1. Radiation and Propagation of Waves
Chapter 10
Standard Text Book
Lecture # 4

2. Objectives Understand wave, electromagnetic waves
Radiation and associated phenomenon
Explain
Reflection
Refraction
Diffraction
Polarization
Describe the Propagation of waves
Ground Waves
Sky waves
Space Waves

3. Wave Wave is a mode of transfer of energy Transverse waves
Longitudinal waves

4. Transverse Waves
Transverse waves are those whose direction of propagation is perpendicular to both the electrical field and the magnetic field The electrical field and the magnetic fields lie in planes that are perpendicular to each other. (x and y planes)
Thus the direction of propagation will be in the z plane or third dimension

5. Electromagnetic Waves
Consist of
Magnetic wave
Electrical wave
Most of the energy is returned to the circuit.
If it isn’t, then some it must be “set free” or radiated. Radiated energy is not desirable.
But if such power is “escaped on purpose” then it is said to be radiated

6. Wave Propagation Example
electric
field
propagation direction
magnetic
field

7. Electrical to Magnetic Conversion
The antennas are the transducers
The transmitting antenna changes the electrical energy into electromagnetic or waves
The receiving antenna changes the electromagnetic energy back into electrical energy
These electromagnetic waves propagate at rates ranging from 150kHz to 300GHz

8. Radio-frequency Interference
If the radiated energy comes from another radio transmitter, then it is considered radio-frequency interference (RFI)
The transmitting antenna should be specifically designed to prevent the energy from being returned to the circuit.
It is desirable that the antenna “free” the energy in order that it might radiate into space

9. Electromagnetic Interference
If the energy comes from else where, then it is electromagnetic interference (EMI)

10. A few Concepts at a glance
Free Space
Which does not interfere with normal radiation and propagation
Point Source
A simple point acting like a source radiating in all directions
Power density
Power per unit area
Isotropic source
one which radiates uniformly in all directions

11. Polarization of the Electrical Field
The polarization of the electrical field is determined by the direction of oscillations
If the oscillations are in the vertical direction then the polarization is said to be vertical
If the oscillations are in the horizontal direction then the polarization is said to be horizontal

12. Polarization of antenna
Thus a “vertical” antenna will result in a vertically polarized wave.
A vertical antenna is one that consists of a vertical tower, wire, or rod, usually a quarter wavelength in length that is fed at the ground and uses the ground as a reflecting surface.

13. Wavefronts
A wavefront is a plane joining all points of equal phase in a wave
Take a point in space. Imagine waves radiating outward in all directions from this point. The result would resemble a sphere. The point of radiation is called the isotropic point source

14. Isotropic Power
Since the power at any point away from the isotropic point is inversely proportional to the square of the distance from the point, then the power decreases rapidly the further away from the point you need.
Although the wavefront is curved in shape, from a distance small sections appear planar and can be thought of as plane wavefronts

15. Reflection Reflection is the abrupt reversal in direction
Caused by any conductive medium such as
Metal surfaces or
Earth’s surface
There will normally be a shift in phase
Coefficient of reflection is less than 1

16. Complete Reflection
Complete reflection will occur only in perfect conductors and when the electric field is perpendicular to the reflecting element or medium
Coefficient of Reflection will be 1
Coefficient of Reflection is the ratio of the reflected wave intensity to the incident wave intensity

17. Refraction
Occurs when the waves pass from one medium to another whose densities are different
Coefficient of reflection is less than 1
The angle of incidence and the angle of refraction is related by Snell’s Law

18. Refraction
Sub-Refraction
Refraction (straight line)
Normal Refraction
Earth
Refraction (or bending) of signals is due to temperature, pressure, and water vapor content in the atmosphere.
Amount of refractivity depends on the height above ground.
Refractivity is usually largest at low elevations.
The refractivity gradient (k-factor) usually causes microwave signals to curve slightly downward toward the earth, making the radio horizon father away than the visual horizon.
This can increase the microwave path by about 15%,

19. Diffraction Waves traveling in straight lines bend around obstacles
Based on Huygen’s principle (1690)
Each point on a wavefront can be thought of as an isotropic point or a source of secondary spherical energy
Concepts explains why radio waves can be heard behind tall mountains or buildings that are normally considered to block line of sight transmissions

20. Attenuation and Obstructions
Shorter the wavelength (higher frequency) of the wireless signal, the more the signal it is attenuated.
Same wavelength (frequency), less amplitude.
Longer the wavelength (lower frequency) of the wireless signal, the less the signal is attenuated.

21. Propagation…..

22. Ground and Space Waves

23. Ground-Wave Propagation
The curved surface of the Earth horizon can diffract long-wavelength
(low frequency) radio waves. The waves can follow the curvature of the
Earth for as much as several hundred miles.

24. Ground-Wave Propagation
Results from a radio wave diffraction along the Earth’s surface.
Primarily affects longer wavelength radio waves that have vertical polarization (electric field is oriented vertically).
Most noticeable on AM broadcast band and the 160 meter and 80 meter amateur bands.
Communication distances often extend to 120 miles or more.
Most useful during the day at 1.8 MHz and 3.5 MHz when the D-Region absorption makes sky-wave propagation impossible.

25. Attenuation related to frequency
Loses increase with increase in frequency
Not very effective at frequencies above 2Mhz
Very reliable communication link
Reception is not affected by daily or seasonal weather changes

26. Used to communicate with submarines
ELF (30 to 300 Hz) propagation is utilized

27. Sky Wave Propagation

28. Atmospheric Regions Region Height Notes Troposphere 7 miles
Region where all weather occurs
Stratosphere
6 to 30 miles
Region where atmospheric gases “spread out” horizontally. The high speed jet stream travels in the stratosphere.
Ionosphere
30 to 400 miles
Region where solar radiation from the sun creates ions. Major influence on HF radio wave propagation.

29. Atmospheric Regions

30. What is the ionosphere?
The ionosphere is the uppermost part of the atmosphere, distinguished because it is ionized by solar radiation.
At heights above 80 km (50 miles), the atmosphere is so thin that free electrons can exist for short periods of time before they are captured by nearby ions.
This part of the atmosphere is ionized and contains a plasma.
In a plasma, negative free electrons and positive ions are attached by the electromagnetic force, but they are too energetic to stay fixed together in neutral molecules.

31. Sky-wave Propagation
Ionization levels in the Earth’s ionosphere can refract (bend) radio waves to return to the surface.
Ions in the Earth’s upper atmosphere are formed when ultraviolet (UV) radiation and other radiation from the sun knocks electrons from gas atoms.
The ionization regions in the Earth’s ionosphere is affected the sunspots on the sun’s surface

32. FIGURE 12-9 Sky-wave propagation.
Gary M. Miller, Jeffrey S. Beasley Modern Electronic Communication, 7e
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

33. Sky Wave Propagation

34. Radio waves radiated from the transmitting antenna in a direction toward the ionosphere
Long distance transmissions
Sky wave strike the ionosphere, is refracted back to ground, strike the ground, reflected back toward the ionosphere, etc until it reaches the receiving antenna
Skipping is the refraction and reflection of sky waves

35. Ionosphere
The layers that form the ionosphere vary greatly in altitude, density, and thickness with the varying degrees of solar activity.
The upper portion of the F layer is most affected by sunspots or solar disturbances
There is a greater concentration of solar radiation during peak sunspot activity.
The greater radiation activity the more dense the F layer and the higher the F layer becomes and the greater the skip distance

36. FIGURE 12-11 Relationship of frequency to refraction by the ionosphere.
Gary M. Miller, Jeffrey S. Beasley Modern Electronic Communication, 7e
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

37. FIGURE 12-12 Relationship of frequency to critical angle.
Gary M. Miller, Jeffrey S. Beasley Modern Electronic Communication, 7e
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

38. Terms Critical Frequency: Critical Angle:
The highest frequency that will be returned to the earth when transmitted vertically under given ionospheric conditions
Critical Angle:
The highest angle with respect to a vertical line at which a radio wave of a specified frequency can be propagated and still be returned to the earth from the ionosphere

39. Maximum usable frequency (MUF)
The highest frequency that is returned to the earth from the ionosphere between two specific points on earth
Optimum Working frequency:
The frequency that provides for the most consistent communication path via sky waves

40. Quiet Zone or Skip Zone:
The space between the point where the ground wave is completely dissipated and the point where the first sky wave is received
Fading:
Variations in signal strength that may occur at the receiver over a period of time.

41. Space Wave
Two types
Direct
Ground reflected

42. FIGURE 12-6 Direct and ground reflected space waves.
Gary M. Miller, Jeffrey S. Beasley Modern Electronic Communication, 7e
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

43. Direct Limited to “line-of sight” transmission distances
Antenna height and curvature of earth are limiting factors
Radio horizon is about 80% greater than line of sight because of diffraction effects

44. FIGURE 12-7 Radio horizon for direct space waves.
Gary M. Miller, Jeffrey S. Beasley Modern Electronic Communication, 7e
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

45. Reflected
Part of the signal from the transmitter is bounced off the ground and reflected back to the receiving antenna
Can cause problems if the phase between the direct wave and the reflected wave are not in phase
Detuning the antenna so that the reflected wave is too weak to receive

46. Tropospheric scattering
Signals are aimed at the troposphere rather than the ionosphere
350 Mhz to 10GHz for paths up to 400 mi
Received signal = 10-6 th of the transmitted power
Fading a problem

47. Line-Of-Sight Propagation
Radio signals travel in a straight line from a transmitting antenna to the receiving antenna.
Provides VHF/UHF communications within a 100 miles or so.
Signals can be reflected by buildings, hills, airplanes, etc.

48. Radio Path Horizon
The distance D to the radio horizon is greater from a higher
antenna. The maximum distance over which two stations may
communicate by space wave is equal to the sum of their
distances to the horizon.

49. VHF/UHF Signals Through Ionosphere
Sporadic E
A type of sky-wave propagation that allows long distance communication on the VHF bands (6 meters, 2 meters and 220 Mhz) through the E region of the atmosphere.
Occurs only sporadically during certain times of the year.

50. Radio Spectrum Symbol Frequency range Wavelength,  Comments ELF
< 300 Hz
> 1000 km
Earth-ionosphere waveguide propagation
ULF
300 Hz – 3 kHz
1000 – 100 km
VLF
3 kHz – 30 kHz
100 – 10 km LF
30 – 300 kHz
10 – 1 km
Ground wave propagation MF
300 kHz – 3 MHz
1 km – 100 m HF
3 – 30 MHz
100 – 10 m
Ionospheric sky-wave propagation
VHF
30 – 300 MHz
10 – 1 m
Space waves, scattering by objects similarly sized to, or bigger than, a free-space wavelength, increasingly affected by tropospheric phenomena
UHF
300 MHz – 3 GHz
1 m – 100 mm
SHF
3 – 30 GHz
100 – 10 mm
EHF
30 – 300 GHz
10 – 1 mm