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Current through coils
Gene Fuller wrote:
... how can you learn anything about the details of a complex system by averaging and netting? Because the conservation of energy principle is about averaging and netting. Because steady-state analysis is about averaging and netting. Because engineers have 200 years of averaging and netting behind us to prove that it works. When you try to track an individual electron's velocity and position, guess what happens? -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Cecil, quit trying to pedal that bull****. The currents at the two
ends of the coil are NOT the same if they are different phases. It is the phase difference that lets you establish different standing wave currents at the two ends, when there's a travelling wave in each direction. So if the phase is different, then clearly there is net current going into the coil half the cycle, and net current coming out of the coil half the cycle. Go ahead and do it with your travelling wave and phasors. It will work just as well as instantaneous currents. You will find a net current into the coil at some phase. Clearly the phasor notation is just a simplification of instantaneous currents for the case of sinusoidal excitation, and the answers darned well better be the same, or you better throw out your phasor notation. OF COURSE the AVERAGE charge in and out is balanced! If it weren't, then you have a DC current with nowhere to go. This is a linear system we're modelling here, with no way to convert a sinewave to DC. So tell us what net AC current into a component represents, and we'll just about be there. |
Current through coils
No, Cecil, I did not try to change the meaning by trimming. I was
simply pointing out a basic flaw in your whole development. You use the differing phase to establish that a travelling wave in each direction results in a difference in the standing wave current at each end, but then you try to use amplitude only to show no net current into the coil. Now use the SAME phase difference you used to develop the standing wave, and use it to determine the net AC current into the coil, AT SOME PHASE. Now use the same phase difference in the other direction to see that it also results in a net AC current AT SOME PHASE. AND for the case where there is a standing-wave current difference between the two ends of the coil, the net coil current is EXACTLY as predicted by the vector sum of the two travelling wave net currents. Now you decide. Can I do phasor math? Do you need a specific example with numbers, or can YOU work that out yourself? Suggest you use the example from your previous posting. If that causes any difficulty, try it with 180 degrees phase shift through the component. I've done it, and it keeps giving me precisely the same answer as a full cycle of instantaneous currents. |
Current through coils
wrote in message Let's focus on one thing at a time. You claim a bug cather coil has "an electrical length at 4MHz of ~60 degrees". That concept is easily proven false, just like the claim a short loaded antenna is "90-degree resonant". Both can be shown to be nonsense pictures of what is happening. Assume I have a 30 degree long antenna. If the loading inductor is 60 electrical degrees long, I could move it anyplace in that antenna and have a 90 degree long antenna. We all know that won't happen, so what is it you are really trying to say? 73 Tom OK lets get me some educating here. I understand that, say quarter wave resonant vertical (say 33 ft at 40m) has 90 electrical degrees. Is that right or wrong? The current distrubution on said (full size) vertical is one quarter of the wave of 360 deg. which would make it 90 degrees. Max current is at the base and then diminishes towards the tip in the cosine function down to zero. Voltage distribution is just opposite, min at the base, feed point and max at the tip. EZNEC modeling shows that to be the case too. Is that right or wrong? If we stick them end to end and turn horizontal, we get dipole, which then would be 180 deg. "long" or "180 degrees resonant". If not, what is the right way? If I insert the coil, say about 2/3 up (at 5 ft. from the bottom) the shortened vertical, I make the coil size, (inductance, phys. dimensions) such that my vertical will shrink in size to 8 ft tall and will resonate at 7.87 MHz. I learned from the good antenna books that this is still 90 electrical "resonant" degrees. Maximum of current is at the feed point, minimum or zero at the tip. If you stick those verticals (resonant) end to end and horizontal, you get shortened dipole, with current distribution equal to 180 degrees or half wave. Max current at the feed point, minima or zero at the tips. (RESONANT radiator) How many electrical degrees would that make? How do you arrive at that? Why is this a nonsense? Can we describe "pieces" or segments of the radiator as having proportional amount of degrees corresponding to their physical length, when excited with particular frequency? If I can be enlightened about this, we can go then to the next step. Answers, corrections please. Yuri, K3BU |
Current through coils
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Current through coils
On Thu, 23 Mar 2006 11:12:54 -0500, "Yuri Blanarovich"
wrote: Can we describe "pieces" or segments of the radiator as having proportional amount of degrees corresponding to their physical length, when excited with particular frequency? If I can be enlightened about this, we can go then to the next step. Hi Yuri, At your page you assert: "The current in a typical loading coil in the shortened antennas drops across the coil roughly corresponding to the segment of the radiator it replaces. " so I must presume this is part and parcel to your question above and the coil is part of that proportionality where all segments combine to 90°. On the other hand, Cecil is only willing to allow: On Wed, 22 Mar 2006 23:48:11 GMT, Cecil Moore wrote: +/- 50% accuracy. Now, given that you might describe a radiator whose vertical sections add to 30°, then it follows from your page's assertion that the coil must represent 60°. Cecil, again, would give pause and restrict that to some value between 30° and (oddly enough) 90°. The total structure then represents a 60° to 120° electrically high verticle. The long and short of this (a pun) is that Cecil has argued you into a rhetorical corner where it is highly unlikely that the whole shebang is ever 90° long - by parts that is. Or as a Hail Mary argument, you could simply assert that the range encompasses the right value for your assertion above, but then anyone could use the same logic to say all loaded antennas are only 70° electrically tall and another could boast 110° and you couldn't dispute them. (Yes, you could, of course, this is a newsgroup afterall.) Perhaps you would like to argue this for yourself (I don't pay much attention to Cecil anyway as this +/- 50% slop factor accounts for). 73's Richard Clark, KB7QHC |
Current through coils
Roy Lewallen wrote:
John Popelish wrote: Roy Lewallen wrote: . . . In my modification to Cecil's EZNEC file I showed how the coil behaves the same with no antenna at all, just a lumped load impedance. As long as the load impedance and external C stay the same, the coil behavior stays the same. Excellent. As long as there is external C, the coil acts in a non lumped way, regardless of whether its current passes to an antenna or a dummy load. This is the same result you would get with any transmission line, also, except that the C is inside the line, instead of all around it. No, the coil is acting in a lumped way whether the C is there or not. A combination of lumped L and lumped C mimics a transmission line over a limited range. And a transmission line mimics a lumped LC network, over a limited range. We are still talking about an antenna loading coil, aren't we? This is a coil made with a length of conductor that is a significant fraction of a wavelength at the frequency of interest, and with low coupling between the most separated turns. And with non zero capacitance of every inch of that length to the rest of the universe and to neighboring inches of the coil. To say it is acting in a lumped way I can only assume that you mean a lumped model of it can be produced that predicts its behavior with an acceptable approximation at a given frequency. Sure, at a single frequency, lots of different models can be useful. I am trying to get inside the black box and understand how the device acts as it acts, not discover what simplified models might approximate it under specific conditions. But neither the L nor C is acting as more or less than a lumped component. All the "transmission line" properties I listed in my last posting for the LC circuit can readily be calculated by considering L and C to be purely lumped components. What can be calculated and what is going on are two different subjects. Perhaps this difference in our interests is the basis of our contention. Its propagation is a lot slower than a normal transmission line based on straight conductors, isn't it? There's more L per unit length than on an equal length line made with straight wire, so yes the propagation speed is slower. But there's nothing magic about that. A lumped LC circuit can be found to have exactly the same delay and other characteristics of a transmission line, and it can do it in zero length. Then we agree on this. Perhaps the words "slow wave transmission line" have been copyrighted to mean a specific mechanism of slow wave propagation, not all mechanisms that propagate significantly slower than straight wire transmission lines do. If so, I missed that. .... A slow wave structure is a type of waveguide in which the fields inside propagate relatively slowly. Ramo and Whinnery is a good reference, and I'm sure I can find others if you're interested. I'll do a bit of looking. Thanks. The propagation velocity of the equivalent transmission line is omega/sqrt(LC), so the speed depends equally on the series L and the shunt C. Per unit of length in the direction of propagation. Helical coils have a lot of L in the direction of propagation, compared to straight wire lines, don't they? Yes indeed, as discussed above. And as I said above, you can get plenty of delay from a lumped L and C of arbitrarily small physical size. You keep going back to how lumped components can mimic actual distributed ones (over a narrow frequency range). I get it. I have no argument with it. But why do you keep bringing it up? We are talking about a case that is at least a border line distributed device case. I am not interested in how it can be modeled approximately by lumped, ideal components. I am interested in understanding what is actually going on inside the distributed device. . . . The question, I think is whether large, air core coils act like a single inductance (with some stray capacitance) that has essentially the same current throughout, or is a series of inductances with distributed stray capacitance) that is capable of having different current at different points, a la a transmission line. And the answer must be that it depends on the conditions. At some frequencies, it is indistinguishable from a lumped inductance, but at other frequencies, it is clearly distinguishable. You have to be aware of the boundary case. Yes. It's a continuum, going from one extreme to the other. As Ian has pointed out several times, any theory should be able to transition from one to the other. Or start with a less simplified theory that covers all cases, so you don't have to decide when to switch tools. The example Cecil posted on his web page was one for which the L could be modeled completely adequately as a lumped L, at least so far as its current input and output properties were concerned. (if you add to that model, the appropriate lumped capacitors at the appropriate places) Being a significant fraction of the antenna's total length, it of course does a substantial amount of radiating which a lumped model does not. Another reason to avoid that model, unless you are just looking for the least amount of math to get an approximation. But computation has gotten very cheap. .... But a continuous coil is not a series of discrete lumped inductances with discrete capacitances between them to ground, but a continuous thing. In that regard, it bears a lot of similarity to a transmission line. But it has flux coupling between nearby turns, so it also has inductive properties different from a simple transmission line. Which effect dominates depends on frequency. Yes, that's correct. But if it's short in terms of wavelength, a more elaborate model than a single lumped inductance won't provide any different results. The coil in the EZNEC model on Cecil's web page acts just like we'd expect an inductor to act. A perfect point sized inductor? I don't think so. Except for the radiation, yes. In what ways do you see it differing? A lumped inductor has no stray capacitance. Those also have to be added to the model, before the effect would mimic the real coil (neglecting radiation). With ground present constituting a C, the circuit acts like an L network made of lumped L and C which behaves similarly to a transmission line. With ground, hence external C, absent, it acts like a lumped L. (There are actually some minor differences, due to imperfect coupling between turns and to coupling to the finite sized external circuit.) The combination of L and C "act like" a transmission line, just like any lumped L and C. And it doesn't care whether the load is a whip or just lumped components. I agree with the last sentence. The ones before that seem self contradictory. First you say it acts just like an inductor, then you say it acts like a transmission line. These things (in the ideal case) act very differently. Let me try again. The combination of L and the C to ground act like a transmission line, just like a lumped LC acts like a transmission line. With the ground removed, there's nearly no C, so there's very little transmission-line like qualities. Of course you could correctly argue that there's still a tiny amount of C to somewhere and so you could still model the circuit as a transmission line. The equivalent transmission line would have very high impedance and a velocity factor very near one. Such a transmission line is difficult to distinguish from a plain inductor. But in the real world, the capacitance is always there. It varies, depending on the location of the coil, but it never approaches zero. |
Current through coils
Roy Lewallen wrote:
John Popelish wrote: No, you're misinterpreting what you're seeing. Imagine an LC L network with theoretically lumped series L and shunt C. Okay, I am imagining an idealized, network made of perfect, impossible components that is simple to analyze. Got it. If you look at the currents at the input and output of the perfect inductor, you'll find that they're exactly the same. Right. If, however, you look at the currents in and out of the *network* you'll see that they're different, because of current going to ground through the C. Got it. Same for any pi, T, or more complicated LC network. And, as I said before, you can even pretend it's a transmission line and measure forward and reverse traveling waves and a standing wave ratio. Yes. Under some specific conditions. But with zero length, there can be no standing waves inside the inductor. Yes. There are no waves in a single ideal lumped component, so there can be no waves inside any of them, only a phase shift between the voltage across them and the current through them. But a network made of them can mimic lots of processes that internally involve propagation of waves, including the phase shift between voltages across the terminals and current into the terminals, and even group delay, but only over narrow frequency range. It is a model with this severe limitation. Yet the terminal characteristics of the network are the same as a transmission line. You don't need to imagine standing waves residing inside the inductor in the LC circuit, and you don't need to imagine them inside the inductor in Cecil's model, either. (snip) Whether or not we need to imagine them to picture what is happening at the terminals is not the question at hand. The question in my mind is what is the actual mechanism, inside the device in question that is causing the effects we see at the terminals. I am not interested in the full range of models that predict the effect, but in the actual cause. I accept that my motivation is not necessarily the same as yours. |
Current through coils
Cecil Moore wrote:
John Popelish wrote: Oh poo. At current nodes charge piles up and spreads out, on alternating half cycles. For one half cycle, the pile is positive, and for the next it is negative. This is a basic transmission line concept. If transmission lines had no shunt capacitance, there would be no place to put this charge. But there is, so it is no problem. Whether the transmission line is coax, twin line or a slow wave helix makes little difference. The process is similar. Isn't this what you have been arguing? If the forward traveling wave is equal in magnitude at both ends of the coil, there is no net storage of energy due to the forward traveling wave. Over a complete cycle, I agree, Within a single cycle, standing waves slosh charge back and forth between adjacent current nodes, piling up positive charge at one and negative charge at the next. This is the reason that the voltage peaks at the tip of a quarter wave antenna. It is a current node (because current has no place to go from there), so charge piles up and produces voltage. But over a complete cycle, the net charge movement is zero (the positive piles are he same size as the negative piles). If the reflected traveling wave is equal in magnitude at both ends of the coil, there is no net storage of energy due to the reflected traveling wave. Same thing I said last paragraph. Superposing those two waves still results in no net storage of energy. Sorry, got to hit the road. I'll put this on hold till you get back. Have fun. |
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