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Roy Lewallen wrote:
A single conductor doesn't have a characteristic impedance -- it's the impedance between the two conductors of a transmission line. You can measure a characteristic impedance between, say, a coil and ground, but its value depends on the spacing between the two. If the coil is tilted with respect to the ground, the impedance of this two-conductor system will change with the position along the coil. Roy: I understand what you are saying. But the derivation of Characteristic Impedance in the Corum Bros. paper depends only on the coil dimensions and number of turns; it is independent of any relationship to other conductors or groundplanes. I also note that ON4AA's inductance calculator predicts the "Characteristic impedance of n=0 sheath helix waveguide mode at design frequency" based purely on the coil geometry. The maths is a bit beyond me (trying to solve Maxwell's equations for a solenoidal helix), but seems to bear analogy to the derivation of the characteristic impedance of a waveguide. I'm inclined to try to understand it better, because it's this derived Characteristic Impedance, along with the axial Velocity Factor, that generates the reactance values which seem such a good match to experimental and modelled results. Regards, Steve G3TXQ |
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