1. Sky Wave Propagation

2. Content Sky Wave Propagation Ionosphere layers Virtual height
Critical Frequency
Critical Angle
Maximum Usable Frequency & OWF
Skip Distance

3. Ground Wave Propagation
Disadvantages
Requires relatively high transmission power.
Frequencies up to 2 MHz.
Require large antennas.
Ground Wave get attenuated.
Attenuation increases with frequency.

4. Sky Wave Propagation
The propagation of radio waves reflected or refracted back toward Earth from the ionosphere.
not limited by the curvature of the Earth.

5. Sky Wave Propagation
Sky wave also Known as Skip/ Ionospheric / Hop wave.
Sky wave propagation can include multiple hops between the Earth and the ionosphere.
Frequency range 2 to 30 MHz.

6. Ionosphere
The ionosphere is a region of Earth's upper atmosphere, from about 50 km to 400 km.
It is ionized by solar radiation.

7. Ionosphere
The molecules of the atmosphere are ionized by Ultra violet rays, alpha, beta & gamma rays from Solar radiation.
Ionization density depends upon “ the molecular density and energy of solar radiation”.
Ionization is the process by which a molecule acquires a negative or positive charge by gaining or losing electrons to form ions.

8. Refractive Index of Ionosphere
Reflection of wave from ionosphere is through refraction of waves.
Snell’s Law of refraction in the ionosphere.
n- is refractive index
θi- is angle of incident
θr- is angle of refraction

9. The Ionosphere layers D – Layer E – Layer Es – Layer F1 – Layer

10. The Ionosphere layers

11. The Ionospheric Layers

12. D-Layer Average height 70km Average thickness 10km
Its exists only in day time.
It is not useful layer for HF communication
It reflects some VLF and LF waves
Its electron density N=400 electrons/cc
fc=180kHz
Almost no refraction (bending) of radio waves.
Its virtual height is 60 to 80km.

13. E-Layer Average height 100km Average thickness 25km
Its exists only in day time
It reflects some HF waves
Its electron density N=5×105 electrons/cc
fc=4MHz
Its virtual height is 110km
Maximum single hop range 2,350km

14. Es-Layer Its exists in both day and night It is a thin layer
Its height normally 90 to 130km
Its electron density is high
It is difficult to know where and when it will occur and how long it will persist.

15. F1-Layer Average height 180km Average thickness 20km
It combines with F2 layer at night
fc=5MHz
Its virtual height is 180km
Maximum single hop range 3,000km
Although some HF waves get reflected from it, most of them pass through it
It affect HF waves by providing more absorption.

16. F2-Layer Average height 325km in day time Thickness is about 200km
It falls to a height of 300km at nights as it combines with F1 layer
It offers better HF reflection
Its electron density N=8×1011 electrons/m3
fc=8MHz in day & fc=6MHz in night
Its virtual height is 300km
Maximum single hop range 4,000km

17. Virtual height

18. The virtual height is the height from which the radio wave appears to be reflecting.
Virtual Height > Actual Height

19. Critical Frequency (fc)

20. Critical Frequency (fc)
Critical frequency for a given layer is the highest frequency that will reflected to earth by that layer at vertical incidence.
Higher frequencies “escape”

21. Critical Frequency (fc)
Angle of incident θi = 0

22. Critical Frequency (fc)

23. Critical Frequency (fc)

24. Critical Angle (θc)

25. Critical Angle (θc)

26. Critical Angle (θc)
Critical Angle is defined as the angle of incidence θ< θc which wave will be reflected, θ> θc which wave will not be reflected.
θc depends on thickness of layer, height and frequency of wave.
As the frequency of a radio wave is increased, the critical angle must be reduced.

27. Maximum Usable Frequency
Maximum Usable Frequency (MUF) is the highest radio frequency that can be used for transmission between two points via reflection from the ionosphere at a specified time.
The highest frequency that will be returned to earth for a given angle of incidence.

29. 1

30. 2

31. Comparing Equ. 1 & 2.

32. Skip Distance

33. Skip Distance
The SKIP DISTANCE is the distance from the transmitter to the point where the sky wave is first returned to Earth.
The size of the skip distance depends on the frequency, angle of incidence and ionization.
The minimum distance from the transmitter, along the surface of the earth, at which a wave above the critical frequency will be returned to earth.

34. Skip Distance The Skip distance, ds is h – height of the layer
θc – Critical angle

35. Skip Distance

36. Optimum Working Frequency (OWF)

37. Optimum Working Frequency (OWF)
Optimum Working frequency that provides the most consistent communication path via sky waves.
It is chosen to be about 15% less than the MUF.

38. Electrical Properties of Ionosphere
VARIATIONS IN THE IONOSPHERE
Regular or cyclic variations- can be predicted in advance
Irregular variations – abnormal behaviour of Sun- cannot be predicted in advance.

39. Regular Variations
Daily
Seasonal
11-year
27-day

40. Daily variations Due to 24 hour rotation of earth around the sun.
D layer- disappears at night. Reflects VLF waves- important for VLF comm. – refracts IF and MF waves for short range comm. Absorbs HF waves; little effect on VHF and above

41. Daily variations
E layer- refracts HF waves during day. Ionization- reduced at night.
F layer- Ionization density depends on time of the day and angle of the sun. Has one layer at night and 2 layers F1 and F2 during day.

42. Seasonal Variations Are due to the Earth revolving around the sun.
Ionization density - D, E and F1 – greatest – summer. F2- greatest- Winter. Operating freq- higher in winter

43. Eleven Year Sun Spot Cycle
SUNSPOTS- phenomena of appearance and disappearance of dark, irregularly shaped areas.
Life span of individual sunspots is variable; however, a regular cycle of sunspot activity has also been observed. This cycle has both a minimum and maximum level of sunspot activity that occur approximately every 11 years.

44. Maximum sunspot activity, the ionization density of all layers increases.
Absorption in the D layer increases - the critical frequencies for the E, F1, and F2 layers are higher. At these times, higher operating frequencies must be used for long distance communications.

45. 27-DAY SUNSPOT CYCLE.
As the sun rotates on its own axis, these sunspots are visible at 27-day intervals, the app. period required for the sun to make one complete rotation.
The 27-day sunspot cycle - variations in the ionization density of the layers on a day-to-day basis.

46. Abnormal Variations Sudden Ionospheric Disturbance Ionospheric storms
Sporadic E layer reflection
Tides and winds
Fadings
Whistlers

47. Sporadic E
Sporadic E- Irregular cloud-like patches of unusually high ionization at heights greater than normal E layer- some times thin layer – other times heavily ionized thick layer.
fc – very high- long distance transmission occurs at unusually high freq.
Form and disappear short time either day or night.

48. Sudden Ionospheric Disturbance(SID)
Sudden appearance of Solar flares.
Causes complete fading- Dellinger fade-out.
D-layer
Not found in E, F1 and F2 layers.

49. Whistlers
Whistlers are transient electromagnetic disturbances which occur naturally.
Consist of EM pulses of Audio Frequency radiation along the direction of the magnetic field of the earth between conjugate points in the northern and southern hemispheres.
Types- Long whistlers, short whistlers, noise whistlers.

50. Tides and Winds Tides and Winds are common in the atmosphere.
Winds in the ionosphere are caused by tides. Winds –due to motion of turbulence in F2-layer.

54. Free-Space Propagation
Radio waves propagate through free space in a straight line with a velocity of the speed of light (300,000,000 m/s)
There is no loss of energy in free space, but there is attenuation due to the spreading of the waves

55. Attenuation of Free Space
An isotropic radiator would produce spherical waves
The power density of an isotropic radiator is simply be the total power divided by the surface area of the sphere, according to the square-law:

56. Transmitting Antenna Gain
In practical communication systems, it is important to know the signal strength at the receiver input
It depends on the transmitter power and the distance from the transmitter to the receiver, but also upon the transmitting and receiving antennas
Two important antenna characteristics are:
Gain for the transmitting antenna
Effective area for the receiving antenna
Antennas are said to have gain in those directions in which the most power is radiated

57. Receiving Antenna Gain
A receiving antenna absorbs some of the energy from radio waves that pass it
A larger antenna receives more power than a smaller antenna (in relation to surface area)
Receiving antennas are considered to have gain just as transmitting antennas do
The power extracted from a receiving antenna is a function of its physical size and its gain

58. Path Loss
Free-space attenuation is the ratio of received power to transmitted power
The decibel gain between transmitter and receiver is negative (loss) and the loss found this way is called free-space loss or path loss

59. Fading

60. Fading
The random variation in the received signal strength is called fading.
The main causes of fading are
Multipath propagation
Variation in ionosphere condition
Types of fading
Multipath fading
Selective fading
Interference fading
Polarization fading
Skip fading
Absorption fading

61. Diversity
Diversity reception is used to minimize the effects of fading.
Diversity techniques are
Frequency diversity
Space diversity
Polarization diversity
Time diversity

62. Q & A

63. Multiple Choice Questionsa b c

64. c c c

65. a d b

66. a a

67. b a

68. c

69. Q & A