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</o:shapelayout></xml><![endif]--></head><body lang=PT link=blue vlink=purple><div class=WordSection1><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>BUILD THE "COBRA" ANTENNA<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>By Raymond A. Cook W4JOH<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'><o:p> </o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>Taken from and re-edited from a project in <i>73 Amateur Radio Today</i>, June, 1997<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'><o:p> </o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>The original Cobra antenna designed by W4JOH is built using standard insulated wire of<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>about 14 gauge and fed with 450 ohm ladder line down to the shack into a tuner.<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>It got it's name from the S shaped configuration of it's multi-conductor elements.<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>It performs on it's primary and harmonic operating frequencies as a standard ladder-line fed<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>doublet.<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black;mso-fareast-language:PT'><img width=829 height=422 id="Imagem_x0020_1" src="cid:image003.png@01CC6CBD.D7574020"></span><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'><o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>The close spaced wire elements on each leg introduces two added resonant responses<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>BELOW the antenna's fundamental operating frequency.<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'><o:p> </o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>The 140 foot(</span><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:red'>42,67m</span><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>) version (80 meters) in picture also resonates at about 2.8 MHz and also on 160<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>meters. A standard dipole at 1.9 MHz is about 246 feet total compared to 140 feet in the<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>Cobra! This fact alone makes this an ideal antenna for restricted space on the TOP BAND!<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>The half sized version, 73 foot (40 meter) also covers 60 and 75 meters! All band operation<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>has been reported in the article to be excellent! (With a tuner of course) This antenna design<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>extends the coverage compared to a G5RV both in bands and performance.<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'><o:p> </o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>On its primary and harmonic operating frequencies, tests show no discernable difference<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>in signal strength between a Cobra and a regular full-sized doublet or dipole. On its subbands<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>bands where the Cobra is physically "short", efficiency is somewhat lower than for a<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>full-sized dipole.<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'><o:p> </o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>If you do the math, you will see that there is actually about 420 feet total wire across the top of<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>the antenna on the 80 meter version, (210 feet per side), and about half that on the 40 meter<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>version. The flattop and lead-in length were strictly determined by the physical limitations of<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>the antenna farm and this project is a result of those limitations and the idea of compressing<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>or folding the wire back on itself to fit the antenna farm. (No formulas were given in the article)<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>Keep in mind that there are actually 3 conductors connected in series per side and folded<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>back on each other..... or another way of saying this is that there is one continuous length of<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>210 feet per side in the 80 meter version connected to one side of the ladder line and the<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>same on the other half.<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt;font-family:"Tahoma","sans-serif";mso-fareast-language:PT'><img width=554 height=199 id="Imagem_x0020_5" src="cid:image010.png@01CC6CBD.D7574020"></span><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'><o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'><o:p> </o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>Because the Cobra antenna is a balanced load, it is recommended to install a 4:1 current style<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>balun at the station end of the feed line (many external tuners provide a built-in balun).<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>Ladder-line feed may have to be trimmed for lowest SWR, but using about a 100’(</span><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:red'>30,48m</span><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>) length<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>seems to make for easier tuning on all bands. Extra feed line should be suspended in loose<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>coils and not in a tight roll.<o:p></o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'><o:p> </o:p></span></p><p class=MsoNormal style='text-autospace:none'><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>Raymond is quoted from the article.....</span><b><i><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:mediumblue'>"Some of our more skeptical, and perhaps<o:p></o:p></span></i></b></p><p class=MsoNormal style='text-autospace:none'><b><i><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:mediumblue'>knowledgeable, friends have expressed concern about impedance, power rating, wavecancellation,<o:p></o:p></span></i></b></p><p class=MsoNormal style='text-autospace:none'><b><i><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:mediumblue'>etc. All that we can offer as an answer is the slogan used for many years<o:p></o:p></span></i></b></p><p class=MsoNormal style='text-autospace:none'><b><i><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:mediumblue'>by the Packard Motorcar Company: "</span></i></b><b><i><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:#CD0000'>Ask the man who owns one.</span></i></b><b><i><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:mediumblue'>"" </span></i></b><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:black'>.........</span><b><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:#009B00'>W4JOH<o:p></o:p></span></b></p><p class=MsoNormal><b><span style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:blue'><a href="http://www.hamuniverse.com/cobraantenna.html"><span lang=EN-US>http://www.hamuniverse.com/cobraantenna.html</span></a></span></b><b><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:blue'><o:p></o:p></span></b></p><p class=MsoNormal><b><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:blue'><o:p> </o:p></span></b></p><p class=MsoNormal><b><span style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:blue;mso-fareast-language:PT'><img border=0 width=659 height=101 id="Imagem_x0020_6" src="cid:image011.png@01CC6CBD.D7574020"></span></b><b><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif";color:blue'><o:p></o:p></span></b></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Tahoma","sans-serif";mso-fareast-language:PT'><img border=0 width=244 height=150 id="Imagem_x0020_2" src="cid:image012.png@01CC6CBD.D7574020"></span><span style='font-size:12.0pt;font-family:"Tahoma","sans-serif"'> </span><span style='font-size:12.0pt;font-family:"Tahoma","sans-serif";mso-fareast-language:PT'><img border=0 width=223 height=150 id="Imagem_x0020_3" src="cid:image013.png@01CC6CBD.D7574020"><img border=0 width=301 height=147 id="Imagem_x0020_4" src="cid:image014.png@01CC6CBD.D7574020"></span><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif"'><o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif"'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:12.0pt;font-family:"Tahoma","sans-serif"'><o:p> </o:p></span></p></div></body></html>