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#1
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fat conductor question
hi
i've got a question about how wires to use when making a fat conductor hf dipole(cage antenna)using wire equally spaced around a circular form. ie target diameter is 8 inches, how many wires around the form, 4, 6, 8? tnx larry kd5foy |
#2
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While you will probably get any number of answers, I personally have always
used 6. The best supports I have found are slices of 12" white plastic sewer pipe. You normally can get a scrap piece at your local utility company. Make them 3/8" thick and put a hole every 60 degrees. If you want to take a look at the effect of increasing wires or diameter, Reg has a program for cage dipoles that lets you determine the best setup for what your trying to accomplish. Anecdotally, the cages are quieter (less electrical noise) than standard dipoles. Don't make this statement on here however, because 9 folks will jump out of the woodwork to tell you how wrong you are. Good Luck! "larry d clark" wrote in message . com... hi i've got a question about how wires to use when making a fat conductor hf dipole(cage antenna)using wire equally spaced around a circular form. ie target diameter is 8 inches, how many wires around the form, 4, 6, 8? tnx larry kd5foy |
#3
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Anecdotally, the cages are quieter (less electrical noise) than standard
dipoles. Don't make this statement on here however, because 9 folks will jump out of the woodwork to tell you how wrong you are. ================================= What few anecdotes I have read fail to say whether the noise is locally generated or is the more distant stuff which comes in with the wanted signals. There is no possible reason why a fat wire will discriminate better between between signals and distant noise than a thin wire. To any antenna everything is just e.m waves. Receiving and radiating patterns, radiation resistance and efficiency are identical for both. But local noise sources are within the near field of the antenna. Or the the antenna can be considered to be within the near fields of a lot of noise sources. One end of the antenna can be nearer to some noise source(s) than the other end. So although the dipole may be accurately balanced relative to ground (almost certainly it isn't) it may be very unbalanced relative to the location of local nise sources. The much greater capacitance of a fat antenna to ground, and to everything else, will shift the local receiving pattern. The fat antenna will also have a smaller capacitative reactance to ground and to everything else. Remember local noise is mainly due to its electric field and so the fat antenna, all other things remaining unchanged, will have a greater tendency to decouple itself from local electric noise fields. The effect is not un-related to screened receiving loops. It will be more noticeable with decreasing frequency and with decreasing antenna sizes. It will not be noticeable at all at low noise receiving sites where the near-fields of the antenna and noise sources do not inter-react. ---- Reg, G4FGQ |
#4
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On Fri, 19 Dec 2003 22:34:39 GMT, "larry d clark"
wrote: hi i've got a question about how wires to use when making a fat conductor hf dipole(cage antenna)using wire equally spaced around a circular form. ie target diameter is 8 inches, how many wires around the form, 4, 6, 8? tnx larry kd5foy Hi Larry, An example design for one configuration may be found at: http://home.comcast.net/~kb7qhc/ante.../Cage/cage.htm As to your question about diameter and count of skirt wires, review: http://www.qsl.net/kb7qhc/antenna/Discone/discone.htm which, although relates to a discone, it serves to illustrate how the "thickness" comes to bear on bandwidth. Much of my other analysis simply suggests the more wires, the closer approximation to the solid cylinder you are trying to build a skeleton of. The question of how many is good enough should be evident that 16 certainly offers a good approximation, fewer, as revealed in my other work brings a corresponding poorer approximation. I've built physical models with up to 6 skirt wires, and although useful, they did show resonances where the two links above reveal wide banded behavior. One design was a 40M Discone, another a wideband vertical dipole (hoping for 20-10M but probably closer to 15-10M). 73's Richard Clark, KB7QHC |
#5
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"larry d clark" wrote in message . com... hi i've got a question about how wires to use when making a fat conductor hf dipole(cage antenna)using wire equally spaced around a circular form. ie target diameter is 8 inches, how many wires around the form, 4, 6, 8? tnx larry kd5foy Larry, If all you are trying to do is to increase the bandwidth, there are better ways. For instance, have two dipoles, connected together at the feedpoint, but with the ends separated either vertically or horizontally. Cut the two dipoles for slightly different frequencies. I ran EZNEC on such an antenna for 75/80, and easily got an SWR of less than 2:1 from 3.5 -4.0. You could mount the support antenna as a dipole, and the other as an inverted V. Tam/WB2TT |
#6
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Why not use Romex, connect all three conductors together at the
feedpoint, and cut each conductor for whatever resonant frequency on each band.. I've done this, and with a little matching it worked out nicely. 73 de AI8W, Chris Tarmo Tammaru wrote: "larry d clark" wrote in message . com... hi i've got a question about how wires to use when making a fat conductor hf dipole(cage antenna)using wire equally spaced around a circular form. ie target diameter is 8 inches, how many wires around the form, 4, 6, 8? tnx larry kd5foy Larry, If all you are trying to do is to increase the bandwidth, there are better ways. For instance, have two dipoles, connected together at the feedpoint, but with the ends separated either vertically or horizontally. Cut the two dipoles for slightly different frequencies. I ran EZNEC on such an antenna for 75/80, and easily got an SWR of less than 2:1 from 3.5 -4.0. You could mount the support antenna as a dipole, and the other as an inverted V. Tam/WB2TT |
#7
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Many tnx for the excellent suggests!!! it is much appreciated larry kd5foy "larry d clark" wrote in message . com... hi i've got a question about how wires to use when making a fat conductor hf dipole(cage antenna)using wire equally spaced around a circular form. ie target diameter is 8 inches, how many wires around the form, 4, 6, 8? tnx larry kd5foy |
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