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Current through coils
John Popelish wrote:
. . . That's exactly the difference. But if you measure a single point, you can't tell whether you are measuring a point on a traveling wave or a standing wave. Agree? There seems to be some confusion about just what a standing wave is. A standing wave is the result of, and the sum of, two or more traveling waves. There aren't points which are "on" one or the other. If you can separately measure or calculate the values of the traveling current waves at any point, you can add them to get the total current (what Cecil calls "standing wave current") at that point. If you add the traveling current waves at each point along the line and plot the amplitude of the sum (that is, of the total current) versus position, you see a periodic relationship between the amplitude and position. It's this relationship which is called a "standing wave". It's so called because its position relative to the line stays fixed. It's simply a graph of the total current (the sum of the traveling waves) vs. position. Roy Lewallen, W7EL |
Current through coils
Roy Lewallen wrote:
The coil in the EZNEC model on Cecil's web page acts just like we'd expect an inductor to act. 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. But the point is that the delay through the coil is somewhere between 40 degrees and 60 degrees. When you tried to measure the phase shift through a coil, you used standing wave current phase to make the measurement. Standing wave current phase is unchanging so you made a measurement blunder. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Roy Lewallen wrote:
A standing wave is the result of, and the sum of, two or more traveling waves. There aren't points which are "on" one or the other. If you can separately measure or calculate the values of the traveling current waves at any point, you can add them to get the total current (what Cecil calls "standing wave current") at that point. If you add the traveling current waves at each point along the line and plot the amplitude of the sum (that is, of the total current) versus position, you see a periodic relationship between the amplitude and position. It's this relationship which is called a "standing wave". It's so called because its position relative to the line stays fixed. It's simply a graph of the total current (the sum of the traveling waves) vs. position. And there's no such thing as current imbalance based on standing wave currents being different at each end of a loading coil. "Current imbalance" is a concept that doesn't apply to standing waves. "Phase rotation with position" is a concept that doesn't apply to standing waves. Standing wave current is NOT ordinary current. It is the superposition of two ordinary currents. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Correction:
Roy Lewallen wrote: (Last paragraph) Important for what? No matter how long the coil or how many turns of the wire, a small (in terms of wavelength) inductor won't act like a slow wave structure or an axial mode helical antenna. . . The word "diameter" should be added: Important for what? No matter how long the coil or how many turns of the wire, a small *diameter* (in terms of wavelength) inductor won't act like a slow wave structure or an axial mode helical antenna. . . Roy Lewallen, W7EL |
Current through coils
Cecil Moore wrote:
John Popelish wrote: K7ITM wrote: What happens to that imbalance in charge? Where does it go? What do we call something that behaves that way? What's so freakin' special about that? The charge briefly piling up and then being sucked out of such an inductor is the same place charge piles up and is sucked out of parts of a transmission lines with standing waves on them. Seems you got sucked in by a myth, John. The forward current is equal at both ends of the coil. Now, cut that out! Standing waves have sinusoidal current swings that vary in amplitude with location. Location includes the two ends of a coil. The reflected current is equal at both ends of the coil. Smile when you say that. That takes care of any question of charge imbalance. There simply isn't any. 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? Assume the coil is 90 degrees long and that the forward current is one amp and the reflected current is one amp. At one end of the coil, the forward and reflected currents are 180 degrees out of phase. The standing wave current is zero. At the other end of the coil, the forward and reflected currents are in phase. The standing wave current is 2 amps. Okay. Now do you see why standing wave current is considered not to be flowing? I see how no current is considered to be flowing. Current is charge flowing. AC current is charge flowing back and forth. But I see how two waves going in opposite directions create a standing wave where the magnitude of the sinusoidal current at different points along the standing wave have different magnitudes. And that between the nodes where the amplitude is zero, the phase of the current variation is constant. |
Current through coils
Roy Lewallen wrote:
John Popelish wrote: (snip) But any real, physical inductor has shunt capacitance to its surroundings. So if you neglect this without considering whether or not this is reasonable, you are going to be blindsided by its effects, eventually. I don't disagree with anything you've said. The point I was trying to make was that the resemblance of a coil to a transmission line depends not only on the coil but also its capacitance to other objects -- and not to its relationship to traveling current waves. One thing I've seen done on this thread is to use the C across the inductor in transmission line formulas, appearing to give the coil a transmission line property all by itself and without any external C. This is incorrect. Yep. It is capacitance between each part of the coil and somewhere other than the coil that makes it act like a transmission line. Remove the shunt C and it ceases looking like a transmission line. How do I remove the shunt C of an inductor? With an active guarding scheme? Actually, you can reduce it to a negligible value by a number of means. One I've done is to wind it as a physically small toroid. Yes, smaller means less shunt capacitance. But less is not zero. There is always some. In the example discussed in the next paragraph, removing ground from the model reduces the external C to a small enough value that the current at the coil ends become nearly equal. Nearly equal, but not equal, yes. In some cases nearly is close enough to equal that you can neglect it and get a reasonable approximation. In other cases the approximation is not so reasonable. It is a matter of degree. That of course isn't an option in a real mobile coil environment, but it illustrates that the current drop from one end to the other, which in some ways mimics a transmission line, is due to external C rather than reaction with traveling waves as Cecil claims. I don't see it as a "rather", but as an effect that becomes non negligible under some circumstances. 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. This isn't, however, to discount the possibility of the coil interacting with the antenna's field. It just wasn't significant in that case. Okay. So whether or not this coil is acting as a slow wave transmission line in addition to being inductive depends on the surrounding fields and connections? I have no trouble with that. Well, not a "slow wave" transmission line. Its propagation is a lot slower than a normal transmission line based on straight conductors, isn't it? We shouldn't confuse an ordinary lumped LC transmission line approximation with a true slow wave structure such as a helical waveguide (next item). Heaven forfend. ;-) I am not clear on the difference. 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? And let's talk for a minute about the coil "acting like" a transmission line. A transmission line is of course a distributed circuit. But you can make a single pi or tee section with lumped series L and shunt C which has all the characteristics of a transmission line at one frequency(*), including time delay, phase shift, characteristic impedance, impedance transformation, and everything else. If put into a black box, you wouldn't be able to tell the difference among the pi, tee, or transmission line -- at one frequency. You could even sample the voltage and current with a Bird wattmeter and conclude that there are traveling voltage and current waves in both cases, and calculate the values of the standing waves on either "transmission line". And this is with a pure inductance and capacitance, smaller than the tiniest components you can really make. With a single section, you can mimic any transmission line Z0 and any length from 0 to a half wavelength. (The limiting cases, however, require some components to be zero or infinite.) So you can say if you wish that the inductor in this network "acts like" a transmission line -- or you can equally correctly say that the capacitor does, because it's actually the combination which mimics a transmission line. But only over a narrow range of frequencies, beyond which it begins deviating more and more from true transmission line behavior. To mimic longer lines or mimic lines over a wider frequency range requires more sections. Hence a description that includes both lumped and distributed attributes. So what can we conclude about inductors from this similar behavior? Certainly not that there's anything special about inductors interacting with traveling waves or that inductors comprise some kind of "slow wave structure". The duality comes simply from the fundamental equations which describe the nature of transmission lines, inductances, and capacitances. 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. Because the LC section's properties are identical to a transmission line's at one frequency, we have our choice in analyzing the circuit. We can pretend it's a transmission line, or we can view it as a lumped LC network. If we go back to the fundamental equations of each circuit element, we'll find that the equations end up exactly the same in either case. And the results from analyzing using each method are identical -- if not, we've made an error. 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. 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. 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. (*) It actually acts like a transmission line at many frequencies, but a different length and Z0 of line at each frequency. To mimic a single line over a wide frequency range requires additional sections. I think I agree with this. Either a simple transmission line or a simple inductance description is incomplete. It does some of both. As far as considering a coil itself as a "slow wave structure", Ramo and Whinnery treat this subject. It's in the chapter on waveguides, and they explain how a helix can operate as a slow wave waveguide structure. To operate in this fashion requires that TM and TE modes be supported inside the structure which in turn requires a coil diameter which is a large part of a wavelength. Axial mode helix antennas, for example, operate in this mode. Coils of the dimensions of loading coils in mobile antennas are orders of magnitude too small to support the TM and TE modes required for slow wave propagation. I'll have to take your word for this limitation. But it seems to me that the length of the coil in relation to the wavelength and even the length of the conductor the coils is made of are important, also. Important for what? No matter how long the coil or how many turns of the wire, a small (in terms of wavelength) inductor won't act like a slow wave structure or an axial mode helical antenna. But its propagation speed will be slower than it would be if the wire were straight. don't know if that qualifies it for a "slow wave" line or not. This is for the same reason that a two inch diameter pipe won't perform as a waveguide at 80 meters -- there's not enough room inside to fit the field distribution required for that mode of signal propagation. There will of course be some point at which it'll no longer act as a lumped inductor but would have to be modeled as a transmission line. But this is when it becomes a significant fraction of a wavelength long. Why can't it be modeled as a transmission line before it is that long? will you get an incorrect result, or is it just a convenience to model it as a lumped inductor, instead? If the turns are very loosely coupled to each other, the wire length becomes more of a determining factor. As I mentioned in earlier postings, there's a continuum between a straight wire and that same wire wound into an inductor. As the straight wire is wound more and more tightly, the behavior transitions from that of a wire to that of an inductance. There's no abrupt point where a sudden change occurs. Yes. |
Current through coils
Roy Lewallen wrote:
John Popelish wrote: . . . That's exactly the difference. But if you measure a single point, you can't tell whether you are measuring a point on a traveling wave or a standing wave. Agree? There seems to be some confusion about just what a standing wave is. A standing wave is the result of, and the sum of, two or more traveling waves. There aren't points which are "on" one or the other. Sure there are. If there is a standing wave on a wire, and you have a tiny current transformer sensor you can slide along the wire, you can measure the instantaneous current (or the RMS) at any point along the wire. If the sensor sits at a single point and sees an AC current, you have no way, from this one measurement, if this current is the result of a standing wave (two oppositely traveling equal waves adding), or a single traveling wave, or any combination of traveling waves of different amplitudes. You know only the net current at that point. If you can separately measure or calculate the values of the traveling current waves at any point, you can add them to get the total current (what Cecil calls "standing wave current") at that point. That is what I mean by the current at a point. If you add the traveling current waves at each point along the line and plot the amplitude of the sum (that is, of the total current) versus position, you see a periodic relationship between the amplitude and position. It's this relationship which is called a "standing wave". It's so called because its position relative to the line stays fixed. It's simply a graph of the total current (the sum of the traveling waves) vs. position. I have no disagreement with this description. |
Current through coils
Cecil Moore wrote:
And there's no such thing as current imbalance based on standing wave currents being different at each end of a loading coil. "Current imbalance" is a concept that doesn't apply to standing waves. "Phase rotation with position" is a concept that doesn't apply to standing waves. Standing wave current is NOT ordinary current. It is the superposition of two ordinary currents. You two are so close to agreement. Standing waves have a current that varies with position. The fact that the EZNEC simulation of a loading coil shows differing current in a situation that is a fairly pure standing wave situation (more energy bouncing up and down the antenna than is radiating from it) means that the RMS current will vary along the standing wave. And, since the simulation shows a different current magnitude at the two ends of the coil, a significant part of a standing wave cycle must reside inside the coil (more than the physical length between the two ends of the coil would account for). In one case (the highest frequency one) the phase of the current even reverses from one end of the coil to the other, as well as an amplitude variation, indicating that a standing wave node occurs some where inside the coil, and the two ends are on opposite ends of that node. If the two currents had been equal, but 180 degrees out of phase, the node would have been in the center of the coil. |
Current through coils
Cecil wrote,
"The forward current is equal at both ends of the coil. The reflected current is equal at both ends of the coil." If that's really true, then the net current is precisely equal at both ends of the coil. I thought you had been claiming that the current is different at each end. Which way is it going to be? If they are different phases, then they are NOT equal. If they are different phases, where does the phase shift COME FROM? If I allow a wave in one direction ONLY and the currents at the two ends are DIFFERENT in phase, WHAT HAPPENS inside the coil to make them different? Where does the extra charge come from, or go to? It's all very simple. Yawn. Hint: Replace the coil with a piece of coaxial transmission line, formed into a loop so the input and output ends are adjacent. Short the outer conductors together and notice that nothing changes in terms of the voltages across each end of the line and currents in the center conductors at each end. Note the difference in current at the two ends of the line, and note the current in the single outer conductor terminal of this three-terminal system. Notice that the sum of all three currents at every instant in time is essentially zero (current direction taken as positive going into each terminal). Got it yet? Do you understand WHAT it is, besides the inductance, that allows a coil to look like a transmission line? Do you understand that the mode is not quite TEM, so some of the usual TEM transmission line behaviour is not going to hold? Cheers, Tom |
Current through coils
Cecil wrote, among other things,
"One amp of forward current is flowing into the coil and one amp of forward current is flowing out of the coil. Charge is balanced." Absolutely NOT! You said the phase difference between the two ends is 45 degrees. Therefore, charge "input" and "output" is balanced ONLY twice during a cycle, when the instantaneous currents are the same. No phase need apply he we're talking INSTANTANEOUS currents. The rest of the time, there's net charge accumulating inside the coil half the time, and net charge coming out the other half the time. GOT IT yet??? You do NOT need phasor math to do this! WHAT does that charge represent, Cecil? C'mon, you can say it... Cheers, Tom (Sorry, no prizes. They've already been awarded to John.) |
Current through coils
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. 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. Well, not a "slow wave" transmission line. 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. We shouldn't confuse an ordinary lumped LC transmission line approximation with a true slow wave structure such as a helical waveguide (next item). Heaven forfend. ;-) I am not clear on the difference. 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. 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. . . . So what can we conclude about inductors from this similar behavior? Certainly not that there's anything special about inductors interacting with traveling waves or that inductors comprise some kind of "slow wave structure". The duality comes simply from the fundamental equations which describe the nature of transmission lines, inductances, and capacitances. 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. 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. 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. Because the LC section's properties are identical to a transmission line's at one frequency, we have our choice in analyzing the circuit. We can pretend it's a transmission line, or we can view it as a lumped LC network. If we go back to the fundamental equations of each circuit element, we'll find that the equations end up exactly the same in either case. And the results from analyzing using each method are identical -- if not, we've made an error. 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? 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. . . . Important for what? No matter how long the coil or how many turns of the wire, a small (in terms of wavelength) inductor won't act like a slow wave structure or an axial mode helical antenna. But its propagation speed will be slower than it would be if the wire were straight. don't know if that qualifies it for a "slow wave" line or not. That's the third time for this. Sure. A theoretical lumped inductor and a theoretical lumped shunt capacitor can have a very slow propagation velocity, and with no physical length at all. I'm failing to see why this has some special relevance. This is for the same reason that a two inch diameter pipe won't perform as a waveguide at 80 meters -- there's not enough room inside to fit the field distribution required for that mode of signal propagation. There will of course be some point at which it'll no longer act as a lumped inductor but would have to be modeled as a transmission line. But this is when it becomes a significant fraction of a wavelength long. Why can't it be modeled as a transmission line before it is that long? will you get an incorrect result, or is it just a convenience to model it as a lumped inductor, instead? Hm, I tried to explain that, but obviously failed. You can model it either way. If you've done your math right, you'll get exactly the same answer, because you'll find that you're actually solving the same equations. . . . Roy Lewallen, W7EL |
Current through coils
John Popelish wrote:
You two are so close to agreement. Standing waves have a current that varies with position. The fact that the EZNEC simulation of a loading coil shows differing current in a situation that is a fairly pure standing wave situation (more energy bouncing up and down the antenna than is radiating from it) means that the RMS current will vary along the standing wave. And, since the simulation shows a different current magnitude at the two ends of the coil, a significant part of a standing wave cycle must reside inside the coil (more than the physical length between the two ends of the coil would account for). No, you're misinterpreting what you're seeing. Imagine an LC L network with theoretically lumped series L and shunt C. If you look at the currents at the input and output of the perfect inductor, you'll find that they're exactly the same. 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. 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. But with zero length, there can be no standing waves inside the inductor. 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. When you look at the currents reported by EZNEC for the model on Cecil's web page, the current at the top of the coil is the equivalent to the *network* current described above. It's the current flowing through the inductance minus the current being shunted to ground via the C between the coil and ground. You can tell just how much this is by looking at my modified model and subtracting the current going into the coil from ground from the current going into ground from the added wire. They're not the same -- the difference is the displacement current through the C from the inductor to ground. When I removed the ground, you could then see the current flowing through the inductor, by itself, without the current being shunted off. And lo and behold, it's nearly the same at both ends of the inductor, showing that the inductor is behaving very much like a lumped L. Only in conjunction with the C to ground does the combination mimic a transmission line -- just like any other lumped LC circuit. Of course, at some length and/or poorness of interturn coupling, a coil will start behaving in a way we can't adequately model as a lumped L. But that's not the case here. . . . Roy Lewallen, W7EL |
Current through coils
Cecil Moore wrote:
That's exactly the difference. But if you measure a single point, you can't tell whether you are measuring a point on a traveling wave or a standing wave. Agree? I agree but who would be stupid enough to measure just a single point? Electronic components are exactly that stupid. They have no conception of traveling or standing waves. They react simply to the voltages and currents they experience at their terminals. As far as current is concerned, that means the simple movement of charge past a single point. You see a larger picture of the whole antenna, so you can choose many different ways to theorize about it. But your theory cannot be correct if it requires that components behave in different, special ways according to the way you happen to be thinking about it at the time. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Current through coils
Roy Lewallen wrote:
Important for what? No matter how long the coil or how many turns of the wire, a small *diameter* (in terms of wavelength) inductor won't act like a slow wave structure or an axial mode helical antenna. . . So many words trying to avoid the real issue which is: What is the percentage of a wavelength occupied by a loading coil. It doesn't matter what the size of the coil is. In the real world, a loading coil occupies a certain percentage of a wavelength. For a small coil, that percentage will be small. For a large coil that percentage will be large. We have had to throw out your phase measurements using the phase of standing wave currents because that phase you used is unchanging whether in a wire or in a coil. Your phase measurements tell us zero information about the delay through a coil. That leaves us only with indirect measurements based on the self- resonant frequency of the coil in the mobile environment or the phase information left in the standing wave current amplitude over the 90 degree antenna. My self-resonant frequency measurements indicate that a 75m loading- coil occupies 40-60 degrees of a 360 degree wavelength. That's 11%-17% of a wavelength. Dr. Corum's papers agree with that estimate. Another way of estimating the percentage of the antenna occupied by the loading coil would be to plot the current segments from feedpoint to tip. Then draw a cosine wave on the same graph with 0 degrees at the feedpoint and 90 degrees at the tip. A rough estimate of the percentage occupied by the coil would be the slice of the cosine wave from the bottom of the coil to the top of the coil. Mere words are not going to change the percentage of a wavelength occupied by a real-world loading coil. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
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. 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. Superposing those two waves still results in no net storage of energy. Sorry, got to hit the road. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
John Popelish wrote:
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. Dr. Corum says the boundary is 15 degrees, or 0.04 wavelength. Another place in his class notes he says that if 1/6 of a wavelength of wire is used to make the coil, the lumped-circuit model will NOT work. My 75m bugcatcher coil is more than 1/6 of a wavelength of wire. 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. Dr. Corum has a test equation to see if his velocity factor equation applies. The test is: 5*N*D^2/lamda(0) = 1 where N is number of turns, D is the diameter of the coil, and lamda(0) is the self-resonant frequency. If this equation is satisfied, then equation (32) applies for velocity factor. For my 75m bugcatcher coil, the test number is 0.4 = 1 and the velocity factor equation yields 0.0175. That's certainly a slow wave device. But its propagation speed will be slower than it would be if the wire were straight. don't know if that qualifies it for a "slow wave" line or not. A velocity factor of 0.0175 for a 75m bugcatcher seems to qualify. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
John Popelish wrote:
If there is a standing wave on a wire, and you have a tiny current transformer sensor you can slide along the wire, you can measure the instantaneous current (or the RMS) at any point along the wire. If the sensor sits at a single point and sees an AC current, you have no way, from this one measurement, if this current is the result of a standing wave (two oppositely traveling equal waves adding), or a single traveling wave, or any combination of traveling waves of different amplitudes. You know only the net current at that point. But if one it smart enough to slide the sensor up and down the wire and note the phase is fixed and unchanging, one knows he is dealing with a standing wave. If you add the traveling current waves at each point along the line and plot the amplitude of the sum (that is, of the total current) versus position, you see a periodic relationship between the amplitude and position. It's this relationship which is called a "standing wave". It's so called because its position relative to the line stays fixed. It's simply a graph of the total current (the sum of the traveling waves) vs. position. And that's all it is - the sum of two traveling waves. A standing wave has no separate existence of its own. It is an artifact of superposition. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
John Popelish wrote:
Standing waves have a current that varies with position. The fact that the EZNEC simulation of a loading coil shows differing current in a situation that is a fairly pure standing wave situation (more energy bouncing up and down the antenna than is radiating from it) means that the RMS current will vary along the standing wave. And, since the simulation shows a different current magnitude at the two ends of the coil, a significant part of a standing wave cycle must reside inside the coil (more than the physical length between the two ends of the coil would account for). And since a significant part of a standing wave cycle resides inside the coil, it occupies a non-negligible percentage of a wavelength. By every valid method, measured or calculated, a 75m bugcatcher coil occupies tens of degrees of a wavelength (out of 360 degrees). My best estimate is 60 degrees in a 75m mobile antenna. In one case (the highest frequency one) the phase of the current even reverses from one end of the coil to the other, as well as an amplitude variation, indicating that a standing wave node occurs some where inside the coil, and the two ends are on opposite ends of that node. If the two currents had been equal, but 180 degrees out of phase, the node would have been in the center of the coil. Yes, if a current node exists inside a coil, the standing wave currents are flowing into the coil at the same time from both ends and 1/2 cycle later they are both flowing out of the coil at the same time. Wonder how a lumped-circuit inductance handles that? :-) -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
K7ITM wrote:
Cecil wrote, "The forward current is equal at both ends of the coil. The reflected current is equal at both ends of the coil." If that's really true, then the net current is precisely equal at both ends of the coil. I was speaking above about the magnitudes only, not the phases. It was clear from the rest of my posting that was the assumption. The fact that you attempted to change the meaning by trimming is noted. So to be perfectly clear, here is my statement re-worded using a 45 degree phase shift through the coil. The forward current magnitude is equal at both ends of the coil. The reflected current magnitude is equal at both ends of the coil. At the bottom of the coil, the forward current is 1 amp at zero deg. At the bottom of the coil, the reflected current is 1 amp at zero deg. At the bottom of the coil, the standing wave current is 2 amps at zero deg. At the top of the coil, the forward current is 1 amp at -45 deg. At the top of the coil, the reflected current is 1 amp at +45 deg. At the bottom of the coil, the standing wave current is 1.4 amp at zero deg. I asked if you knew how to do phasor math but you trimmed out that phasor math part of my posting. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
K7ITM wrote:
Cecil wrote, among other things, "One amp of forward current is flowing into the coil and one amp of forward current is flowing out of the coil. Charge is balanced." Absolutely NOT! You said the phase difference between the two ends is 45 degrees. Therefore, charge "input" and "output" is balanced ONLY twice during a cycle, when the instantaneous currents are the same. No phase need apply he we're talking INSTANTANEOUS currents. Give us a break, Tom. Of course, we are *NOT* and never have been talking instantaneous currents. All currents ever discussed concerning this subject have been RMS currents. That's just your instantaneous strawman. Long term charge accumulation is averaged over many cycles. There is simply none of that because the traveling waves are not storing any net charge inside the coil. How can you get so desperate as to play such silly games? My statement obviously meant: One amp of RMS forward current is flowing into the coil and one amp of RMS forward current is flowing out of the coil. Average charge is balanced. Even though the standing wave current is different at each end of the coil, the average charge into and out of the coil is still balanced. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Roy Lewallen wrote:
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. The main effect in a standing wave environment are the forward and reflected phasors rotating in opposite directions. The standing wave current is ZERO when those phasors are 180 degrees out of phase. The standing wave current is maximum when those phasors are in phase. "Current going to ground through the C" is not even required. But with zero length, there can be no standing waves inside the inductor. You keep saying stuff like this as if a zero length inductor actually existed in reality. Wake up, Roy, and smell the roses. That zero length inductor exists only in human minds. When you look at the currents reported by EZNEC for the model on Cecil's web page, the current at the top of the coil is the equivalent to the *network* current described above. It's the current flowing through the inductance minus the current being shunted to ground via the C between the coil and ground. Huh? How do you explain the current at the top being greater than the current at the bottom of the coil? Is the coil sucking current from the ground? -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Cecil Moore wrote: Roy Lewallen wrote: 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. The main effect in a standing wave environment are the forward and reflected phasors rotating in opposite directions. The standing wave current is ZERO when those phasors are 180 degrees out of phase. The standing wave current is maximum when those phasors are in phase. "Current going to ground through the C" is not even required. That's utter nonsense Cecil, and why people aren't buying into your misconceived theories. Maybe you can take some time to rethink your position while on vacation. A two-terminal network that transforms impedance, now there's a concept! An inductor behaves exactly the same way in or out of your so-called standing wave environment. It follows the same rules all the time. Since your theory says otherwise, it has to be wrong. Wave theory is just another way of analyzing a complex system. It doesn't change how things inside the system behave. 73 Tom |
Current through coils
Cecil Moore wrote: wrote: The point I (and others) tried to make was that in a small inductor current was essentially equal at both ends of the coil, ... A 75m bugcatcher coil is NOT a small inductor. It is a slow- wave structure with a velocity factor of about 0.017, both measured and calculated. That gives my bugcatcher coil an electrical length at 4 MHz of about ~60 degrees. 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 |
Current through coils
Cecil Moore wrote:
Ian White GM3SEK wrote: You see a larger picture of the whole antenna, so you can choose many different ways to theorize about it. But your theory cannot be correct if it requires that components behave in different, special ways according to the way you happen to be thinking about it at the time. Inuendo devoid of any technical content, Ian? Precisely and specifically NOT that! Let me have one last try: The human observer sees a larger picture of the whole antenna, and can choose many different ways to theorize about it. But a theory cannot be correct if it requires that components behave in different, special ways according to the way a person happens to be thinking about it at the time. If you cannot see that statement as a fundamental principle of scientific logic, then I have run out of ways to tell you. Replacing the part of my previous message that you snipped: Electronic components... have no conception of traveling or standing waves. They react simply to the voltages and currents they experience at their terminals. They cannot behave in different ways for different types of current. If you want to analyse the current into different parts and give them different labels, a pure, lumped loading inductance MUST still respond to every kind of current in the same way. It is not my theory. It is the distributed network model which you apparently reject. No, I reject your incorrect applications. The reasons may look simple but they are absolutely fundamental. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Current through coils
Ian White GM3SEK wrote:
Precisely and specifically NOT that! :-) "My theory"? It's not my theory. Components behaving differently? No. Special ways according to my thinking? Of course not. There's nothing special. The "special magic thinking" is yours in thinking that standing wave current is the same as traveling wave current. If you cannot see that statement as a fundamental principle of scientific logic, then I have run out of ways to tell you. I see your statement for exactly what it is, Ian, full of inuendo and ignorance of the nature of standing wave current. Have you no clue what func(kx)*func(wt) really means? It is not my theory. It is the distributed network model which you apparently reject. No, I reject your incorrect applications. You reject the distributed network analysis because you are completely technically ignorant of the nature of standing wave current. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
wrote:
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. Well, I've been challenging you to do just that for weeks now and so far, nothing. Please note the contradiction between your statement above which says an antenna doesn't have to be 90 degrees long to be resonant and your statement below which says it does. Would you please make up your mind? 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. This again demonstrates your misconceptions. Please pay attention this time. When my 75m bugcatcher coil is configured as a base-loaded coil with a 7 foot whip, it occupies ~60 degrees of antenna. The 7 foot whip occupies ~10 degrees of the antenna. The total length is only ~70 degrees, not 90 degrees. That 90 degrees is just your strawman and you even contradicted yourself above. The antenna doesn't have to be 90 degrees long. What has to happen is for (Vfor+Vref)/(Ifor+Iref) to be resistive at the feedpoint. There are many possibilities for that in antennas not 90 degrees long. I gave one such example possibility weeks ago. Perhaps you missed it. I haven't measured the number of degrees occupied by a center- loaded 75m bugcatcher coil. Since the inductance of the center-loaded coil must be increased when moved from the base to the center, it would occupy more of the antenna at the center than it does at the base for the same resonant frequency. The 70uH 75m bugcatcher coil occupies ~60 degrees when installed at the base. For the same resonant frequency and same length for the rest of the antenna, a center-loaded coil would need about double that reactance, making it about 1.4 times the size of the base-loaded coil. So I would estimate that the center-loaded coil is occupying ~80 degrees of the antenna, much closer to a total of 90 degrees than the base-loaded version. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Cecil Moore wrote:
Ian White GM3SEK wrote: Precisely and specifically NOT that! :-) "My theory"? It's not my theory. Components behaving differently? No. Special ways according to my thinking? Of course not. There's nothing special. The "special magic thinking" is yours in thinking that standing wave current is the same as traveling wave current. If you cannot see that statement as a fundamental principle of scientific logic, then I have run out of ways to tell you. I see your statement for exactly what it is, Ian, full of inuendo Let us repeat the statement, then: The human observer sees a larger picture of the whole antenna, and can choose many different ways to theorize about it. But a theory cannot be correct if it requires that components behave in different, special ways according to the way a person happens to be thinking about it at the time. That statement was not innuendo at all. It means nothing more than what it literally says. It applies to any and every observer who attempts to construct a theory about something. Everybody is included; but nobody is exempt. and ignorance of the nature of standing wave current. Have you no clue what func(kx)*func(wt) really means? It is not my theory. It is the distributed network model which you apparently reject. No, I reject your incorrect applications. You reject the distributed network analysis because you are completely technically ignorant of the nature of standing wave current. That is a close to perfect mirror-image of my views on the positions you take. The difference is that my views join up with the rest of human knowledge about antennas and circuit behaviour. Yours don't. They fail that crucial test. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Current through coils
Cecil,
Well, I guess it's back to the math books for me. I mistakenly thought that currents described by cos(kz-wt) and cos(kz).cos(wt) would be considered "instantaneous" currents. If they're really RMS, well . . . I am curious about one thing, however. It would seem that all of this "averaging", "RMS", and "net" is a bit inconsistent with digging into a distributed network problem, which you insist is the only valid description. Everything can vary in time and space in a distributed network. Certainly these consolidating functions are useful for a general overview, but how can you learn anything about the details of a complex system by averaging and netting? 73, Gene W4SZ Cecil Moore wrote: Give us a break, Tom. Of course, we are *NOT* and never have been talking instantaneous currents. All currents ever discussed concerning this subject have been RMS currents. That's just your instantaneous strawman. Long term charge accumulation is averaged over many cycles. There is simply none of that because the traveling waves are not storing any net charge inside the coil. How can you get so desperate as to play such silly games? My statement obviously meant: One amp of RMS forward current is flowing into the coil and one amp of RMS forward current is flowing out of the coil. Average charge is balanced. Even though the standing wave current is different at each end of the coil, the average charge into and out of the coil is still balanced. |
Current through coils
Ian White GM3SEK wrote:
That statement was not innuendo at all. It means nothing more than what it literally says. It applies to any and every observer who attempts to construct a theory about something. Everybody is included; but nobody is exempt. It means the lumped-circuit model works where the distributed- network model fails. That is false. It is just the opposite. the distributed-network model works where the lumped-circuit model fails. The difference is that my views join up with the rest of human knowledge about antennas and circuit behaviour. Only up to where the coils are 15 degrees long. Then the distributed network model must be engaged to avoid blunders exactly like you and others are making. Yours don't. They fail that crucial test. Distributed network analysis fails the test??? Please provide an example. The IEEE would probably publish a paper on such. -- 73, Cecil http://www.qsl.net/w5dxp |
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
John Popelish wrote:
wrote: A two-terminal network that transforms impedance, now there's a concept! (My opinion follows, please correct me. Dang, I should put that in my sig.) In reality, there is no such thing as a two terminal network, unless one of those terminals is grounded. For all other cases, there is an unavoidable implied ground terminal that covers all the stray capacitance of the device. So the bug catcher coil is recognized as a 3 terminal device, with ground being the third terminal. It can be modeled as a pi, T or transmission line structure, as long as you want to understand what to quantify it at only one frequency (or a narrow band), and the choice is arbitrary. If you are concerned with modeling a large frequency range (that goes well past the first self resonance), one of those models (or a more complicated one) will be superior. You fellows lack imagination. As long as you're trying to morph a coil into a transmission line, why not just imagine it as a shorted stub? There's more than one way to make an inductive reactance. 73, Tom Donaly, KA6RUH |
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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