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#1
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Cecil Moore wrote: Reg Edwards wrote: So in an antenna system, in general, we have 3 consectutive transmission lines sections with the loading coil forming the center section. Because of the high inductance of the loading coil, Zo = Sqrt(L/C) will have a much higher value than that of a wire of the same straight length. (Rhetorical Question) Given that the coil section resembles a high-Zo transmission line with reflections, how can the current into the coil section be equal in magnitude and phase to the current out of the coil section? -- 73, Cecil http://www.qsl.net/w5dxp Maybe it's a half wavelength long? ;-) 73, Jim AC6XG |
#2
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Jim Kelley wrote:
Cecil Moore wrote: (Rhetorical Question) Given that the coil section resembles a high-Zo transmission line with reflections, how can the current into the coil section be equal in magnitude and phase to the current out of the coil section? Maybe it's a half wavelength long? ;-) Strangely enough, that won't do it, Jim. A coil equivalent to 1/2WL reverses the phase of the current such that current is flowing into both ends at the same time. According to some gurus, that violates Kirchhoff's laws. But Kraus shows how phase-reversing coils are used in collinear arrays. Reckon Kraus knows he is violating Kirchhoff's laws? For a real-world air-core coil to exhibit identical currents at the input and output, it would have to be one wavelength long, lossless, and non-radiating. That's a pretty tall order just to support an old wives' tale. :-) -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#3
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Cecil Moore wrote: Jim Kelley wrote: Maybe it's a half wavelength long? ;-) For a real-world air-core coil to exhibit identical currents at the input and output, it would have to be one wavelength long, lossless, and non-radiating. That's a pretty tall order just to support an old wives' tale. :-) I happily stand corrected. 73 de jk |
#4
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Jim Kelly wrote,
Cecil Moore wrote: Jim Kelley wrote: Maybe it's a half wavelength long? ;-) For a real-world air-core coil to exhibit identical currents at the input and output, it would have to be one wavelength long, lossless, and non-radiating. That's a pretty tall order just to support an old wives' tale. :-) I happily stand corrected. 73 de jk Pathetic. 73, Tom Donaly KA6RUH |
#5
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Tdonaly wrote: Jim Kelly wrote, Cecil Moore wrote: Jim Kelley wrote: Maybe it's a half wavelength long? ;-) For a real-world air-core coil to exhibit identical currents at the input and output, it would have to be one wavelength long, lossless, and non-radiating. That's a pretty tall order just to support an old wives' tale. :-) I happily stand corrected. 73 de jk Pathetic. 73, Pathetic, best regards? Please elaborate. 73, Jim AC6XG |
#6
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Cecil wrote,
Strangely enough, that won't do it, Jim. A coil equivalent to 1/2WL reverses the phase of the current such that current is flowing into both ends at the same time. According to some gurus, that violates Kirchhoff's laws. But Kraus shows how phase-reversing coils are used in collinear arrays. Reckon Kraus knows he is violating Kirchhoff's laws? For a real-world air-core coil to exhibit identical currents at the input and output, it would have to be one wavelength long, lossless, and non-radiating. That's a pretty tall order just to support an old wives' tale. :-) -- 73, Cecil http://www.qsl.net/w5dxp You need to go back to school, Cecil. Anyone can stick a coil - or a capacitor for that matter - between two similar antiresonant circuits and find a frequency where the two circuits are in phase without resorting to calling the coil - or capacitor - 1/2WL. There's also a frequency where the two circuits are 180 deg. out of phase, but current isn't flowing into two ends of the coil - or capacitor - at the same time. I hope there's no on this newsgroup gullible enough to take your fractured circuit theory seriously. 73, Tom Donaly, KA6RUH |
#7
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Tdonaly wrote:
There's also a frequency where the two circuits are 180 deg. out of phase, but current isn't flowing into two ends of the coil - or capacitor - at the same time. I hope there's no one on this newsgroup gullible enough to take your fractured circuit theory seriously. Kraus says: "It is generally assumed that the current distribution of an infinitesimally thin antenna is sinusoidal, and that the phase is constant over a 1/2WL interval, changing abruptly by 180 degrees between intervals." If I locate a loading-coil such that the phase change node is in the middle of the coil, current-in will be 180 degrees out of phase with current-out. In a two-terminal series circuit, if the net current-in is 180 degrees out of phase with the net current-out, those two currents are either flowing into the coil at the same time or flowing out of the coil at the same time 1/2 cycle later. Consider a 1.5WL helical dipole. The two net currents at 0.4WL and 0.6WL are flowing in opposite directions. Consider something even more bizarre. If the coil is exactly 1/2WL and each end is located at a current node, assuming the forward current is equal to the reflected current (Kraus' assumption) then zero net current is flowing in and out of both ends of the coil even though there is a current maximum point in the middle of the coil. This is how Kraus' phase-reversing coil works in his collinear array antenna. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#8
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Cecil Moore wrote:
snip Kraus says: "It is generally assumed that the current distribution of an infinitesimally thin antenna is sinusoidal, and that the phase is constant over a 1/2WL interval, changing abruptly by 180 degrees between intervals." Kraus says something quite similar in the second edition of "Antennas". (I believe you are quoting from the third edition, which was co-authored by someone else.) However, Kraus is merely being careless with terminology. (It is likely that he did not fully anticipate that he would be quoted out of context.) If one studies the accompanying diagrams it is clear that Kraus is simply referring to the standard functional form of a sinusoidal curve. For reasons not clear to me he decides to call the natural progression from positive to negative as the sine function passes through zero an abrupt 180 degree phase change. This is misleading at best. A true phase change would be, for example, an abrupt transition from +1 to -1 in the sine function. This sort of phase change is used in numerous communication schemes, such as PSK31. snip Consider something even more bizarre. If the coil is exactly 1/2WL and each end is located at a current node, assuming the forward current is equal to the reflected current (Kraus' assumption) then zero net current is flowing in and out of both ends of the coil even though there is a current maximum point in the middle of the coil. This is how Kraus' phase-reversing coil works in his collinear array antenna. Why is this even the least bit bizarre? Your favorite example of an ideal transmission line with a perfectly reflecting termination shows exactly the same thing. Are you suggesting that any node on an ideal standing wave cuts off everything further downstream? If so, then you might want to consider Tom's suggestion and head back to school. 73, Gene W4SZ |
#9
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Gene Fuller wrote:
Consider something even more bizarre. If the coil is exactly 1/2WL and each end is located at a current node, assuming the forward current is equal to the reflected current (Kraus' assumption) then zero net current is flowing in and out of both ends of the coil even though there is a current maximum point in the middle of the coil. This is how Kraus' phase-reversing coil works in his collinear array antenna. Why is this even the least bit bizarre? Your favorite example of an ideal transmission line with a perfectly reflecting termination shows exactly the same thing. Are you suggesting that any node on an ideal standing wave cuts off everything further downstream? No, no, no. I'm saying that if Tom finds current flowing into both ends of a coil at the same time to be a bizarre thought, then a coil with no current flowing into the ends at all, even though current is maximum at the center of the coil, would be an even more bizarre thought *FOR HIM*. Tom seems to have a sacred cow that he doesn't want to barbecue. Assume a 180 degree phase shifting coil with a current node at each end as I described above. Also assume one misses the current nodes by 6 degrees and that the maximum current loop is 1 amp. The current at one end of the coil will be ~0.1 amp at zero degrees while the current at the other end of the coil is ~0.1 amp at 180 degrees. That means current is flowing into both ends of the coil at the same time and then flowing out of both ends at the same time 1/2 cycle later. Tom calls anyone who believes that "gullible" and that tells me I should go back to school. I am merely demonstrating the laws of physics operating outside of Tom's sacred cow box. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#10
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Gene Fuller wrote:
Cecil Moore wrote: Kraus says: "It is generally assumed that the current distribution of an infinitesimally thin antenna is sinusoidal, and that the phase is constant over a 1/2WL interval, changing abruptly by 180 degrees between intervals." Kraus says something quite similar in the second edition of "Antennas". (I believe you are quoting from the third edition, which was co-authored by someone else.) However, Kraus is merely being careless with terminology. (It is likely that he did not fully anticipate that he would be quoted out of context.) It is within the context of physics. It is only out of context when the context is sacred cows and old wives' tales. If one studies the accompanying diagrams it is clear that Kraus is simply referring to the standard functional form of a sinusoidal curve. For reasons not clear to me he decides to call the natural progression from positive to negative as the sine function passes through zero an abrupt 180 degree phase change. This is misleading at best. Kraus is merely following convention. The sign of the real part of the current at 89 degrees is positive. The sign of the real part of the current at 91 degrees is negative. A positive sign indicates current flowing in one direction. A negative sign indicates current flowing in the opposite direction. Since there are only two possible directions in a wire, those two directions are 180 degrees apart, by definition. A true phase change would be, for example, an abrupt transition from +1 to -1 in the sine function. This sort of phase change is used in numerous communication schemes, such as PSK31. A true phase change would also be, a smooth transition from +0.001 through zero to -0.001. When current equals zero at a standing wave node, the phase of the real component of current on each side of that zero is 180 degrees different. For the real component of the current, a 180 degree phase reversal occurs between 89 degrees and 91 degrees. Cos(89) = +0.017, Cos(91) = -0.017 -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
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