Practical_Antenna_Handbook_0071639586

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528 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 Switched-Pattern RDF <strong>Antenna</strong>s If an advancing wavefront arrives at two identical antennas from a direction perpendicular to the line connecting the antennas, the phase of the wavefront will be the same at both antennas. If signals from the two antennas are “chopped” at an audio rate with a square wave and alternately fed to a simple receiver, the resulting phase modulation will appear as an audio tone at the receiver output. Rotation of the antenna array until the tone is nulled out, which occurs when there is no phase difference between the signals at the two antennas, provides an accurate bidirectional “fix” on the source of the wavefront. Double-Ducky Direction Finder (DDDF) Figure 23.16 shows in simplified form a system originally presented to amateur radio operators by David T. Geiser, W5IXM, for use with “rubber ducky” VHF antennas; he called it the double-ducky direction finder (DDDF). Because of the sharpness of the null, Geiser chose to use these short, loaded, and rubber-damped antennas rather than full l/4 or 5/8l whips because vibration and wind can cause enough relative motion between elements of an array built with full-length whips to make it difficult to get a clean null in normal outdoor use. Spacing of the two antennas is not critical; 0.25l to 1l apart over a good ground plane (such as the roof of a car or truck) should work well. As with all vertical monopoles, if no ground plane exists, a sheet metal ground plane should be provided. In Fig. 23.16 the antennas are fed from a common transmission line to the receiver. In order to keep them electrically the same distance apart, identical half-wavelength sections of transmission line are used to connect the antennas to the shared section of line. Switching is accomplished by using a bipolar square wave and PIN diodes. The bipolar square wave (see inset to Fig. 23.16) has equal positive and negative peak voltages on opposite halves of the cycle. The PIN diodes (D 1 and D 2 ) are connected in opposition such that diode D 1 conducts on negative excursions of the square wave and D 2 conducts on positive excursions. In any given half-cycle of the audio frequency square wave, one antenna is connected to the receiver when its diode is conducting and the other diode is back-biased, creating a high impedance between its antenna and the shared line. Thus, the active antenna alternates rapidly between ANT1 and ANT2. The combined signal from the two antennas is coupled to the receiver through a small-value capacitor (C 1 ) that blocks the baseband audio frequency square wave from appearing at the receiver input. This allows use of the shared transmission line segment for both the RF signal and the switching signal. An RF choke (RFC 3 ) is used to keep RF from the antenna from entering the square-wave generator. The DDDF antenna phase-modulates the incoming signal at the frequency of the square wave, and the resulting tone can be heard in the receiver output. When the signal’s direction of arrival is perpendicular to the line between the two antennas, the phase difference is zero and the audio tone disappears. The pattern of the DDDF antenna is bidirectional, so the ambiguity problem found with loop antennas exists here, as well. In this system the ambiguity can be resolved by either of two methods: • Place a reflector l/4 behind each antenna. This is an attractive solution, but it tends to distort the antenna pattern a little bit. • Rotate the antenna through 90 degrees (or walk an L-shaped path).
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 529 Figure 23.16 Double-ducky switched-pattern RDF system. Summary This chapter has touched on the basics of radio direction finding. Much more depth can be found on the Internet and in newsletters from clubs and others specializing in RDF technology and techniques. RDF can be very useful for locating RF sources, both good and bad; as such, it is an important tool for locating illegal stations or undesired noise sources. RDF is a great way to build club spirit and teach a wide variety of technical concepts through fox-hunting training exercises and group construction projects. It can also be used to determine your own location if you can get bearings on at least two RF sources at known positions. Try it . . . you’ll like it!
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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 529 and 530: Figure 22.9 Interferometer pattern.
Page 531 and 532: CHAPTER 23 Radio Direction-Finding
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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 767 Yagi antennas (Cont.)
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