Practical_Antenna_Handbook_0071639586

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462 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 Z Impedance N Intrinsic impedance of dielectric F c Figure 20.10 Impedance versus frequency. Frequency F The propagation constant b for rectangular waveguide is a function of both cutoff frequency and operating frequency: ⎛ β = ω µε 1 – 0111057 FIG 18-10 ⎝ ⎜ f ⎞ c f ⎠ ⎟ 2 (20.16) from which we can express the TE-mode impedance: Z TE = 1 µε ⎛ f ⎞ ⎜ ⎟ ⎝ f ⎠ – c 2 (20.17)
C h a p t e r 2 0 : M i c r o w a v e W a v e g u i d e s a n d A n t e n n a s 463 and the TM-mode impedance: Z TM f 377 1 ⎛ ⎝ ⎜ ⎞ c = – ⎟ f ⎠ 2 (20.18) Waveguide Terminations When an electromagnetic wave propagates along a waveguide, it must eventually reach the end of the guide. If the end is open, then the wave will propagate into free space. The horn radiator is an example of an unterminated waveguide. If the waveguide terminates in a metallic wall, then the wave reflects back down the waveguide, from whence it came. The interference between incident and reflected waves forms standing waves (see Chap. 4). Such waves are stationary in space but vary in the time domain. In order to prevent standing waves or, more properly, the reflections that give rise to standing waves, the waveguide must be terminated in a matching impedance. When a properly designed antenna is used to terminate the waveguide, it forms the matched load required to prevent reflections. Otherwise, a dummy load must be provided. Figure 20.11 shows several types of dummy load. The classic termination is shown in Fig. 20.11A. The “resistor” making up the dummy load is a mixture of sand and graphite. When the fields of the propagated wave enter the load, they cause currents to flow, which in turn cause heating. Thus, the RF power dissipates in the sand-graphite mixture rather than being reflected back down the waveguide. A second type of dummy load is shown in Fig. 20.11B. The resistor element is a carbonized rod critically placed at the center of the electric field. The E-field causes currents to flow, resulting in I 2 R losses that dissipate the power. Bulk loads, similar to the graphite-sand chamber, are shown in Fig. 20.11C, D, and E. Using bulk material such as graphite or a carbonized synthetic material, these loads are used in much the same way as the sand load (i.e., currents set up, and I 2 R losses dissipate the power). The resistive vane load is shown in Fig. 20.11F. The plane of the element is orthogonal to the magnetic lines of force. When the magnetic lines cut across the vane, currents are induced, which gives rise to the I 2 R losses. Very little RF energy reaches the metallic end of the waveguide, so there is little reflected energy and a low VSWR. There are situations where it isn’t desirable to terminate the waveguide in a dummy load. Several reflective terminations are shown in Fig. 20.12. Perhaps the simplest form is the permanent end plate shown in Fig. 20.12A. The metal cover must be welded or otherwise affixed through a very low resistance joint. At the substantial power levels typically handled in transmitter waveguides, even small resistances can be important. The end plate (shown in Fig. 20.12B) uses a quarter-wavelength cup to reduce the effect of joint resistances. The cup places the contact joint at a point that is a quarterwavelength from the end. This point is a minimum-current node, so I 2 R losses in the contact resistance become less important. The adjustable short circuit is shown in Fig. 20.12C. The walls of the waveguide and the surface of the plunger form a half-wavelength channel. Because the metallic end of the channel is a short circuit, the impedance reflected back to the front of the plunger is
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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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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 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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