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Asimov wrote:
Since a portion of the EM field in open wire line is free to travel outside the conductor into the environment then we may safely assume there is an exchange between the environment and the conductor. If the conductors are perfectly conducting, no part of the field at all exists within the conductor. With good conductors like copper and at HF and above, there's very little penetration of the conductor by the fields, either electric or magnetic. As far as an "exchange" goes, it sounds like you're trying to describe radiation. If not, what's the phenomenon you're referring to? If the impedance of each is approximately the same then there is less loss in the interface between the two. No, that's not true. First of all, a mismatch doesn't cause loss. Secondly, as I explained in my last posting, the characteristic impedance of a transmission line isn't the same thing as the characteristic impedance of free space. If you were to construct a transmission line with 377 ohms characteristic impedance (numerically the same as the characteristic impedance of free space), the ratio of E/H fields between the conductors probably won't be anywhere near 377 ohms, as it is in a plane wave propagating without wires. It has to do with the reflective coefficient where the energy is returned. Well, no. There isn't a bundle of energy trying to escape the line and bouncing off the air, or bouncing off the air as it travels along the line, or bouncing off the conductors into the air. So reflection coefficient isn't applicable here. You will note 300 ohm open line has less loss than 100 ohm open line. Yes, and 600 ohm line has less loss than 377 ohm line. You'll have to find a way to fit this into your theory if you want to pursue it. RL The loss in coax is a trade off to achieve stability. RL Coax is more stable than open wire line? Does open wire line drift in RL some way? It is susceptible to ambient humidity and proximity to conductive objects (birds, snow, rfi). That is a source of drift in practical terms. Thanks for the clarification. Because the differential fields are completely confined within a coaxial cable, they are indeed more immune to external influences. I'm afraid that the conclusions you've reached about loss and characteristic impedance are based on a poor understanding of fundamental transmission line operation. The result is some conclusions that are, and are well known to be, untrue. If you really feel that you have a viable theory, you should be able to provide some equations and formulas to quantify the extra loss you're talking about. The existing theory, formulas and equations, in daily use for over a hundred years, have been shown countless times to accurately predict transmission line loss, and they don't include the phenomena you're describing. So although I think it's highly doubtful that your formulations will prove more accurate, if you post them they can pretty easily be tested by actual cable measurement. Roy Lewallen, W7EL |
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