Practical_Antenna_Handbook_0071639586
C h a p t e r 2 : r a d i o - W a v e P r o p a g a t i o n 37 Direction of travel Surface of earth Ground currents Figure 2.17 Distortion of vertically polarized electric field by lossy ground resistance. cally polarized waves, however, the earth offers electrical resistance to the E-field and returns currents to subsequent waves (Fig. 2.17). The conductivity of the soil determines how much energy is returned. Table 2.1 shows the typical conductivity values for several different forms of soil, as well as saltwater and freshwater. Type of Soil Dielectric Constant Conductivity (Siemens/Meter) Relative Quality Saltwater 81 5 Best Freshwater 80 0.001 Very poor Pastoral hills 14–20 0.03–0.01 Very good Marshy, wooded 12 0.0075 Average/poor Rocky hills 12–14 10 Poor Sandy 10 0.002 Poor Cities 3–5 0.001 Very poor Table 2.1 Sample Soil Conductivity Values The wavefront of a surface wave is tilted because of the losses in the ground that tend to retard the lower end of the wavefront (also shown in Fig. 2.17). The tilt angle is a function of the frequency, as shown in Table 2.2. Frequency (kHz) Tilt Angle Ratio Earth/Seawater (Degrees) 20 207 4.3/0.021 200 104 13.4/0.13 2,000 64 32.3/0.5 20,000 25 35/1.38 Table 2.2 Tilt Angle as a Function of Frequency
38 p a r t I I : F u n d a m e n t a l s Ground-Wave Propagation Frequency Dependence The frequency of a radio signal in large measure determines its surface-wave behavior. In the low-frequency (LF) band (30 to 300 kHz), ground losses are small for vertically polarized signals, so medium-distance communication (up to several hundred miles) is possible. In the medium-wave (MW) band (300 to 3000 kHz, which includes both the AM broadcast band and the 160-m amateur band), distances of a few hundred miles are common. In the high-frequency (HF) band, ground losses are typically greater, so the surface-wave distance reduces drastically. It is possible, in the upper end of the HF band (3000 to 30,000 kHz), for surface-wave signals to die out within a few dozen miles. This phenomenon is often seen in the 15- and 10-m amateur radio bands, as well as the 11-m (27-MHz) citizens band. Stations only 20 mi apart may not be able to communicate, but both can talk to a third station across the continent via ionospheric skip! Tropospheric Propagation The troposphere is the portion of the atmosphere between the surface of the earth and the stratosphere (or about 4 to 7 mi up). Some older texts group tropospheric propaga- Direction of propagation Low dielectric constant (low moisture content) Surface of earth High dielectric constant (high moisture content) A High dielectric constant Direction of propagation Low dielectric constant Surface of earth B Figure 2.18 Refraction in the troposphere.
- Page 4 and 5: Practical Antenna Handbook Joseph J
- Page 6 and 7: Contents Preface . . . . . . . . .
- Page 8 and 9: C o n t e n t s vii 12 The Yagi-Uda
- Page 10 and 11: C o n t e n t s ix Switched-Pattern
- Page 12 and 13: Preface My paternal grandfather was
- Page 14 and 15: P r e f a c e xiii this book repres
- Page 16 and 17: Acknowledgments As with other field
- Page 18 and 19: Background and History Part I Chapt
- Page 20 and 21: CHAPTER 1 Introduction to Radio Com
- Page 22 and 23: C h a p t e r 1 : I n t r o d u c t
- Page 24 and 25: Fundamentals Part II Chapter 2 Radi
- Page 26 and 27: CHAPTER 2 Radio-Wave Propagation To
- Page 28 and 29: C h a p t e r 2 : r a d i o - W a v
- Page 30 and 31: C h a p t e r 2 : r a d i o - W a v
- Page 32 and 33: C h a p t e r 2 : r a d i o - W a v
- Page 34 and 35: C h a p t e r 2 : r a d i o - W a v
- Page 36 and 37: C h a p t e r 2 : r a d i o - W a v
- Page 38 and 39: C h a p t e r 2 : r a d i o - W a v
- Page 40 and 41: C h a p t e r 2 : r a d i o - W a v
- Page 42 and 43: R N-1 C h a p t e r 2 : r a d i o -
- Page 44 and 45: C h a p t e r 2 : r a d i o - W a v
- Page 46 and 47: C h a p t e r 2 : r a d i o - W a v
- Page 48 and 49: C h a p t e r 2 : r a d i o - W a v
- Page 50 and 51: C h a p t e r 2 : r a d i o - W a v
- Page 52 and 53: C h a p t e r 2 : r a d i o - W a v
- Page 56 and 57: C h a p t e r 2 : r a d i o - W a v
- Page 58 and 59: C h a p t e r 2 : r a d i o - W a v
- Page 60 and 61: C h a p t e r 2 : r a d i o - W a v
- Page 62 and 63: C h a p t e r 2 : r a d i o - W a v
- Page 64 and 65: C h a p t e r 2 : r a d i o - W a v
- Page 66 and 67: C h a p t e r 2 : r a d i o - W a v
- Page 68 and 69: Figure 2.29C Monthly averaged sunsp
- Page 70 and 71: C h a p t e r 2 : r a d i o - W a v
- Page 72 and 73: C h a p t e r 2 : r a d i o - W a v
- Page 74 and 75: C h a p t e r 2 : r a d i o - W a v
- Page 76 and 77: C h a p t e r 2 : r a d i o - W a v
- Page 78 and 79: C h a p t e r 2 : r a d i o - W a v
- Page 80 and 81: C h a p t e r 2 : r a d i o - W a v
- Page 82 and 83: C h a p t e r 2 : r a d i o - W a v
- Page 84 and 85: C h a p t e r 2 : r a d i o - W a v
- Page 86 and 87: C h a p t e r 2 : r a d i o - W a v
- Page 88 and 89: C h a p t e r 2 : r a d i o - W a v
- Page 90 and 91: C h a p t e r 2 : r a d i o - W a v
- Page 92 and 93: C h a p t e r 2 : r a d i o - W a v
- Page 94 and 95: C h a p t e r 2 : r a d i o - W a v
- Page 96 and 97: C h a p t e r 2 : r a d i o - W a v
- Page 98 and 99: CHAPTER 3 Antenna Basics An antenna
- Page 100 and 101: C h a p t e r 3 : A n t e n n a B a
- Page 102 and 103: C h a p t e r 3 : A n t e n n a B a
C h a p t e r 2 : r a d i o - W a v e P r o p a g a t i o n 37<br />
Direction of travel<br />
Surface of<br />
earth<br />
Ground currents<br />
Figure 2.17 Distortion of vertically polarized electric field by lossy ground resistance.<br />
cally polarized waves, however, the earth offers electrical resistance to the E-field and<br />
returns currents to subsequent waves (Fig. 2.17). The conductivity of the soil determines<br />
how much energy is returned. Table 2.1 shows the typical conductivity values for<br />
several different forms of soil, as well as saltwater and freshwater.<br />
Type of Soil<br />
Dielectric<br />
Constant<br />
Conductivity<br />
(Siemens/Meter) Relative Quality<br />
Saltwater 81 5 Best<br />
Freshwater 80 0.001 Very poor<br />
Pastoral hills 14–20 0.03–0.01 Very good<br />
Marshy, wooded 12 0.0075 Average/poor<br />
Rocky hills 12–14 10 Poor<br />
Sandy 10 0.002 Poor<br />
Cities 3–5 0.001 Very poor<br />
Table 2.1 Sample Soil Conductivity Values<br />
The wavefront of a surface wave is tilted because of the losses in the ground that<br />
tend to retard the lower end of the wavefront (also shown in Fig. 2.17). The tilt angle is<br />
a function of the frequency, as shown in Table 2.2.<br />
Frequency (kHz) Tilt Angle Ratio<br />
Earth/Seawater<br />
(Degrees)<br />
20 207 4.3/0.021<br />
200 104 13.4/0.13<br />
2,000 64 32.3/0.5<br />
20,000 25 35/1.38<br />
Table 2.2 Tilt Angle as a Function of Frequency
Change language