Practical_Antenna_Handbook_0071639586

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408 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 where b is the distance (usually half the total boom length) from the mast to where the elements farthest from the mast are attached, and e is half the average length of elements nearest the ends of the boom. Only elements directly grounded to the boom should be considered as fully contributing to the top hat effect. Note If it is to ever be used as a radiating monopole, any guyed tower must have at least one insulator in each and every guy wire attached to the tower, including guy wires attached at guy stations farther down the tower. With the exception of any initial lengths established in the top set of guy wires for top hat purposes, the distance from the tower attachment point to the uppermost insulator in each guy wire should not exceed 3 ft or so. Even though the tower itself may be grounded at its base, failure to insulate each and every guy wire near the point where it attaches to the tower completely changes the electrical characteristics of the structure, complicates calculations and assumptions, and may well destroy the overall efficacy of the tower as a radiator. Even if you believe you have no intention of ever using your towers as low-frequency antennas, interests and plans can change—insulate your guy wires when you erect your towers—it’s a whole lot easier to do it then than it is to do it later! As discussed in the chapter on verticals (Chap. 9), if a grounded vertical monopole is increased in length somewhat beyond l/4, its elevation pattern is “squashed”; that is, radiation (and reception) at low vertical angles is enhanced at the expense of the higher angles. For most intended uses of an efficient transmitting vertical on 160 or 80 m, this is desirable—hence, the suggestion that it’s okay to let the total equivalent electrical length exceed l/4. When this is the case, the feedpoint impedance at the base of the antenna will increase and the reactive component will appear as a series inductance, which is quite easily canceled with a transmitting air variable capacitor in series with the feedpoint. When the base of a vertical is insulated from ground so that it can be fed directly from the transmission line, the technique is called series feed, and it is most often employed in conjunction with wire verticals. But for mechanical simplicity and low cost, towers and rigid 160-m verticals in most amateur installations are grounded at the base. For these, shunt feed must be used. Here’s how it works. In general, the impedance at any given point on a grounded vertical monopole increases with height up to l/2. Numerous ways of connecting a feedline to such a structure have been employed over the years. Figure 18.2 shows one of the simplest: an unbalanced output ATU feeds a wire connected to the side of the tower at an appropriate point. Electrically, this is half of the delta match often employed to provide a balanced feed to the center of dipoles and Yagi driven elements. The “correct” height for the tap on the tower depends on the design and tuning range of the ATU; the objective is to find a height where both the resistive and the reactive parts of the impedance seen at the lower end of the wire by the ATU are within its tuning range. Similar to the sloping wire or half-delta feed of Fig. 18.2 is the shunt feed of Fig. 18.3. This is known as a gamma match; it is basically the unbalanced form of the T-match often used to feed HF and VHF Yagi driven elements. Physically, the only difference between this and the delta match is that the gamma wire or rod is held a constant distance from the side of the tower, allowing the ATU and gamma rod to be placed very close to the tower. The gamma match is adjusted to provide minimum SWR on the transmission line at the center of the operating frequency range by alternately changing the tap point on the vertical and the spacing of the gamma rod from the radiating element while retuning the series capacitor.
C h a p t e r 1 8 : a n t e n n a s f o r 1 6 0 M e t e r s 409 Single-conductor feedline (delta feed) ATU Coax to XMTR Figure 18.2 Delta-matching a grounded vertical tower antenna. Electrically, the shunt feed acts much like an autotransformer, converting the (higher) impedance of the tap on the tower to the (lower) impedance of the feedline. If dimensioned properly, the need for a separate ATU can be eliminated completely or the ATU can be replaced with a single air variable capacitor having adequate spacing between its plates for the transmitter power levels involved. The capacitor is placed in series between the bottom end of the shunt rod and the hot side of the coaxial cable or unbalanced transmission line feeding the antenna. <strong>Antenna</strong> modeling software (Chap. 25) is an excellent way to experiment with tower heights and top loading schemes. But its greatest value here is in determining the “proper” length and spacing for the gamma rod or wire. Properly used, antenna modeling software can eliminate literally dozens of trips up and down the tower! Here’s a true story involving a 90-ft guyed tower built with Rohn 45 triangular sections and topped off with a 10-ft heavy-duty mast (Fig. 18.4). At the 92-ft point is a four-
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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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Page 409 and 410: CHAPTER 17 Emergency and Portable A
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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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C h a p t e r 2 9 : T o w e r s 655
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C h a p t e r 2 9 : T o w e r s 667
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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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