High-frequency radio wave absorption in the D- region

Transcription

1 Utah State University From the SelectedWorks of David Smith Spring 2017 High-frequency radio wave absorption in the D- region David Alan Smith, Utah State University This work is licensed under a Creative Commons CC_BY International License. Available at:

2 High-frequency Radio Wave Absorption A research report presented by David Alan Smith Utah State University Department of Physics March 30,

3 Game Plan Radio Wave Propagation Sky Waves Properties of Ionosphere Geometric Optics High-frequency Radio Wave Absorption Basic Absorption Equation Types of Absorption Absorption Coefficient Absorption Equation Special Cases Electron Density Collision Frequency Conclusions/Discussion Questions 2

4 Essential Points Absorption is frequency-dependent 3

5 Essential Points Absorption is frequency-dependent Most HF absorption takes place within the D-region 4

6 Essential Points Absorption is frequency-dependent Most HF absorption takes place within the D-region Within the D-region non-deviative absorption dominates (Assuming HF) 5

7 Essential Points Absorption is frequency-dependent Most HF absorption takes place within the D-region Within the D-region non-deviative absorption dominates (Assuming HF) Electron density is critical 6

8 Propagation via Sky Wave Ground Wave vs. Sky Wave 7

9 Ground Wave Stays close to the earth Doesn t leave lower atmosphere Direct ray and ground-reflected ray combine to form space wave AM broadcast band has range of about 160 km (100 miles) Generally ineffective for long-range communications Typical HF ground-wave range Images courtesy of ARRL Antenna Book, p

10 Sky Wave Leaves lower atmosphere Passes through ionized region Refracted according to geometric optics Radio waves entering ionosphere at angles above critical angle go off into space Range up to 4000 km (2500 miles) per hop Efficient long-range communication Subject to various atmospheric conditions Images courtesy of ARRL Antenna Book, pp 23-13, Distance vs. wave angle for onehop transmission 9

11 Ground Wave vs. Sky Wave Ground Wave Sky Wave By Own work (Own work) [CC BY 3.0 ( via Wikimedia Commons 10

12 Definitions 11

13 Definitions Plasma: A macroscopically neutral assembly of charged and possibly also uncharged particles. IEEE Standard Definitions of Terms for Radio Wave Propagation," in IEEE Std , vol., no., pp.i-, 1998 doi: /IEEESTD

14 Definitions Plasma: A macroscopically neutral assembly of charged and possibly also uncharged particles. Dispersive medium: A medium in which one or more of the constitutive parameters vary with frequency. IEEE Standard Definitions of Terms for Radio Wave Propagation," in IEEE Std , vol., no., pp.i-, 1998 doi: /IEEESTD

15 Definitions Plasma: A macroscopically neutral assembly of charged and possibly also uncharged particles. Dispersive medium: A medium in which one or more of the constitutive parameters vary with frequency. Ionosphere: That part of a planetary atmosphere where ions and free electrons are present in quantities sufficient to affect the propagation of radio waves. IEEE Standard Definitions of Terms for Radio Wave Propagation," in IEEE Std , vol., no., pp.i-, 1998 doi: /IEEESTD

16 Definitions Plasma: A macroscopically neutral assembly of charged and possibly also uncharged particles. Dispersive medium: A medium in which one or more of the constitutive parameters vary with frequency. Ionosphere: That part of a planetary atmosphere where ions and free electrons are present in quantities sufficient to affect the propagation of radio waves. D region: The region of the terrestrial ionosphere between about 50 km and 90 km altitude. IEEE Standard Definitions of Terms for Radio Wave Propagation," in IEEE Std , vol., no., pp.i-, 1998 doi: /IEEESTD

17 Definitions Plasma: A macroscopically neutral assembly of charged and possibly also uncharged particles. Dispersive medium: A medium in which one or more of the constitutive parameters vary with frequency. Ionosphere: That part of a planetary atmosphere where ions and free electrons are present in quantities sufficient to affect the propagation of radio waves. D region: The region of the terrestrial ionosphere between about 50 km and 90 km altitude. E region: The region of the terrestrial ionosphere between about 90 km and 150 km altitude. IEEE Standard Definitions of Terms for Radio Wave Propagation," in IEEE Std , vol., no., pp.i-, 1998 doi: /IEEESTD

18 Definitions Plasma: A macroscopically neutral assembly of charged and possibly also uncharged particles. Dispersive medium: A medium in which one or more of the constitutive parameters vary with frequency. Ionosphere: That part of a planetary atmosphere where ions and free electrons are present in quantities sufficient to affect the propagation of radio waves. D region: The region of the terrestrial ionosphere between about 50 km and 90 km altitude. E region: The region of the terrestrial ionosphere between about 90 km and 150 km altitude. F region: The region of the terrestrial ionosphere from about km altitude. IEEE Standard Definitions of Terms for Radio Wave Propagation," in IEEE Std , vol., no., pp.i-, 1998 doi: /IEEESTD

19 Definitions Plasma: A macroscopically neutral assembly of charged and possibly also uncharged particles. Dispersive medium: A medium in which one or more of the constitutive parameters vary with frequency. Ionosphere: That part of a planetary atmosphere where ions and free electrons are present in quantities sufficient to affect the propagation of radio waves. D region: The region of the terrestrial ionosphere between about 50 km and 90 km altitude. E region: The region of the terrestrial ionosphere between about 90 km and 150 km altitude. F region: The region of the terrestrial ionosphere from about km altitude. High-frequency Spectrum: 3.0 MHz-30 MHz IEEE Standard Definitions of Terms for Radio Wave Propagation," in IEEE Std , vol., no., pp.i-, 1998 doi: /IEEESTD

20 Ionospheric Properties 19

21 Ionosphere The ionosphere is considered a weakly-ionized plasma For a fully-ionized plasma the ratio of charged particles to neutral particles is about 1 Within the ionized region of the atmosphere this ratio is always much less than 1. Hence the ionosphere is a weakly-ionized plasma 20

22 D-region Height: About 90 km Thickness: About 40 km Significant diurnal variations Typical daytime electron density 21

23 E-region Height: About 150 km Thickness: About 60 km Diurnal variations though not as pronounced as D-region Typical daytime electron density: 22

24 F1-region Height: About 350 km Thickness: About 200 km Diurnal variations Typical daytime electron density: 23

25 F2-region Height: About 1000 km Thickness: About 750 km Diurnal variations, though not as pronounced Electron Density: Unlike previous regions, F2 electron density decreases with height Important note: F2 becomes the F-region after sunset. 24

26 Ionospheric Properties 25

27 Electron Density: Function of Height Electron concentration per cubic centimeter (Daytime) Image from Kelley p

28 Electron Density: Function of Height Maybe doesn t seem interesting but lots going on Electron concentration per cubic centimeter (Daytime) Image from Kelley p

29 Geometric Approach 28

30 Geometric Approach Ray path in a continuously varying medium (Ionosphere) (Lied p 4) Bends away from the normal 29

31 Geometric Approach Snell s Law: 30

32 Geometric Approach Snell s Law: Index of Refraction: 31

33 Geometric Approach Snell s Law: Index of Refraction: 32

34 Geometric Approach Snell s Law: Index of Refraction: Dielectric Constant of Weakly-Ionized Gas 33

35 Geometric Approach Snell s Law: Index of Refraction: Dielectric Constant of Weakly-Ionized Gas 34

36 Geometric Approach Snell s Law: Index of Refraction: Dielectric Constant of Weakly-Ionized Gas 35

37 Geometric Approach Snell s Law: Index of Refraction: Dielectric Constant of Weakly-Ionized Gas Note dependence on electron density 36

38 Geometric Approach Within a dispersive media such as the ionosphere: 37

39 Geometric Approach Within a dispersive media such as the ionosphere: For a given frequency, as electron density increases index of refraction decreases 38

40 Geometric Approach Within a dispersive media such as the ionosphere: For a given frequency, as electron density increases index of refraction decreases For a given electron density as frequency increases index of refraction approaches unity 39

41 Index of Refraction Index of Refraction Index of Refraction as Function of Frequency E E E E E E+07 Frequency (Hz) 1.00 Index of Refraction as Function of Electron Density E E E E+11 Number of electrons per cubic meter 40

42 Geometric Approach Three Cases: 41

43 Geometric Approach Three Cases: 42

44 Geometric Approach Three Cases: 43

45 Geometric Approach Three Cases: 44

46 The Basic Absorption Equation 45

47 Basic Absorption Equation Equation for total system loss Note: Each term is a base-10 log. Hence, we add them 46

48 Basic Absorption Equation Equation for total system loss Transmitting Receiving What goes on in between Critical term is path loss Note: Each term is a base-10 log. Hence, we add them 47

49 Basic Absorption Equation Equation for path loss 48

50 Basic Absorption Equation Equation for path loss Critical term is absorption. Hence, we focus on the absorption term 49

51 Basic Absorption Equation Equation for absorption 50

52 Basic Absorption Equation Equation for absorption 51

53 Basic Absorption Equation Equation for absorption 52

54 Basic Absorption Equation Equation for absorption 53

55 Basic Absorption Equation Equation for absorption 54

56 Basic Absorption Equation 55

57 Basic Absorption Equation Kappa has units of nepers per unit length. Hence, the above equation has units of nepers 56

58 Basic Absorption Equation Kappa has units of nepers per unit length. Hence, the above equation has units of nepers From the rules of logarithms, 57

59 Basic Absorption Equation Kappa has units of nepers per unit length. Hence, the above equation has units of nepers From the rules of logarithms, 58

60 Basic Absorption Equation Kappa has units of nepers per unit length. Hence, the above equation has units of nepers From the rules of logarithms, 59

61 Basic Absorption Equation Kappa has units of nepers per unit length. Hence, the above equation has units of nepers From the rules of logarithms, Since there are roughly 8.69 db per neper the absorption equation has units of db per unit length 60

62 Types of Absorption 61

63 Types of Absorption Type of absorption depends on relationship between radio wave frequency and plasma frequency 62

64 Types of Absorption Type of absorption depends on relationship between radio wave frequency and plasma frequency Type one: Radio wave frequency about the same as plasma frequency 63

65 Types of Absorption Type of absorption depends on relationship between radio wave frequency and plasma frequency Type one: Radio wave frequency about the same as plasma frequency 64

66 Types of Absorption Type of absorption depends on relationship between radio wave frequency and plasma frequency Type one: Radio wave frequency about the same as plasma frequency Hence, the wave propagates slowly at the group velocity through ionosphere This type of absorption is called Deviative Absorption 65

67 Types of Absorption Type of absorption depends on relationship between radio wave frequency and plasma frequency Type one: Radio wave frequency about the same as plasma frequency Hence, the wave propagates slowly at the group velocity through ionosphere This type of absorption is called Deviative Absorption Deviative absorption uncommon in D-region 66

68 Types of Absorption Type of absorption depends on relationship between radio wave frequency and plasma frequency Type two: Radio wave frequency greater than plasma frequency 67

69 Types of Absorption Type of absorption depends on relationship between radio wave frequency and plasma frequency Type two: Radio wave frequency greater than plasma frequency 68

70 Types of Absorption Type of absorption depends on relationship between radio wave frequency and plasma frequency Type two: Radio wave frequency greater than plasma frequency Hence, wave propagates at about speed of light This is called non-deviative absorption Very common in D-region Note: Appendix 1 of my report presents a discussion/derivation of group and phase velocities. 69

71 Altitude (km) Types of Absorption Plasma Frequency Profile Plasma Frequency (MHz) Profile of plasma frequency from km. But we re really interested in D-region 70

72 Height (km) Types of Absorption Typical Plasma Frequency Profile within D-region Plasma Frequency (MHz) Profile of plasma frequency from km. Note plasma frequency nearly always less than 3.0 MHz 71

73 Height (km) Types of Absorption Typical Plasma Frequency Profile within D-region Plasma Frequency (MHz) Thus we see that non-deviative absorption dominates in the D-region (Assuming 3.0 < f < 30.0 MHz) 72

74 Absorption Coefficient 73

75 Absorption Coefficient In the absorption equation kappa is defined as the absorption coefficient 74

76 Absorption Coefficient In the absorption equation kappa is defined as the absorption coefficient Recall that kappa is also defined as the measure of the decay of amplitude per unit distance 75

77 Absorption Coefficient In the absorption equation kappa is defined as the absorption coefficient Recall that kappa is also defined as the measure of the decay of amplitude per unit distance I show in appendix 2 of my report that kappa is derived from Maxwell s equations Hence, the absorption equation is based on first principles 76

78 Absorption Coefficient In the absorption equation kappa is defined as the absorption coefficient Recall that kappa is also defined as the measure of the decay of amplitude per unit distance I show in appendix 2 of my report that kappa is derived from Maxwell s equations Hence, the absorption equation is based on first principles In chapter 2 of Ionospheric Radio Propagation Davies spends many pages discussing the theory of wave propagation. Starting with Maxwell s equations it can be shown that the absorption coefficient can be described by, 77

79 Absorption Coefficient In the absorption equation kappa is defined as the absorption coefficient Recall that kappa is also defined as the measure of the decay of amplitude per unit distance I show in appendix 2 of my report that kappa is derived from Maxwell s equations Hence, the absorption equation is based on first principles In chapter 2 of Ionospheric Radio Propagation Davies spends many pages discussing the theory of wave propagation. Starting with Maxwell s equations it can be shown that the absorption coefficient can be described by, 78

80 Absorption Coefficient 79

81 Absorption Coefficient Units of kappa are nepers per unit length Defining Terms: New important term! 80

82 Absorption Coefficient Units of kappa are nepers per unit length Plugging in constant values we find that, 81

83 Absorption Equation Revisited 82

84 Absorption Equation 83

85 Absorption Equation 84

86 Absorption Equation 85

87 Absorption Equation In this form integral is over path length 86

88 Absorption Equation In this form integral is over path length Electron density and collision frequency can be functions of height 87

89 Absorption Equation In this form integral is over path length Electron density and collision frequency can be functions of height 88

90 Absorption Equation 89

91 Absorption Equation 90

92 Absorption Equation Now integrated over height 91

93 Absorption Equation For special case of vertical transmission: 92

94 Absorption Equation For special case of vertical transmission: 93

95 Special Cases 94

96 Special Cases Case 1: Radio wave frequency greater than collision frequency. 95

97 Special Cases Case 1: Radio wave frequency greater than collision frequency. Case 2: Radio wave frequency less than collision frequency. 96

98 Special Cases Case 1: Radio wave frequency greater than collision frequency. Case 2: Radio wave frequency less than collision frequency. Case 3: Radio wave frequency about equal to collision frequency. 97

99 Special Cases Case 1: Radio wave frequency greater than collision frequency. Case 2: Radio wave frequency less than collision frequency. Case 3: Radio wave frequency about equal to collision frequency. According to Davies and Lied Case 1 applies generally for HF radio waves at mid-latitudes 98

100 Special Cases Case 1: Radio wave frequency greater than collision frequency. Case 2: Radio wave frequency less than collision frequency. Case 3: Radio wave frequency about equal to collision frequency. According to Davies and Lied Case 1 applies generally for HF radio waves at mid-latitudes 99

101 Special Cases Case 1: Radio wave frequency greater than collision frequency. Case 2: Radio wave frequency less than collision frequency. Case 3: Radio wave frequency about equal to collision frequency. According to Davies and Lied Case 1 applies generally for HF radio waves at mid-latitudes The absorption equation in terms of radio wave frequency in cycles per second. 100

102 Total Absorption (db) Absorption Equation Total Absorption by Frequency Frequency (MHz) Thus we see the frequency-dependence of absorption Based on data from Bain and Harrison as well as Kelley and also a HW assignment from L. Scherliess 101

103 Absorption (db0 Absorption (db) Absorption (db) Absorption (db) Absorption at 3.0 MHz Absorption at 6.0 MHz D E F Total 0 D E F Total Region Region Absorption at 15.0 MHz Absorption at 30.0 MHz D E F Total Region D E F Total Region Thus we see that most absorption takes place within the D-region Based on data from Bain and Harrison as well as Kelley and also a HW assignment from L. Scherliess 102

104 Absorption (db0 Absorption (db) Absorption (db) Absorption (db) Absorption at 3.0 MHz Absorption at 6.0 MHz D E F Total 0 D E F Total Region Region Absorption at 15.0 MHz Absorption at 30.0 MHz D E F Total Region D E F Total Region Thus we see that most absorption takes place within the D-region Based on data from Bain and Harrison as well as Kelley and also a HW assignment from L. Scherliess 103

105 Electron Density 104

106 Electron Density Typical electron concentration per cubic centimeter (Daytime) Image from Kelley p

107 Electron Density Again, not too interesting Typical electron concentration per cubic centimeter (Daytime) Image from Kelley p

108 Altitude (km) Electron Density Electron Density Profile E E E E E E E+12 Electron Density (per cubic meter) Based on Bain and Harrison [1972] Electron density profile below 100 km 107

109 Electron Density Everything depends on electron density 108

110 Electron Density Everything depends on electron density Plasma Frequency 109

111 Electron Density Everything depends on electron density Plasma Frequency Index of Refraction 110

112 Electron Density Everything depends on electron density Plasma Frequency Index of Refraction Absorption Coefficient 111

113 Electron Density Everything depends on electron density Plasma Frequency Index of Refraction Absorption Coefficient Thus we see that electron density is the most critical component 112

114 Collision Frequency 113

115 Collision Frequency We are concerned with two collision types: Electron- ion Electron-neutral 114

116 Collision Frequency We are concerned with two collision types: Electron- ion Electron-neutral We find the following equations for collision frequencies: 115

117 Collision Frequency We are concerned with two collision types: Electron- ion Electron-neutral We find the following equations for collision frequencies: 116

118 Collision Frequency We are concerned with two collision types: Electron- ion Electron-neutral We find the following equations for collision frequencies: Electron-Ion: 117

119 Collision Frequency We are concerned with two collision types: Electron- ion Electron-neutral We find the following equations for collision frequencies: Electron-Ion: Electron-Neutral: 118

120 Collision Frequency We are concerned with two collision types: Electron- ion Electron-neutral We find the following equations for collision frequencies: Electron-Ion: Electron-Neutral: 119

121 Collision Frequency We are able to make the following simplifying assumptions: 120

122 Collision Frequency We are able to make the following simplifying assumptions: Within the D-region, the neutral atmosphere density is fairly consistent 121

123 Collision Frequency We are able to make the following simplifying assumptions: Within the D-region, the neutral atmosphere density is fairly consistent Within the D-region,. Hence we need only consider electron-neutral collisions 122

124 Collision Frequency We are able to make the following simplifying assumptions: Within the D-region, the neutral atmosphere density is fairly consistent Within the D-region, Within the D-region. Hence we need only consider electron-neutral collisions. Hence it is sufficient to us the neutral temperature 123

125 Collision Frequency We are able to make the following simplifying assumptions: Within the D-region, the neutral atmosphere density is fairly consistent Within the D-region, Within the D-region. Hence we need only consider electron-neutral collisions. Hence it is sufficient to us the neutral temperature 124

126 Collision Frequency We are able to make the following simplifying assumptions: Within the D-region, the neutral atmosphere density is fairly consistent Within the D-region, Within the D-region. Hence we need only consider electron-neutral collisions. Hence it is sufficient to us the neutral temperature 125

127 Conclusions/Discussion 126

128 Conclusions/Discussion We showed the following to be true: 127

129 Conclusions/Discussion We showed the following to be true: Absorption is frequency-dependent 128

130 Conclusions/Discussion We showed the following to be true: Absorption is frequency-dependent Most HF absorption takes place within the D-region 129

131 Conclusions/Discussion We showed the following to be true: Absorption is frequency-dependent Most HF absorption takes place within the D-region Within the D-region non-deviative absorption dominates 130

132 Conclusions/Discussion We showed the following to be true: Absorption is frequency-dependent Most HF absorption takes place within the D-region Within the D-region non-deviative absorption dominates The electron density is the most critical component 131

133 Conclusions/Discussion We showed the following to be true: Non-deviative absorption within the D-region can be described mathematically in terms of neutral density or collision frequency, 132

134 Conclusions/Discussion We showed the following to be true: Non-deviative absorption within the D-region can be described mathematically in terms of neutral density or collision frequency, 133

135 Acknowledgments Special thanks to the following who assisted in the preparation of the presentation Dr. Jan J. Sojka Dr. Vince Eccles And thank you to my supervisory committee: Doctors J. Sojka, D. Peak, B. Fejer, M. Taylor, R. Fullmer 134

136 Key Sources References 135

137 fin 136

138 Questions? 137

139 138

140 139

141 140

142 Frequency (MHz) D-region absorption values using data from Bain and Harrison 141