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Does reactance of dipole depend on diameter ??
On Sun, 27 Jul 2008 19:46:40 +0100, Dave wrote:
I wish to know if the reactance of a dipole that is physically 0.5000 wavelengths in length depends on the diameter of the wire or not. Hi Dave, Yes, it does. You are working with source material with conflicting agendas. One is simply interested in what is called a dipole for the sake of field studies and the characteristics of that dipole are a good first order approximation. This means thin-wire by and large. The other source is examining the antenna itself (or so it seems by both accounts). The fatter the wire, the lower the inductance. Naturally the reactance must follow. The fatter the wire, the more wavelength it encompasses for a given length, hence the length can be shorter for resonance. This shorthand hardly matters for conventional wire antennas as "fat" is in the extreme, and wire is hardly the proper nomenclature when we get into these gross dimensions. Approximations of "fat" come with cage structures that attempt to mimic a solid of revolution. If you want to find the author who developed the first principles of thin vs. fat, that is Dr. Sergei Alexander Schelkunoff (with Friis). In what has been decried in this forum as the failed metaphor of an antenna as transmission line, the antenna formulas from Schelkunoff were derived from (beat) a transmission line, albeit a special one. To attempt to draw parallels between transmission lines and antennas is fraught with failures, true. Specifically, the traditional dipole in its thin-wire implementation has no linear Impedance relationship along its length. The wire separation is always growing with distance from the feed point and thus the Z varies with distance. This failure was anticipated by Schelkunoff, and folded into field theory through using conic sections for the dipole arms. Hence the biconical dipole, the conical monopole, and the discone. The transmission line analogy survives through this legacy. All formulas that you have probably recited are the degenerative forms for his based on the conic sections. Now as to that degeneration of the conic section into "thick" wire to "thin" wire. The conic section is certainly thick at the distal end, no doubt there. It is also thin at the feed point. The advantage is lowered capacitance bridging the feedpoint compared to that if the thickness were constant from the distal end - for a given thickness/length/resonance. Also the conic sections most nearly approach the shape of the emerging wave's initial spherical front. Well, the long and short of it is to seek: "Antennas: Theory and Practice," Sergei A. Schelkunoff and Harald T. Friis, Bell Telephone Laboratories, New York : John Wiley & Sons, 1952. 73's Richard Clark, KB7QHC |
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