t
D
18. Free Space Electric Field
Electric field strength is relatively easy to
measure
Often used to specify signal strength
Unit: volts/meter
r
P
E t
30
19. Absorption
No absorption in free space
EM wave are absorbed in atmosphere as energy is
transferred to atoms and molecules
Electromagnetic waves are absorbed in the
atmosphere according to wavelength. Two
compounds are responsible for the majority of
signal absorption: oxygen (O2) and water vapor
(H2O).
Absorption below 10 Ghz is quit insignificant
20.
21. Reflection
Specular reflection: smooth surface
Angle of incidence = angle of reflection
Diffuse reflection: rough surface
Reflection in all directions because angle of
incidence varies over the surface due to its
roughness
24. Polarization
Polarization of a wave is the direction of the
electric field vector
Linearly polarized waves have the vector in
the same direction at all times
Horizontal and vertical polarization are common
Circular and elliptical polarization are also
possible
It is a physical orientation of radiated waves
in space
28. Cross Polarization
If transmitting and receiving antennas have
different polarization, some signal is lost
Theoretically, if the transmitting and
receiving polarization angles differ by 90
degrees, no signal will be received
A circularly polarized signal can be received,
though with some loss, by any linearly
polarized antenna
30. Atmospheric density changes with height
Slight refraction of wave
Increases Radio horizon
31. Refraction
Occurs when waves move from one medium to
another with a different propagation velocity
Index of refraction n is used in refraction
calculations
r
n
32. Snell’s Law
Angles are measured with respect to the
normal to the interface
2211
sinsin nn
34. Angle of Refraction
If n1<n2 then ray bends toward the normal (away
from the interface)
If n1>n2 then ray bends away from the normal
(toward the interface)
35. Diffraction
Occurs when radiation passes an object with
dimensions small compared with wavelength
The object appears to act as a source of radiation
Allows radio stations to be received on the shadow
side of obstacles
39. Terrestrial Propagation
Propagation over earth’s surface
Different from free-space propagation
Curvature of the earth
Effects of the ground
Obstacles in the path from transmitter to receiver
Effects of the atmosphere, especially the
ionosphere
40. Ground-Wave Propagation
Happens at relatively low frequencies
up to about 2 MHz
Only works with vertically polarized waves
Waves follow the curvature of earth
range varies from worldwide at 100 kHz and less to
about 100 km at AM broadcast band frequencies
(approx. 1 MHz)
41.
42. Ionospheric Propagation
Useful mainly in HF range (3-30 MHz)
Signals are refracted in ionosphere and returned
to earth
Worldwide communication is possible using
multiple “hops”
43. Ionospheric Layers
D layer: height approx. 60-90 km
E layer: height approx. 90-150 km
F1 layer: height approx. 150-250 km
F2 layer: height approx. 250-400 km
D, E layers disappear at night
F layers combine into one at night
44. Ionospheric Activity
More ionization causes signals to bend more
Ionization caused by solar radiation
greater during daytime
greater during sunspot cycle peaks (we are about at
a decreasing value now-2004)
D,E layers are less highly ionized than F layer and
usually just absorb signals
45. Refraction of Signals
Bending of signals by atmosphere decreases with
increasing frequency
Bending of signals by atmosphere increases with
increasing ionization
46. Daytime Propagation
D and E layers absorb lower frequencies, below
about 8-10 MHz
F layers return signals from about 10-30 MHz
47. Nighttime Propagation
D, E layers disappear
F layer returns signals from about 2-10 MHz
Higher frequencies pass through ionosphere into
space
48. Ionospheric Sounding
Transmit signal straight up
Note the maximum frequency that is returned
This is the critical frequency
49. Important Frequencies in HF
Propagation
Critical frequency
Highest frequency that is returned to earth
Maximum Usable Frequency (MUF)
Highest frequency that is returned at a given point
MUF= fcsecθ
Optimum Working Frequency (OWF)
85% of MUF for more reliable communication
50. Skip Zone
Region between maximum ground-wave distance
and closest point where sky waves are returned
from the ionosphere,