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 31 Figure 2.12B Relation of field strength to signal field power. Figure 2.13 Out-of-phase waves cancel. where h t and h r are the heights of the transmitting and receiving antennas, respectively. The reflected signal contains both amplitude change and phase change. The phase change is typically π radians (180 degrees). The amplitude change is a function of frequency and the nature of the reflecting surface. The reflection coefficient can be characterized as γ = pe jφ (2.16)
32 p a r t I I : F u n d a m e n t a l s where g = reflection coefficient p = amplitude change ϕ = phase change j = imaginary operator ( −1) For smooth, high-reflectivity surfaces and a horizontally polarized microwave signal that has a shallow angle of incidence, the value of the reflection coefficient is close to –1. The phase change of the reflected signal at the receiving antenna is at least π radians because of the reflection. Added to this change is an additional phase shift that is a function of the difference in path lengths. This phase shift can be expressed in terms of the two antenna heights and path length: ⎛ h h ⎞ s = π + ⎜ 4π t r ⎟ ⎝ λD ⎠ 1 (2.17) Multipath reception problems exist because of interference between the direct and reflected components of the space wave. The multipath phenomenon that is perhaps most familiar to many readers is ghosting of over-the-air analog television signals. Some multipath events are transitory in nature (as when an aircraft flies over the direct transmission path), while others are permanent (as when a large building or hill reflects the signal). In mobile communications, multipath phenomena are responsible for reception dead zones and picket fencing. A dead zone exists when destructive interference between direct and reflected (or multiply reflected) waves drastically reduces signal strengths. This problem is most often noticed at VHF and above when the vehicle is stopped; the solution is to move the antenna one half wavelength (which at VHF and UHF may be a matter of a few inches). Picket fencing occurs as a mobile unit moves through successive dead zones and signal enhancement (or normal) zones; it is hard to describe via written word, but it sometimes sounds like a series of short noise bursts. At VHF, UHF, and microwave frequencies, the space wave is limited to so-called line-of-sight distances. The horizon is theoretically the limiting distance for communications, but the radio horizon is actually about 15 percent farther than the optical horizon (Fig. 2.14). This phenomenon is caused by refractive bending in the atmosphere around the curvature of the earth, and it makes the geometry of the situation look as if the earth’s radius is 4 3 its actual radius. Refraction occurs at VHF through microwave frequencies but not in the visible light spectrum because water and atmospheric pressure (which relates to the effects of atmospheric gases on microwave signals) become important contributors to the phenomenon. The K factor expresses the degree of curvature along any given path, while the Figure 2.14 Radio horizon versus optical horizon.
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- 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
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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 31<br />
Figure 2.12B Relation of field strength to signal field power.<br />
Figure 2.13 Out-of-phase waves cancel.<br />
where h t and h r are the heights of the transmitting and receiving antennas, respectively.<br />
The reflected signal contains both amplitude change and phase change. The phase<br />
change is typically π radians (180 degrees). The amplitude change is a function of frequency<br />
and the nature of the reflecting surface. The reflection coefficient can be characterized<br />
as<br />
γ = pe<br />
jφ<br />
(2.16)
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