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1. 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
2. Electromagnetic Radiation Includes radio waves, light,
X-rays, gamma rays VLF 3 30 kHz LF 30 300 kHz MF 300 3000 kHz HF 3
30 MHz VHF 30 300 MHz UHF 300 3000 MHz Radio waves of our
interest
3. 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
4. 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
5. Electromagnetic Waves Wavelength is : fV p / Where, Vp is
the phase velocity is the wavelength f is the frequency
6. Ohms Law in Space HEZ /
7. 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) Ohms law holds if characteristic
impedance Z of medium is used For free space, Z = 377 Ohm
8. Power Density EH H E PD Z Z 2 2
9. 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
10. Isotropic antenna radiating equally in every direction
11. 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
12. Power flux density Power flux density= E X H
13. Geometrical loss 2 4r PPD Because of the power P on the
spherical surface is constant for every spherical surface (4 r2 )
we consider, the power flux density at the distance r from the
isotropic antenna must decrease as 1/4r2. 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
14. 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
15. 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
16. 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
17. 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
18. Specular Reflection
19. Diffuse reflection
20. 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
21. Circular polarization
22. linear polarization
23. 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
24. Refraction Refraction takes place when EM wave pass from
one medium to another medium with diff density.
25. Atmospheric density changes with height Slight refraction
of wave Increases Radio horizon
26. 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
27. Snells Law Angles are measured with respect to the normal
to the interface 2211 sinsin nn
28. refraction
29. Angle of Refraction If n1n2 then ray bends away from the
normal (toward the interface)
30. 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
31. EM WAVE PROPAGATION
32. Layer of atmosphere
33. Terrestrial Propagation Propagation over earths 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
34. 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)
35. 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
36. 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
37. 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
38. Refraction of Signals Bending of signals by atmosphere
decreases with increasing frequency Bending of signals by
atmosphere increases with increasing ionization
39. Daytime Propagation D and E layers absorb lower
frequencies, below about 8-10 MHz F layers return signals from
about 10-30 MHz
40. Nighttime Propagation D, E layers disappear F layer returns
signals from about 2-10 MHz Higher frequencies pass through
ionosphere into space
41. Ionospheric Sounding Transmit signal straight up Note the
maximum frequency that is returned This is the critical
frequency
42. 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= fcsec Optimum Working Frequency (OWF) 85% of MUF for more
reliable communication
43. Skip Zone Region between maximum ground-wave distance and
closest point where sky waves are returned from the
ionosphere,