Practical_Antenna_Handbook_0071639586
262 P a r t I V : D i r e c t i o n a l H i g h - F r e q u e n c y A n t e n n a A r r a y s R ATU XMTR Figure 10.10 Terminated rhombic antenna. Bobtail Curtain The bobtail curtain of Fig. 10.11 is a fixed array consisting of three individual quarterwavelength vertical elements in a line; adjacent elements are spaced a half-wavelength apart and attached electrically to a horizontal wire across their tops that serves both as a phasing wire and as the other half of an L-shaped dipole. The array is fed at the bottom end of the center element. This is a high impedance point, so a parallel-tuned tank circuit or an ATU capable of matching very high impedances is required. The array must be kept high enough in the air that the bottoms of the l/4 vertical elements are some distance above ground—ideally high enough in the air that humans and animals cannot touch their high-voltage tips, although, clearly, the feed wire to the center element may require special protection. A bobtail curtain for 80 m should thus be suspended about 75 or 80 ft up; with catenary effects causing the phasing line to sag, end supports of at least 100 ft are appropriate. RF energy introduced at the bottom of the center element splits and goes equally to the two end elements when it reaches the phasing line at the top. Since each end element is l/2 from the center element, the currents of all three elements are in phase, although the current in the center element is twice that of either of the others. (This is an
C h a p t e r 1 0 : W i r e A r r a y s 263 Figure 10.11 Bobtail curtain. example of a 1:2:1 binomial array, as discussed in Chap. 5.) Since the currents feeding the two end elements are traveling in opposite directions along the top, and since the current to either end element goes through a reversal at the middle of the horizontal half-section, the horizontal radiation is partially canceled out. Because all three elements are in phase, the principal lobe is broadside to the plane of the three radiating elements. For maximum signal north and south, for instance, the array should be suspended between supports on an east-west line. The radiating elements are l/4 monopoles and normally would require a low-Â resistance ground path for their return currents. However, if each vertical element is visualized as one side of a l/2 dipole and half the horizontal distance along the phasing line to the adjacent element is seen as the other side of the dipole (or as an elevated groundplane), the vertical elements themselves have no particular need for a good ground system. However, unless there is an excellent RF ground at the base of the middle element for the matching network to connect to, there is no low-loss return path for RF currents flowing to/from the ground end of the matching network, and excessive transmitter power will be wasted in some vague but very lossy return path. Thus, the matching network at the base of the middle element may require a good system of radials. Since radials lying on the surface of the earth are not resonant, their exact length is not particularly critical. It might be helpful, depending on the quality of your ground, to put radial fields under the end elements, as well. The bobtail curtain has an extremely low angle of radiation. At one time the author had an 80-m bobtail curtain aimed north and south at his home in the northeastern United States; it was a “killer” antenna for very long haul contacts over the north pole into the far reaches of Asiatic Russia and the western Pacific region—especially ÂIndonesia—but it was totally useless for anything much closer. Half-Square If one end element of the bobtail curtain of Fig. 10.11 and its associated phasing line are removed, the resulting antenna is known as a half-square. Now we have two vertical radiators joined by a l/2 horizontal phasing line. As with the original bobtail curtain, the antenna is fed at the bottom of one element—a high impedance point. (Keep in mind that both the fed end and the far end are high-voltage points.) We can consider that element and half the horizontal wire as constituting an L-shaped l/2 dipole feeding a second l/2 dipole. But since the current in consecutive l/2 sections reverses, the
- Page 229 and 230: C h a p t e r 7 : L a r g e W i r e
- Page 231 and 232: C h a p t e r 7 : L a r g e W i r e
- Page 233 and 234: C h a p t e r 7 : L a r g e W i r e
- Page 235 and 236: CHAPTER 8 Multiband and Tunable Wir
- Page 237 and 238: C h a p t e r 8 : M u l t i b a n d
- Page 239 and 240: C h a p t e r 8 : M u l t i b a n d
- Page 241 and 242: C h a p t e r 8 : M u l t i b a n d
- Page 243 and 244: C h a p t e r 8 : M u l t i b a n d
- Page 245 and 246: C h a p t e r 8 : M u l t i b a n d
- Page 247 and 248: CHAPTER 9 Vertically Polarized Ante
- Page 249 and 250: C h a p t e r 9 : V e r t i c a l l
- Page 251 and 252: C h a p t e r 9 : V e r t i c a l l
- Page 253 and 254: Quarter-wave vertical radiator Insu
- Page 255 and 256: C h a p t e r 9 : V e r t i c a l l
- Page 257 and 258: C h a p t e r 9 : V e r t i c a l l
- Page 259 and 260: C h a p t e r 9 : V e r t i c a l l
- Page 261 and 262: C h a p t e r 9 : V e r t i c a l l
- Page 263 and 264: C h a p t e r 9 : V e r t i c a l l
- Page 265 and 266: C h a p t e r 9 : V e r t i c a l l
- Page 267 and 268: C h a p t e r 9 : V e r t i c a l l
- Page 269 and 270: Directional High-Frequency Antenna
- Page 271 and 272: CHAPTER 10 Wire Arrays When a singl
- Page 273 and 274: C h a p t e r 1 0 : W i r e A r r a
- Page 275 and 276: C h a p t e r 1 0 : W i r e A r r a
- Page 277 and 278: Wire 2 C h a p t e r 1 0 : W i r e
- Page 279: C h a p t e r 1 0 : W i r e A r r a
- Page 283 and 284: CHAPTER 11 Vertical Arrays Despite
- Page 285 and 286: C h a p t e r 1 1 : V e r t i c a l
- Page 287 and 288: C h a p t e r 1 1 : V e r t i c a l
- Page 289 and 290: C h a p t e r 1 1 : V e r t i c a l
- Page 291 and 292: C h a p t e r 1 1 : V e r t i c a l
- Page 293 and 294: C h a p t e r 1 1 : V e r t i c a l
- Page 295 and 296: CHAPTER 12 The Yagi-Uda Beam Antenn
- Page 297 and 298: C h a p t e r 1 2 : T h e Y a g i -
- Page 299 and 300: C h a p t e r 1 2 : T h e Y a g i -
- Page 301 and 302: C h a p t e r 1 2 : T h e Y a g i -
- Page 303 and 304: C h a p t e r 1 2 : T h e Y a g i -
- Page 305 and 306: C h a p t e r 1 2 : T h e Y a g i -
- Page 307 and 308: C h a p t e r 1 2 : T h e Y a g i -
- Page 309 and 310: C h a p t e r 1 2 : T h e Y a g i -
- Page 311 and 312: C h a p t e r 1 2 : T h e Y a g i -
- Page 313 and 314: C h a p t e r 1 2 : T h e Y a g i -
- Page 315 and 316: C h a p t e r 1 2 : T h e Y a g i -
- Page 317 and 318: C h a p t e r 1 2 : T h e Y a g i -
- Page 319 and 320: C h a p t e r 1 2 : T h e Y a g i -
- Page 321 and 322: C h a p t e r 1 2 : T h e Y a g i -
- Page 323 and 324: C h a p t e r 1 2 : T h e Y a g i -
- Page 325 and 326: C h a p t e r 1 2 : T h e Y a g i -
- Page 327 and 328: C h a p t e r 1 2 : T h e Y a g i -
- Page 329 and 330: C h a p t e r 1 2 : T h e Y a g i -
262 P a r t I V : D i r e c t i o n a l H i g h - F r e q u e n c y A n t e n n a A r r a y s<br />
<br />
<br />
<br />
<br />
R<br />
<br />
<br />
ATU<br />
XMTR<br />
Figure 10.10 Terminated rhombic antenna.<br />
Bobtail Curtain<br />
The bobtail curtain of Fig. 10.11 is a fixed array consisting of three individual quarterwavelength<br />
vertical elements in a line; adjacent elements are spaced a half-wavelength<br />
apart and attached electrically to a horizontal wire across their tops that serves both as<br />
a phasing wire and as the other half of an L-shaped dipole. The array is fed at the bottom<br />
end of the center element. This is a high impedance point, so a parallel-tuned tank<br />
circuit or an ATU capable of matching very high impedances is required.<br />
The array must be kept high enough in the air that the bottoms of the l/4 vertical<br />
elements are some distance above ground—ideally high enough in the air that humans<br />
and animals cannot touch their high-voltage tips, although, clearly, the feed wire to the<br />
center element may require special protection. A bobtail curtain for 80 m should thus be<br />
suspended about 75 or 80 ft up; with catenary effects causing the phasing line to sag,<br />
end supports of at least 100 ft are appropriate.<br />
RF energy introduced at the bottom of the center element splits and goes equally to<br />
the two end elements when it reaches the phasing line at the top. Since each end element<br />
is l/2 from the center element, the currents of all three elements are in phase, although<br />
the current in the center element is twice that of either of the others. (This is an