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Current through coils
Roy Lewallen wrote:
John Popelish wrote: . . . I think I agree with just about every conclusion you are making about treating coils as slow wave transmission lines. . . A coil itself isn't a slow wave transmission line. Not at all? It seems to me that any real, physical inductor must have some lumped properties and some transmission line properties, and it is the balance of these that must be considered in any particular case to decide which analysis is the more accurate way to deal with it in a circuit. Solenoidal air core inductors have a lot of transmission line properties if the frequency is high enough. If this were not so, they would look exactly like fixed capacitors above self resonance, instead of having multiple impedance peaks and valleys. In conjunction with shunt C, it can be analyzed as a transmission line, but only in conjunction with shunt C. 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. 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? The earlier example of the modification to Cecil's EZNEC model illustrated this -- when the ground (the other side of the shunt capacitor) was removed, the current drop across the coil disappeared. 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. 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. |
Current through coils
Cecil Moore wrote:
John Popelish wrote: It may be you or I may have an untied shoe. In any case, Jim, oops, I mean John, here's the IEEE Dictionary's definition of current. "current - The flow of electrons within a wire or a circuit: measured in ampheres." And no, there is no definition for "current flow" in the IEEE Dictionary. "Current flow" and "power flow" are commonly used terms to signify "charge movement" and "energy movement". Objecting to the use of the words "current flow" is really picking at infinitessimal nits. Current flow is an informal expression that is used by people who only conversationally acquainted with electricity. That, of course, doesn't explain why I occasionally slip up and say it. I am just trying to eliminate as many pick points from your position as possible, to reduce the side trips into strawman wars. |
Current through coils
Yuri Blanarovich wrote: The conclusion was: "There IS a drop, difference in RF current across the antenna loading coil." The significance (to me anyway): Efficiency of the radiator, antenna is proportional to the area under the (cosine) curve of the current distribution across the radiator. That is where we disagree. While it has been years, as I recall you claimed the electrical degrees the inductor replaced caused the slope across the inductor. The point I (and others) tried to make was that in a small inductor current was essentially equal at both ends of the coil, and any change had to be caused by capacitance from the coil to the outside world that was large compared to the termination impedance at the top of the inductor. It really is a shame you flew off the handle so fast and we didn't talk through the problem. That's why misunderstandings start and drag on for years. This has become a "let's get him" thing instead of "let's figure out how it works". So in the typical loaded (mobile or shortened) vertical we are trying to maximize the efficiency and it is important to know what is the current distribution across the radiator. If the coil has a drop in order of 40 - 60% as it appears to be, than that is significant to me to take it into the account. Knowing how to apply this effect will allow me to optimize, maximize the antenna performance. If you look at the measurements at: http://www.w8ji.com/mobile_antenna_c...ts_at_w8ji.htm you'll see for a given antenna structure, I can change the current distrution all around. The current in a small loading coil of reasonable form factor is essentially uniform at both ends of the inductor. This is because the inductor does not replace a certain "electrical degrees" and have a cosine current drop related to those degrees. Any drop in current is caused by displacement current from the inductor to the outside world. By the way, this is DIFFERENT than the self-resonance capacitance Cecil refers to. The capacitance causing a self-resonance is actually a mixture of capacitance to the outside world (that DOES change distribution) and capacitance from turn to turn (that does NOT change slope of current except by how it affects effective inductance). 3. Then Tom, W8JI and his followers, with some "backing" from literature (plenty are wrong), some experiments, modeling, came to "prove" that it can't be so. His conclusion: "The current in the antenna loading coil is the same at both ends". Then the "fight" and controversy started. It appears to me that JI camp is coming from the theoretical end of it, applying laws of physics and theories that do not apply to the case in question. First, it is not "my camp". I know people like to make things like this personal issues, but they really are not. How things work are how things work. I like to learn how things work just as much as anyone else. The problem is when people start getting personal and saying things they would never dare say to another person's face, I get uncooperative. Most people behave that way. Putting personal issues aside, anything can be resolved. 5. Not so fast. JI camp vehemently defended their "equal current" case, using examples, modeling, tailored to support their claims, for some reason ignoring the reality, measurements, experiments done to set the coil in the spot where current can be, and is the same (no argument with that). I can make current virtually equal at virtually any spot, and make it very unequal at virtually any spot, just by changing the quality and physical size of the loading inductor. I'll bet money on this, provided we use real instruments. The only time current will be substantially unequal will be when the inductor has a large amount of capacitance to the outside world (acting like a distributed network of displacement C's and series L's with poor coupling) compared to the termination impedance at the inductor's top. I can take an antenna of specific height and vary current taper in the inductor quite a bit just by changing the style of loading coil. It is the idea that the loading coil drops a certain current because of "electrical degrees" that is so untrue. 10. According W8JI camp, looking at the quarter wave loaded whip, the current goes up the radiator according to cosine curve, then is the same across the coil, then tapers to zero at the tip in the triangular shape (should be the rest of the cosine curve, but close enough approximation). We are talking about typical loaded resonant quarter wave ant, (not any coil in any circuit). Again, this isn't my camp. Repeatedly trying to make this a personal issue really just stops the scientific process. The current distribution described above is indeed how an antenna works. This of course assumes the inductor is compact and has minimal distributed capaciatnce to the outside world compared to the termination impedance presented by the whip. It can be proven. 73 Tom |
Current through coils
K7ITM wrote:
(Yawn) So, I have this system where there's a wave in each direction and they are identical amplitudes so that there is zero loss to radiation or thermal dissipation. And in this system there is a series coil through which the waves pass, and the current at each end of the coil is different amplitude. That means that the coulombs/second passing a point at one end of the coil is different than the coulombs/second passing the other end of the coil. The currents can be in phase or counter-phase. In fact, if the phases of the currents at the two ends of the coil were not the same, then even equal-amplitude currents at each end would imply that, except at certain instants of time, there are differing coulombs/second passing the points at either end of the coil. 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. That is the shunt capacitance to the rest of the universe from each part of the coil or transmission line that momentarily stores this charge. So, I guess the word you are trying to get me to say is "capacitance". Nobody says it is "freakin' special", though. Its common as dirt. What do I win? |
Current through coils
John Popelish wrote:
If you can measure phase, you can see that it is opposite on opposite sides of a node. There is a 180 degree phase shift each time the measurement passes over a node. Do you disagree? Yes, but you can tell that from the amplitude being zero. 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? One could wear a blindfold and use no hands and have an even greater challenge. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
K7ITM wrote:
(Yawn) So, I have this system where there's a wave in each direction and they are identical amplitudes so that there is zero loss to radiation or thermal dissipation. And in this system there is a series coil through which the waves pass, and the current at each end of the coil is different amplitude. That means that the coulombs/second passing a point at one end of the coil is different than the coulombs/second passing the other end of the coil. Sorry, you are wrong about that. Here's why. For simplicity, let's assume the coil is lossless, 45 degrees long, and the forward and reflected current magnitudes are both equal to one amp. These assumptions are for purposes of illustration only. 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. One amp of reflected current is flowing into the coil and one amp of reflected current is flowing out of the coil. Charge is balanced. Note there is ZERO charge imbalance in the coil. The forward and reflected currents are all there are and they are balanced. The forward current at the bottom of the coil is 1 amp at zero degrees. The reflected current at the bottom of the coil is 1 amp at zero degrees. Adding them together yields a standing wave current of 2 amps at zero degrees. Do you know how to do phasor math? The forward current at the top of the coil is 1 amp at -45 degrees. The reflected current at the top of the coil is 1 amp at +45 degrees. Adding them together yields a standing wave current of 1.414 amps at zero degrees. Do you know how to do phasor math? The standing wave current at the bottom of the coil is 2 amps. The standing wave current at the top of the coil is 1.414 amps. THERE IS *NO CURRENT IMBALANCE* BECAUSE THAT STANDING WAVE CURRENT IS NOT REALLY FLOWING. IT IS JUST STANDING THERE. That's the entire point. What happens to that imbalance in charge? Imbalance in charge is a myth, an old wives' tale. There is NO imbalance in charge. SEE ABOVE! -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
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. The reflected current is equal at both ends of the coil. That takes care of any question of charge imbalance. There simply isn't any. 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. Now do you see why standing wave current is considered not to be flowing? -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
John Popelish wrote:
Roy Lewallen wrote: John Popelish wrote: . . . I think I agree with just about every conclusion you are making about treating coils as slow wave transmission lines. . . A coil itself isn't a slow wave transmission line. Not at all? It seems to me that any real, physical inductor must have some lumped properties and some transmission line properties, and it is the balance of these that must be considered in any particular case to decide which analysis is the more accurate way to deal with it in a circuit. Solenoidal air core inductors have a lot of transmission line properties if the frequency is high enough. If this were not so, they would look exactly like fixed capacitors above self resonance, instead of having multiple impedance peaks and valleys. In conjunction with shunt C, it can be analyzed as a transmission line, but only in conjunction with shunt C. 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. 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. 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. 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. 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. 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. The earlier example of the modification to Cecil's EZNEC model illustrated this -- when the ground (the other side of the shunt capacitor) was removed, the current drop across the coil disappeared. 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. We shouldn't confuse an ordinary lumped LC transmission line approximation with a true slow wave structure such as a helical waveguide (next item). 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. 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. 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. 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. 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. (*) 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. 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. 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. 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. Roy Lewallen, W7EL |
Current through coils
Roy Lewallen wrote:
Well, not a "slow wave" transmission line. We shouldn't confuse an ordinary lumped LC transmission line approximation with a true slow wave structure such as a helical waveguide (next item). 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. Dr. Corum gives a formula for calculating the velocity factor of coils which meet a certain criteria. My 75m bugcatcher coil meets that criteria. It's velocity factor calculates out to be 0.0175. It's measured velocity factor is 0.015. That sounds like a "slow wave" device to me. 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. The subject is 75m bugcatcher loading coils mounted on GMC pickups. How the heck does the ground get removed? 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 ... A 75m bugcatcher coil is not small. -- 73, Cecil http://www.qsl.net/w5dxp |
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