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#21
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Current across the antenna loading coil - from scratch
Cecil Moore wrote:
Tom Donaly wrote: Hecht was talking about two opposing waves of the same phase and amplitude interfering with each other. You can't guarantee, in a real antenna, that the two waves do have the same phase and magnitude. :-) Hecht was talking about two coherent EM waves traveling in opposite directions. We are talking about two coherent EM waves traveling in opposite directions. There is a small traveling wave component but it doesn't affect the standing wave. It is what is left over from the standing wave. This discussion has not been about coils. We need to discuss an unterminated lossless transmission line and then move on to 1/2 wavelength thin-wire standing wave antennas. Has it ever occurred to you, Cecil, that a half wave dipole with equal current and voltage waves traveling in opposite directions wouldn't accept power? 73, Tom Donaly, KA6RUH |
#22
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Current across the antenna loading coil - from scratch
John Popelish wrote:
. . . It is obvious to me that you are one of them. Every point on a line carrying a standing wave (except the node points) has AC voltage on it, and AC current through it. The amplitude and phase of those voltages and currents can be described as a phasor, with respect to some reference phase of the same frequency. As you move along the line, the amplitude changes and when you pass through a node the phase reverses. So the phasor does not rotate with position change, except for a step change of 180 degrees at nodes, rather than smooth rotation with respect to position. For a traveling wave, every point on the line has an AC voltage on it, and an AC current passing through it. The amplitude is constant along the line, but the phasor rotates as you move along the line (the phase is linearly dependent on position). But at any single point on the line, a non rotating phasor describes the amplitude and phase with respect to a reference phase of the same frequency. There's a potential for ambiguity here, and that ambiguity has been used a number of times in this thread to cause confusion. So let me try to clarify things. All phasors "rotate", in that every one contains an implicit term e^jwt. That term describes a rotation of the complex phasor quantity at the rotational frequency w (omega), but no change in amplitude. If a quantity doesn't include this implicit term, it's not a phasor, by definition. We can look at any phasor quantity in a system and compare the phase of its rotation with the phase of a reference, and from this assign a phase angle to it. In steady state, the phase angle doesn't change with time -- it's the phase difference between the w - rotating phasor and the w - rotating reference. Phasors of different rotational rates (that is, of different frequencies) can't be combined in the same analysis, unless the implicit term is made explicit, in which case they're no longer phasors. The use of "rotation" in John's posting is talking about a change of phase with physical position. This usage has been confused with the time rotation of the phasor which comes from the implicit e^jwt term. I'd prefer to use the term "phase", which doesn't change with time in a steady state system, directly rather than "rotation" to describe a change in phase with position. With that convention, we see that the phase of a pure traveling wave changes linearly with position. But when we sum forward and reverse traveling waves together to get a total current (or voltage), the phase of the total current (or voltage) is no longer a linear function of position. In the special case of an open or short circuited transmission line, where the forward and reverse traveling waves are equal in amplitude, the phase doesn't change with position at all (except for a periodic reversal in current and voltage direction, which can be interpreted as a 180 degree phase change). But the phasor representing total voltage or current (which Cecil refers to as "standing wave current") at any point, which is the sum of two phasors representing forward and reverse traveling waves, does indeed rotate at w (omega) radians/second rate, just like its constituent phasors. The constant phase with position (of an open or shorted line) simply means that if you froze time at some instant and looked at the angles of the rotating phasors representing the total current at each point along the line, you'd find them all to be at the same angle. They're all rotating. This isn't revolutionary or controversial -- you can find phasors discussed in any elementary circuit analysis text.[*] And it's not difficult to do the summation of forward and reverse traveling waves to see the result, but if you'd like to see how someone else did it, one of the clearest discussions I've found is in Chipman's _Transmission Lines_, a Schaum's Outline book. [*] You have to be a little careful, though. In most introductions to phasors, the author introduces the e^jwt term early on, and quickly drops it from the phasor notation as is customary. So it's easy to forget it's there. But remembering that it is there is vital to understanding this topic, and to keep from being misled by misdirection which takes advantage of confusion and abbreviated notation. Roy Lewallen, W7EL |
#23
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Current across the antenna loading coil - from scratch
Cecil Moore wrote:
John Popelish wrote: Of course. no one is talking about the red herring of charge stored over a whole cycle. Of course, *everyone* except you and Tom Donaly are talking about charge stored over a whole cycle. Bull. That's the entire base of their arguments. The unbalance in the *RMS* current at the bottom of the coil and the *RMS* current at the top of the coil is what the entire discussion is all about. The currents measured by W8JI and W7EL were *RMS* currents. The currents reported by EZNEC are *RMS* currents. And the capacitive currents can also be measured in RMS terms. So what? And no one but you brings up "net storage". We are all talking about ordinary capacitive charge storage within a cycle. If so, that is completely irrelevant to the discussion since W8JI and W7EL are using *RMS* currents for their measurements and EZNEC is reporting *RMS* currents. Let me summarize it for you. W8JI and W7EL apparently think that the RMS current value of zero at the bottom of the coil Vs the RMS current value of one amp at the top of the coil means energy is being sucked into the coil from some external source. I don't read their responses that way. I read their responses as saying that the current leaving or entering an end of an inductor includes a capacitive component and an inductive component. The capacitive current branches out of the coil to the surrounding space, and is what allows a measured difference in the currents passing through its two ends. The path through the wire to the other end is not the only path for current. How about assisting in a tutorial on standing waves rather than diverting and obfuscating the issues? I'm trying. |
#24
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Current across the antenna loading coil - from scratch
Roy Lewallen wrote:
John Popelish wrote: . . . It is obvious to me that you are one of them. Every point on a line carrying a standing wave (except the node points) has AC voltage on it, and AC current through it. The amplitude and phase of those voltages and currents can be described as a phasor, with respect to some reference phase of the same frequency. As you move along the line, the amplitude changes and when you pass through a node the phase reverses. So the phasor does not rotate with position change, except for a step change of 180 degrees at nodes, rather than smooth rotation with respect to position. For a traveling wave, every point on the line has an AC voltage on it, and an AC current passing through it. The amplitude is constant along the line, but the phasor rotates as you move along the line (the phase is linearly dependent on position). But at any single point on the line, a non rotating phasor describes the amplitude and phase with respect to a reference phase of the same frequency. There's a potential for ambiguity here, and that ambiguity has been used a number of times in this thread to cause confusion. So let me try to clarify things. All phasors "rotate", in that every one contains an implicit term e^jwt. That term describes a rotation of the complex phasor quantity at the rotational frequency w (omega), but no change in amplitude. If a quantity doesn't include this implicit term, it's not a phasor, by definition. We can look at any phasor quantity in a system and compare the phase of its rotation with the phase of a reference, and from this assign a phase angle to it. In steady state, the phase angle doesn't change with time -- it's the phase difference between the w - rotating phasor and the w - rotating reference. Phasors of different rotational rates (that is, of different frequencies) can't be combined in the same analysis, unless the implicit term is made explicit, in which case they're no longer phasors. The use of "rotation" in John's posting is talking about a change of phase with physical position. This usage has been confused with the time rotation of the phasor which comes from the implicit e^jwt term. I'd prefer to use the term "phase", which doesn't change with time in a steady state system, directly rather than "rotation" to describe a change in phase with position. With that convention, we see that the phase of a pure traveling wave changes linearly with position. But when we sum forward and reverse traveling waves together to get a total current (or voltage), the phase of the total current (or voltage) is no longer a linear function of position. In the special case of an open or short circuited transmission line, where the forward and reverse traveling waves are equal in amplitude, the phase doesn't change with position at all (except for a periodic reversal in current and voltage direction, which can be interpreted as a 180 degree phase change). But the phasor representing total voltage or current (which Cecil refers to as "standing wave current") at any point, which is the sum of two phasors representing forward and reverse traveling waves, does indeed rotate at w (omega) radians/second rate, just like its constituent phasors. The constant phase with position (of an open or shorted line) simply means that if you froze time at some instant and looked at the angles of the rotating phasors representing the total current at each point along the line, you'd find them all to be at the same angle. They're all rotating. This isn't revolutionary or controversial -- you can find phasors discussed in any elementary circuit analysis text.[*] And it's not difficult to do the summation of forward and reverse traveling waves to see the result, but if you'd like to see how someone else did it, one of the clearest discussions I've found is in Chipman's _Transmission Lines_, a Schaum's Outline book. [*] You have to be a little careful, though. In most introductions to phasors, the author introduces the e^jwt term early on, and quickly drops it from the phasor notation as is customary. So it's easy to forget it's there. But remembering that it is there is vital to understanding this topic, and to keep from being misled by misdirection which takes advantage of confusion and abbreviated notation. Excellent! |
#25
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Current across the antenna loading coil - from scratch
Cecil Moore wrote: Let me summarize it for you. W8JI and W7EL apparently think that the RMS current value of zero at the bottom of the coil Vs the RMS current value of one amp at the top of the coil means energy is being sucked into the coil from some external source. John Popelish wrote: I don't read their responses that way. I read their responses as saying that the current leaving or entering an end of an inductor includes a capacitive component and an inductive component. The capacitive current branches out of the coil to the surrounding space, and is what allows a measured difference in the currents passing through its two ends. The path through the wire to the other end is not the only path for current. You read what I wrote and what Roy wrote correctly John. Cecil changes what other people write to suit his own needs. He changes what other people say, and then points out why the creatively edited text he invented is wrong. That's his debating style. Watch out for it! 73 Tom |
#26
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Current across the antenna loading coil - from scratch
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#27
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Current across the antenna loading coil - from scratch
Tom Donaly wrote:
Has it ever occurred to you, Cecil, that a half wave dipole with equal current and voltage waves traveling in opposite directions wouldn't accept power? It is an approximation, Tom, like a lossless line. For real world dipoles, the voltage and current decay by about 10% between the forward wave and the arrival of the reflected wave. Kraus and Terman both use that approximation in their examples. We aren't saying anything about the traveling wave part of the waves. The discussion is about the standing wave portion of the wave which, by definition, requires equal magnitudes. -- 73, Cecil http://www.qsl.net/w5dxp |
#28
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Current across the antenna loading coil - from scratch
Roy Lewallen wrote:
The constant phase with position (of an open or shorted line) simply means that if you froze time at some instant and looked at the angles of the rotating phasors representing the total current at each point along the line, you'd find them all to be at the same angle. They're all rotating. Yes, when I said standing wave current phase doesn't rotate, I meant with respect to the source current phase. At any instant in time, the phase of the standing wave current is unchanging up and down the line. Assume the standing wave current all up and down the dipole is of constant phase with no variation with 'x'. Roy, you used that current to try to measure the delay through a coil. How did you plan to measure that delay with a signal known to be the same phase not only at both ends of the coil but all up and down the antenna? -- 73, Cecil http://www.qsl.net/w5dxp |
#29
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Current across the antenna loading coil - from scratch
John Popelish wrote:
Cecil Moore wrote: Of course, *everyone* except you and Tom Donaly are talking about charge stored over a whole cycle. Bull. If that's what you think and you can find someone to discuss energy exchange within a cycle, be my guest. As far as I know, Tom Donaly introduced the subject as a diversion. I don't read their responses that way. I couldn't believe it either but after years of arguing with them, it is apparent that many of the gurus here on r.r.a.a are simply ignorant of the nature of standing waves. I really expected them to shout, "April Fool, we have been pulling your leg!" But, sad to say, they are serious about standing wave current "flowing" into the bottom of the coil and out the top. They apparently haven't read "Optics", by Hecht where he says: "E(x,t) = 2Eo*sin(kx)*cos(wt) This is the equation for a standing wave, as opposed to a traveling wave. Its profile does not move through space. ... [The standing wave] phasor doesn't rotate at all, and the resultant wave it represents doesn't progress through space - its a standing wave." If standing waves of light don't move through space, standing waves of RF don't move through a wire. I read their responses as saying that the current leaving or entering an end of an inductor includes a capacitive component and an inductive component. The capacitive current branches out of the coil to the surrounding space, and is what allows a measured difference in the currents passing through its two ends. That is a secondary effect. The primary effect is the phasor addition of the forward current and reflected current which you provided. Compared to zero amps of standing wave current when the forward current phasor and the reflected current phasor are 180 degrees out of phase, just how much effect can capacitance have? -- 73, Cecil http://www.qsl.net/w5dxp |
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