Practical_Antenna_Handbook_0071639586

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520 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s Shortwave and AM BCB “Skip” RDF Radio direction finding is most accurate over relatively short distances. If you have the choice, it’s better to use the ground wave (which is what you use during daylight hours for nearly all AM BCB stations). Skip rolls in on the AM BCB after local sundown, so you can hear all manner of stations up and down the dial. However, with a little practice, you can even RDF distant stations. Unfortunately, there are some practical difficulties associated with skip RDFing. When we look at propagation drawings of skip in textbooks we usually see what’s happening in a single plane at a time. Typically, a single imagined “pencil beam” radio signal is shown traveling at some angle up the ionosphere, where it is “reflected” (actually, it’s a refraction phenomenon but looks like reflection to an observer on the surface) back to earth. We can tell from the drawing that the angle of incidence equals the angle of reflection, just like they told us in high school science classes. Unfortunately, the real world is not so neat and crisp. In the real world the skywave signal we hear is an amalgam of many such “pencil beams”, coming in from a near-infinite number of directions and elevation angles. The strongest component of such a signal aggregate is always the path that has the least total loss from start to end, but for skip distances that path is always shifting because the ionosphere that makes skip reception possible is a continuously shifting medium. A radio wave refracting through the ionosphere must feel like you and I do when taking a ride through the house of warped mirrors at an amusement park. It’s entirely possible, therefore, that at any given instant the preferred path to your antenna for a signal source located to the northwest of you might be a so-called skew path, coming in from as far around the compass rose as the southwest! Terrestrial reflections also cause problems, especially when RDFing a station in the high end of the HF band or the VHF/UHF bands. Radio waves will reflect from geological features such as mountains and from man-made structures (e.g., buildings). If the reflection is strong enough, it might appear to be the real signal and cause a severe error in RDFing. Bottom line: Be wary of RDF results on HF when the “skip is in”. Similarly, don’t let the canyons of New York City or the mountains of upstate fool you on VHF and UHF. Adcock <strong>Antenna</strong>s The Adcock antenna (Fig. 23.9) has been around since 1919, when it was patented by British Royal Engineer and Lieutenant Frank Adcock as a way of overcoming the tendency of RDF loops to lose directional accuracy at nighttime. Adcock surmised that the horizontally polarized component of incoming skywave signals induced loop output voltages indicating certain spurious directions when the output voltage due to the vertically polarized ground wave was near zero. To minimize this error, Adcock designed a number of different configurations based on the use of vertical elements, both monopole and dipole. The Adcock antenna shown here once found use in the United States by the FCC at their old monitoring sites. It consists of two center-fed, nonresonant (but identical) vertical radiators. Each side of each element is at least 0.1l long, but need not be reso-
Figure 23.9 Adcock array RDF antenna. C h a p t e r 2 3 : R a d i o D i r e c t i o n - F i n d i n g ( R D F ) A n t e n n a s 521
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Practical Antenna Handbook
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Practical Antenna Handbook Joseph J
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Contents Preface . . . . . . . . .
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C o n t e n t s vii 12 The Yagi-Uda
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C o n t e n t s ix Switched-Pattern
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Preface My paternal grandfather was
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P r e f a c e xiii this book repres
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Acknowledgments As with other field
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Background and History Part I Chapt
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CHAPTER 1 Introduction to Radio Com
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C h a p t e r 1 : I n t r o d u c t
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Fundamentals Part II Chapter 2 Radi
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CHAPTER 2 Radio-Wave Propagation To
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C h a p t e r 2 : r a d i o - W a v
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R N-1 C h a p t e r 2 : r a d i o -
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Figure 2.29C Monthly averaged sunsp
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CHAPTER 3 Antenna Basics An antenna
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C h a p t e r 3 : A n t e n n a B a
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CHAPTER 4 Transmission Lines and Im
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C h a p t e r 4 : T r a n s m i s s
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Chapter 5 Antenna Arrays and Array
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C h a p t e r 5 : a n t e n n a A r
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A. B. C. D. E. F. G. H. Figure 5.9
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B. C. D. E. F. G. H. Figure 5.10 Gr
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High-Frequency Building-Block Anten
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CHAPTER 6 Dipoles and Doublets A co
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C h a p t e r 6 : D i p o l e s a n
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CHAPTER 7 Large Wire Loop Antennas
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C h a p t e r 7 : L a r g e W i r e
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CHAPTER 8 Multiband and Tunable Wir
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C h a p t e r 8 : M u l t i b a n d
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CHAPTER 9 Vertically Polarized Ante
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C h a p t e r 9 : V e r t i c a l l
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Quarter-wave vertical radiator Insu
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Directional High-Frequency Antenna
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CHAPTER 10 Wire Arrays When a singl
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C h a p t e r 1 0 : W i r e A r r a
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Wire 2 C h a p t e r 1 0 : W i r e
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CHAPTER 11 Vertical Arrays Despite
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C h a p t e r 1 1 : V e r t i c a l
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CHAPTER 12 The Yagi-Uda Beam Antenn
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C h a p t e r 1 2 : T h e Y a g i -
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CHAPTER 13 Cubical Quads and Delta
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C h a p t e r 1 3 : C u b i c a l Q
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Figure 13.5 Multiband quad “spide
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High-Frequency Antennas for Special
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CHAPTER 14 Receiving Antennas for H
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C h a p t e r 1 4 : r e c e i v i n
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351 Figure 14.12 Remote tuning sche
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CHAPTER 15 Hidden and Limited-Space
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C h a p t e r 1 5 : H i d d e n a n
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CHAPTER 16 Mobile and Marine Antenn
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C h a p t e r 1 6 : M o b i l e a n
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CHAPTER 17 Emergency and Portable A
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C h a p t e r 1 7 : E m e r g e n c
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Antennas for Other Frequencies Part
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CHAPTER 18 Antennas for 160 Meters
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C h a p t e r 1 8 : a n t e n n a s
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CHAPTER 19 VHF and UHF Antennas The
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C h a p t e r 1 9 : V H F a n d U H
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CHAPTER 20 Microwave Waveguides and
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C h a p t e r 2 0 : M i c r o w a v
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λ o = c / f 8 3× 10 m / s = 9 C h
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Page 531 and 532: CHAPTER 23 Radio Direction-Finding
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C h a p t e r 2 6 : T h e S m i t h
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D C B A Figure 26.5 Radially scaled
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0.165 Y ' 1.0 j1.1 G' 1.0 Y ' 0.2
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CHAPTER 27 Testing and Troubleshoot
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C h a p t e r 2 7 : T e s t i n g a
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Mechanical Construction and Install
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CHAPTER 28 Supports for Wires and V
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C h a p t e r 2 8 : S u p p o r t s
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CHAPTER 29 Towers At some point, th
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CHAPTER 30 Grounding for Safety and
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C h a p t e r 3 0 : G r o u n d i n
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CHAPTER 31 Zoning, Restrictive Cove
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C h a p t e r 3 1 : Z o n i n g , R
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C h a p t e r 3 1 : Z o n i n g , R
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Appendices
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APPENDIX A Useful Math The material
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A p p e n d i x A : U s e f u l M a
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Appendix B Suppliers and Other Reso
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A p p e n d i x B : S u p p l i e r
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B.1.6 Rotators and Controllers Alfa
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Index Note: Boldface numbers denote
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I n d e x 747 Binns, Jack, first ma
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I n d e x 749 dipole (Cont.): slopi
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I n d e x 751 grounding and ground
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I n d e x 753 ionospheric propagati
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I n d e x 755 maximum effective len
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I n d e x 757 noise (Cont.): sky no
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I n d e x 765 vertically polarized
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I n d e x 767 Yagi antennas (Cont.)
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