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## [TowerTalk] On the phase shift in current between ends of a coil

 To: "Towertalk" [TowerTalk] On the phase shift in current between ends of a coil "Jim Lux" Sat, 22 Nov 2003 08:07:11 -0800
 ```For closewound coils, with length to diameter ratios around 5:1, a series of fairly careful measurements have been made with the coils arranged vertically above a ground plane, fed at the base, with a capacitive load on the other end, and the driving frequency arranged to be at the resonant frequency of the whole assembly. The measurements were made with carefully designed fiberoptic probes that were specifically designed to avoid perturbing the magnetic and electric fields. These measurements were made by tesla coil researchers trying to establish whether the secondary coil acts as an open circuited quarter wavelength transmission line (where there would be a 90 degree phase shift between top and bottom) or whether it was best modeled as a lumped circuit (big L C circuit) In most cases, the phase shift in the current at top and bottom was on the order of 10-20 degrees. Subsequently, rigorous calculation and numerical modeling were done to understand the exact voltage and current distribution within the system; i.e. to explicitly calculate the capacitance and inductance of each segment of the coil, and, as well, to calculate the mutual coupling of each segment of the coil to the others. For inductance the signficant thing is that the magnetic field of one segment pretty much links to the adjacent segments, and less so for the rest. For capacitance, the significant thing is the capacitance to "ground" or "free space" (self C), and less so the capacitance to adjacent segments. At this time, the models are sufficiently well developed that they predict the actual currents and voltages to substantially better than one percent (if not several more significant figures... I've not been following the developments that closely). The models have also been developed to model the current flow on the surface of and within the inductors, using rigorously (in the mathematical sense) validated closed form analytical expressions. Over the past 5 or 6 years, there has been an extensive amount of quite painstaking experimental verification of these models with a lot of attention to experimental methods and quantifying uncertainty. This is a huge amount of work. The take home message here, regarding loading coils, is that simple lumped approximations of a loading coil may do just fine for an initial design cut, but do not adequately reflect reality. The other message is that measurement uncertainties can be quite large in an experimental setup. I'd venture to guess that there has been NO ham measurements or analysis of the loading coil problem that even approaches what has been done in the case of the tesla coil secondary world, especially in terms of experimental and analytical rigor. I'd also give my opinion that it probably wouldn't be worth it to do such analysis or experiments, except perhaps to settle a "bar bet". Most all tesla coils are essentially identical in design and form, so the analysis that has been done is generally applicable to tesla coils, but that's a very narrow applicability. Amateur antennas vary so much in installation and design that a rigorous treatment of one case would not, in general, be applicable to others. I think it's best to leave it at: Loading coils are not isolated lumped elements and cannot be modeled as such. A lumped model serves as a decent design tool to determine a starting point, which will have to be further iterated empirically. To give credit where it's due, the research work I've described above was primarily done by Terry Fritz (in the US), Paul Nicholson (in the UK), and Antonio Carlos M. de Queiroz (in Brazil). I can come up with some links to papers if needed. Jim W6RMK _______________________________________________ See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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