Practical_Antenna_Handbook_0071639586

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C h a p t e r 6 : D i p o l e s a n d D o u b l e t s 193 Figure 6.9 Continued. S R I Space permitting, some operators like to arrange four sloping dipoles from the same mast such that they point in different directions around the compass (Fig. 6.10). A single four-position coaxial switch midway up the center support facilitates switching a directional beam around the compass to favor four different compass headings. If the coaxial cables to the four-position switch are of identical length that is chosen based on whether the remote switch short-circuits the unused cables or leaves them open, additional frontto-back rejection (in the order of 10 dB) from the three idle slopers is possible. I (Top view) I I S R I I R S Mast I I R S Coaxial switch halfway up mast To equipment I = Insulator S = Support Stake R = Rope Broadbanded Dipoles One of the rarely discussed aspects of antenna construction is that the length/diameter ratio of the conductor used for the antenna element is a factor in determining the bandwidth of the antenna. In general, a larger cross-sectional area makes the antenna more broadbanded. When mechanical considerations permit, this suggests the use of aluminum tubing instead of copper wire for the antenna radiator. In fact, on 14 MHz and above, that is almost always the Figure 6.10 Directional antenna made of four slopers (top view).
194 p a r t I I I : h i g h - F r e q u e n c y B u i l d i n g - B l o c k A n t e n n a s preferred solution. At those frequencies, dipoles constructed from aluminum tubing typically require support only at their centers, so a dipole made of aluminum tubing may be relatively inexpensive when compared to the cost of wire plus supporting materials such as rope and pulleys. As frequency decreases, the weight becomes greater because the tubing is both longer and (for structural strength) of greater diameter. On 40 m and below, dipoles of aluminum tubing require special supporting mechanisms and are generally not cost effective. Yet coverage of the entire 80-m band (in the United States, at least) is a significant problem, especially for newer transmitting equipment, because the band is 500 kHz wide, which corresponds to a bandwidth of ±7 percent if the antenna is tuned for 3.75 MHz. Most solid-state exciters and transceivers today include fold-back circuitry that shuts down the amplifier stages or greatly reduces their output power levels in the presence of high VSWR. High-power amplifiers, whether employing vacuum tubes or solid-state devices, include VSWR-driven shutdown protection circuits that often require the operator to resolve the problem and manually reset the amplifier controls before they can be used again. Of course, an ATU can be inserted between the output of the transmitter and the feedline. But such units either need to be manually tuned when changing frequency or, if they are self-tuning, are very expensive. What other alternatives are there? Here are four approaches to widening the inherent bandwidth of a center-fed dipole antenna: folded dipole, bowtie dipole, cage dipole, and fan dipole. Folded Dipole Figure 6.11A shows the folded dipole antenna. This antenna consists of two or more halfwavelength conductors of identical diameter shorted together at both ends and fed in the middle of one conductor. Except at the ends, the two conductors are held a few inches apart (at HF, at least) by insulated spacers. The two-wire folded dipole is often made with 300-Ω television antenna twin-lead transmission line. Because the free-space feedpoint impedance is nearly 300 Ω, or four times that of a conventional dipole, the antenna is a reasonably good match (depending on the height of the antenna above ground) for the same type of twin-lead used as the transmission line. Such a dipole will exhibit a 50 percent improvement in 2:1 VWSR bandwidth compared to a single-wire dipole. On 80 m, for instance, a folded dipole resonant at 3.750 MHz will cover from 3.6 to 3.9 MHz with an SWR of 2:1 or lower. Why is the input impedance four times the input impedance of a conventional dipole when one of the two wires is fed? Recognize that the folded dipole is a continuous 1-l loop. At each end of the fed wire, the current does a U-turn and continues into the unfed wire. If this current were of the same phase everywhere, it would thus be flowing in a spatial direction opposite to the current near the feedpoint. But it’s also true that at or near the antenna design frequency the current in the fed half-wave element goes to zero at each end. Since the current in a wire that is longer than l/2 reverses direction in adjacent l/2 sections (i.e., at each current node), the current reverses at both ends of the folded dipole. When viewed from inside the wire (if that were possible), the current flowing in the unfed wire is thus 180 degrees out of phase electrically with the drive current, but its nominal spatial direction is the opposite of the current in the driven wire. The two effects (wire path and phase reversal) combine to put the unfed wire’s current in phase with the drive current, as viewed from outside the wires. The current in the unfed wire is virtually identical in amplitude to that in the fed wire, suffering only a very slight reduction in amplitude from ohmic and radiation losses.
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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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Page 247 and 248: CHAPTER 9 Vertically Polarized Ante
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Page 253 and 254: Quarter-wave vertical radiator Insu
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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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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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CHAPTER 21 Antenna Noise Temperatur
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C h a p t e r 2 1 : A n t e n n a N
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C h a p t e r 2 1 : A n t e n n a N
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CHAPTER 22 Radio Astronomy Antennas
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C h a p t e r 2 2 : R a d i o A s t
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Figure 22.9 Interferometer pattern.
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CHAPTER 23 Radio Direction-Finding
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C h a p t e r 2 3 : R a d i o D i r
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Figure 23.9 Adcock array RDF antenn
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Figure 23.11 Watson-Watt Adcock arr
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Tuning, Troubleshooting, and Design
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CHAPTER 24 Antenna Tuners (ATUs) Th
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C h a p t e r 2 4 : a n t e n n a T
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CHAPTER 25 Antenna Modeling Softwar
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C h a p t e r 2 5 : A n t e n n a M
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Figure 25.3B Bent dipole wire and s
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CHAPTER 26 The Smith Chart The math
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Figure 26.2 Normalized impedance li
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A Figure 26.3 Constant resistance c
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A Figure 26.4A Constant inductive r
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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 759 reactance: cancellati
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I n d e x 761 standing waves: on an
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I n d e x 763 transmission lines, 1
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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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