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Current through coils
Tom Donaly wrote:
This whole thing boils down to an engineering question, anyway, which is, is it possible to engineer a loading coil to be small enough at the lower end of the HF spectrum so that it can be modeled using network analysis? That situation can be called pure single-point inductive loading. It may not be totally practical, but it is still vital to this discussion. There is an infinite range of real-life loading coils of various shapes and sizes. Pure single-point inductive loading is the limiting case that marks one end of that range. Any successful theory has GOT to get this case right - and if it can't, it fails. Regardless of the actual method used, any correct analysis of the whole antenna MUST conclude that, for the limiting case of pure inductive loading, the voltage/current/phase relationships at the loading inductance are the SAME as those predicted by conventional circuit analysis. This limiting case is where the two kinds of analysis come together, and here they MUST agree. That means a correct analysis for the whole antenna MUST predict zero phase shift in the current (It = I0 cos wt) between the terminals of the loading inductance. This requirement only applies for pure inductance, and only at that single point where the inductor is inserted into the antenna; but for that limiting case the requirement to join up with circuit theory is real, absolute and non-negotiable. Let's be clear: in this context, "current" is the plain ordinary alternating current that we learned about in school: It = I0 cos wt. It is the simple back-and-forth movement of electrons (charge) past a given point. Nobody denies that for real-life loading coils there can be a phase shift in the current from end to end, and that it will become larger as the coil becomes longer and skinnier. That isn't the question I'm addressing here. But the question of what happens when the coil shrinks down to become a single-point loading inductance is equally important: it cannot be evaded, and it is a definitive deal-breaker. It's hard to tell for sure from the avalanche of messages, but Cecil's analysis apparently fails in the limiting case of pure inductance - or rather, he seems to deny that the test is even a valid one. In principle there is nothing wrong with attempting a traveling-wave analysis for a loaded whip. Done correctly, it will give the right results that join up seamlessly with circuit theory as well. It's just that Cecil has NOT done it correctly. I think there are several reasons, and until he corrects them all, his theory will continue to fail... and he will continue in denial of that. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Current through coils
Ian White GM3SEK wrote:
. . . In principle there is nothing wrong with attempting a traveling-wave analysis for a loaded whip. Done correctly, it will give the right results that join up seamlessly with circuit theory as well. . . . One of the tests the traveling wave analysis must pass is that the results from forward current wave excitation plus the results from reverse current wave excitation must equal the results from excitation by the sum of the two, i.e., the total current. This is required by superposition, whether the network is lumped or distributed. And analysis based on a distributed model, as Ian says, must converge to the same results as a model with lumped components as the physical sizes of the components get very small. Analyses of the examples using lumped models with total current have been entirely adequate to explain the observed inductor currents. Roy Lewallen, W7EL |
Current through coils
Roy Lewallen wrote: Ian White GM3SEK wrote: . . . In principle there is nothing wrong with attempting a traveling-wave analysis for a loaded whip. Done correctly, it will give the right results that join up seamlessly with circuit theory as well. . . . One of the tests the traveling wave analysis must pass is that the results from forward current wave excitation plus the results from reverse current wave excitation must equal the results from excitation by the sum of the two, i.e., the total current. This is required by superposition, whether the network is lumped or distributed. Cecil claims I'm the ONLY one who disagrees with him! Richard C., Ian, Roy, Gene, and Tom D. also appear to not be on Cecil's team. 73 Tom |
Current through coils
wrote:
Roy Lewallen wrote: Ian White GM3SEK wrote: . . . In principle there is nothing wrong with attempting a traveling-wave analysis for a loaded whip. Done correctly, it will give the right results that join up seamlessly with circuit theory as well. . . . One of the tests the traveling wave analysis must pass is that the results from forward current wave excitation plus the results from reverse current wave excitation must equal the results from excitation by the sum of the two, i.e., the total current. This is required by superposition, whether the network is lumped or distributed. Cecil claims I'm the ONLY one who disagrees with him! Richard C., Ian, Roy, Gene, and Tom D. also appear to not be on Cecil's team. The only thing those people have in common is a desire to get the facts right... and physical reality allows no compromises. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Current through coils
Ian White GM3SEK wrote:
To all: Ian is not addressing the issue which is: Can a standing wave current phase measurement be used to tell us anything about the phase shift through a loading coil? The answer is NO! W7EL's phase measurements are flawed. Therefore, they cannot be presented as evidence of anything valid. They certainly cannot be used to prove that a coil is a point inductance. The question is not whether the two models agree on the low end - they do agree. The question is whether they agree on the high end - they don't and they are not supposed to. The differences I have presented in the two models is where the lumped circuit model fails and the distributed network model is valid. There is an infinite range of real-life loading coils of various shapes and sizes. Pure single-point inductive loading is the limiting case that marks one end of that range. Any successful theory has GOT to get this case right - and if it can't, it fails. Since the distributed network model is a superset of the lumped circuit model, it does get that case right. It also gets the case right at the other end of the range where the lumped circuit model fails. 75m mobile loading coils cannot be validly modeled using the lumped circuit model. Regardless of the actual method used, any correct analysis of the whole antenna MUST conclude that, for the limiting case of pure inductive loading, the voltage/current/phase relationships at the loading inductance are the SAME as those predicted by conventional circuit analysis. This limiting case is where the two kinds of analysis come together, and here they MUST agree. And they do, no question about that. That means a correct analysis for the whole antenna MUST predict zero phase shift in the current (It = I0 cos wt) between the terminals of the loading inductance. Here you are allowing your model to dictate reality, not vice versa. A practical antenna is a large structure, usually at least an electrical 1/4 wavelength. There is no such thing as a point inductance in a real world mobile ham antenna. Let's be clear: in this context, "current" is the plain ordinary alternating current that we learned about in school: It = I0 cos wt. It is the simple back-and-forth movement of electrons (charge) past a given point. Let's be clear. That model fails in a transmission line as it does in standing wave antennas. Taking a simple-minded approach to physics is where the air, earth, fire, and water elements came from. Nobody denies that for real-life loading coils there can be a phase shift in the current from end to end, and that it will become larger as the coil becomes longer and skinnier. That isn't the question I'm addressing here. But the question of what happens when the coil shrinks down to become a single-point loading inductance is equally important: it cannot be evaded, and it is a definitive deal-breaker. In that case, both models give the same answer. But that case doesn't exist in reality in real-world antennas. In reality, the lumped circuit model fails when it is extended to large structures like transmission lines and antennas. It's hard to tell for sure from the avalanche of messages, but Cecil's analysis apparently fails in the limiting case of pure inductance - or rather, he seems to deny that the test is even a valid one. The distributed network analysis works perfectly in the limiting case since it is a superset of the lumped circuit model. There is absolutely no disagreement between the distributed network analysis and the lumped circuit analysis for point inductors. Anyone who says there is is just attempting to set up a strawman. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Roy Lewallen wrote:
One of the tests the traveling wave analysis must pass is that the results from forward current wave excitation plus the results from reverse current wave excitation must equal the results from excitation by the sum of the two, i.e., the total current. This is required by superposition, whether the network is lumped or distributed. It certainly does that within the bounds of the principle of superposition. But as I earlier pointed out, like two superposed PSK modem signals, phase information is lost in the superposition. You used standing wave current phase to try to measure the phase shift through a coil. Your attempt was futile since the standing wave current phase doesn't contain any phase information. And analysis based on a distributed model, as Ian says, must converge to the same results as a model with lumped components as the physical sizes of the components get very small. And it certainly does. But the distributed network model works for antennas and transmission lines where the lumped circuit model fails. The lumped circuit model is supposed to fail for transmission lines and antennas. Analyses of the examples using lumped models with total current have been entirely adequate to explain the observed inductor currents. Maybe for you, Roy, but not for me and others. Are the four elements of earth, air, water, and fire adequate to explain the physical world to your satisfaction? -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Ian White GM3SEK wrote:
The only thing those people have in common is a desire to get the facts right... I have asked you guys to explain the technical facts behind about a dozen technical questions of mine. The silence has been deafening. Many readers of this newsgroup have noticed the same thing. Here are the technical facts about the two models. At one end of the spectrum, we have lumped inductance. At the other end we have physically huge coils. The crossover point where the lumped circuit model becomes invalid is about 0.04 wavelength. A mobile antenna is a lot longer than that. At lengths above, 0.04 wavelength, standing waves have to be taken into account. The lumped inductance model cannot take standing waves into account. It assumes instantaneous faster-than-light propagation of current. DC|-------------distributed network model valid----------------| DC|---lumped circuit model valid---| 0.04WL cutoff Here are the two main technical points: 1. Nothing valid is proven by using standing wave current phase to measure anything. Gene F. and Tom D. seem to realize that. 2. The taper of the standing wave current through a coil depends upon where the coil is placed in the standing wave environment. You guys have looked only at mobile antennas. Try looking at longer antennas like the one at: http://www.qsl.net/w5dxp/current.htm You will find that the current taper through a coil can be zero, positive, negative, or even reverse phase. The phase reversal can be considered to be current flowing into both ends of the coil at the same time. Please come out of the deep dark lumped circuit cave and see what the rest of the world is like. You guys have been seduced by your model. You have assumed the presuppositions of the model are valid without technical proof. Here's an analogy: Ian W. says: I believe water is one of the basic elements and as proof, I offer a quart of water. Roy L. says: I believe earth is one of the basic elements and as proof, I offer a bucket of earth. Tom R. says: I believe fire is one of the basic elements and as proof, I offer this burning torch. Richard C. says: I believe air is one of the basic elements and as proof, I offer this balloon full of it. Cecil says: I believe there are over 100 elements and as proof, I offer this periodic chart of those elements. I will be on spring break until Monday so don't think I have given up on getting the technical facts out there for all to see. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Cecil,
Sorry, Ian had it exactly correct. I cannot speak for Roy or others, but I am quite sure I did not take any "side" in this topic. I believe in lumped circuit models, and I believe in network models. My aim was to try to correct some of the basic math and physics flaws, not to argue over the exact regimes of applicability for lumped and non-lumped models. If you choose to use my postings as support for your "side", so be it. At the same time, if proving your "side" includes mobile antennas with 48 foot whips and giant bugcatcher coils working at three times their self-resonant frequency, then please leave me off of your "side". 73, Gene W4SZ Cecil Moore wrote: wrote: Cecil claims I'm the ONLY one who disagrees with him! False! Gene Fuller, Tom Donaly, and Roy Lewallen have all made postings that support my side of the argument against yours. That doesn't mean they agree with me 100%. They agree with me on some technical things that are not in dispute except by you. |
Current through coils
Cecil Moore wrote:
To all: Ian is not addressing the issue which is: Can a standing wave current phase measurement be used to tell us anything about the phase shift through a loading coil? The answer is NO! I flatly do not accept your notion of a special kind of "standing wave current" that has its own special kind of phase properties. The current that the loading coil experiences is plain old ordinary alternating current flowing in the wire - the simple movement of electrons back and forth past a point, according to It = I0 cos wt. That is also the current that an RF ammeter reports, and the current that a computer simulation reports too. Any special kind of current that requires electronic components to behave in some different way from normal is simply not real. You have a fundamental misconception of what a standing wave of current really is. You repeat all the words about "standing waves", "cos kz", "scientific logic", "laws of physics" etc; but you don't actually let any of it into your mind. All the questions you ask other people are rooted in your own misconceptions. In other words, the questions are rigged so that they cannot be answered except by agreeing with you. And if someone very sensibly does not answer - why, you "win" that way too. I don't believe you realise how regularly you do this. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Current through coils
Gene Fuller wrote:
I am quite sure I did not take any "side" in this topic. I assume you are on the side of technical facts as am I. I believe in lumped circuit models, and I believe in network models. My aim was to try to correct some of the basic math and physics flaws, not to argue over the exact regimes of applicability for lumped and non-lumped models. Your func(kx)*func(wt) Vs func(kx +/- wt) posting contributed a good deal of technical accuracy to the discussion and I thank you for that. At the same time, if proving your "side" includes mobile antennas with 48 foot whips ... then please leave me off of your "side". And there's the blind spot. The two antennas are identical except for the 40 foot extension. The coil doesn't know the extension is there. Why did the slope of the current taper reverse between those two configurations? How does the lumped-circuit model explain more current "flowing" out of the top of the coil than is "flowing" into the bottom of the coil? It's a simple question but the answer has been conspicuous by its absence. I will keep asking that question until someone answers it. The distributed network analysis handles both of those configurations in a valid way. Adding 40 feet to an antenna is no problem. The relative phases of the forward and reflected currents changed - that's all that happened. But by adding 40 feet of wire to the antenna, the lumped circuit analysis falls completely apart. So how do we know it was valid for the 8 foot antenna where everything is the same except for length? We don't! Since the lumped circuit analysis falls apart by adding 40 feet to an antenna, I contend that the lumped circuit analysis fell apart with the 8 foot antenna but you guys don't realize it yet. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Ian White GM3SEK wrote:
I flatly do not accept your notion of a special kind of "standing wave current" that has its own special kind of phase properties. We already know that, Ian. Please drag out your dusty math book and try to understand the difference between the standing wave current function, func(kx)*func(wt), and the traveling wave current function, func(kx +/- wt). They are obviously different. Calling the standing wave current a "current" is something of a misnomer since it doesn't exhibit the characteristics of a normal current at all. What are the implications of a "fixed phase" for a current, i.e. its phasor doesn't rotate? Do you disagree with Gene's technically accurate posting on the subject? ************************************************** *************************** Gene Fuller, W4SZ wrote: In a standing wave antenna problem, such as the one you describe, there is no remaining phase information. Any specific phase characteristics of the traveling waves died out when the startup transients died out. Phase is gone. Kaput. Vanished. Cannot be recovered. Never to be seen again. The only "phase" remaining is the cos (kz) term, which is really an amplitude description, not a phase. The so-called "phase reversal" in longer antennas is not really about phase either. It is merely a representation of the periodic sign reversal seen in a cosine function. ************************************************** **************************** The current that the loading coil experiences is plain old ordinary alternating current flowing in the wire ... False! Standing wave current is different from DC, AC, or RF traveling waves. Please take time out to understand the implications of a non-rotating phasor for a current. All other AC currents have rotating phasors but the standing wave current phasor doesn't rotate all up and down a 1/2WL thin-wire dipole. That makes it extremely different from any other AC current. Any special kind of current that requires electronic components to behave in some different way from normal is simply not real. The forward current and the reflected current are not special. The superposed standing wave current doesn't behave as normal current at all. It's phase doesn't change along the entire length of a 1/2WL thin- wire dipole. I have said this a dozen times and it hasn't yet soaked in yet so I will continue to repeat it. What does unchanging phase imply about a standing wave current? All other AC currents change phase. You have a fundamental misconception of what a standing wave of current really is. You repeat all the words about "standing waves", "cos kz", "scientific logic", "laws of physics" etc; but you don't actually let any of it into your mind. I am open-minded, Ian, and use the scientific method to correct my mistakes and thus zero in on the technical facts. One of a guru's presuppositions is that he already knows everything. I have no such misconceptions about myself. All the questions you ask other people are rooted in your own misconceptions. In other words, the questions are rigged so that they cannot be answered except by agreeing with you. No, Ian, my questions are rigged so they cannot be answered except by agreeing with the laws of physics and gurus cannot afford to show their ignorance of the laws of physics. That leaves them between a rock and a hard place as far as answering my questions are concerned. That's the only reason for the "Silence of the Gurus". -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
On Wed, 22 Mar 2006 13:08:37 GMT, Cecil Moore
wrote: False! Gene Fuller, Tom Donaly, and Roy Lewallen have all made postings that support my side of the argument against yours. I, on the other hand (barring these misappropriated citations), have busted this thread four times. It lingers on in soap opera format for amusement purposes only. |
Current through coils
Richard Clark wrote:
I, on the other hand (barring these misappropriated citations), have busted this thread four times. In your mind, delusions of grandeur certainly seem to work. :-) -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
On Wed, 22 Mar 2006 16:23:42 GMT, Cecil Moore
wrote: I, on the other hand (barring these misappropriated citations), have busted this thread four times. In your mind, delusions of grandeur certainly seem to work. :-) Through logic, on paper, at the bench, by gum. It certainly shut you up. |
Current through coils
Cecil Moore wrote:
We already know that, Ian. Please drag out your dusty math book and try to understand the difference between the standing wave current function, func(kx)*func(wt), and the traveling wave current function, func(kx +/- wt). They are obviously different. Calling the standing wave current a "current" is something of a misnomer since it doesn't exhibit the characteristics of a normal current at all. What are the implications of a "fixed phase" for a current, i.e. its phasor doesn't rotate? This can be improved. Current is charge movement. DC, traveling waves and standing waves all are *exactly* charge moving past a given point, and nothing more. A phasor is not a real thing, but a mathematical abstraction that relates how the sinusoidal change in current magnitude and direction relate to a reference periodic cycle in time. Since, in both standing waves and traveling waves, current at a point, changes magnitude and sign in exactly the same way (at a point, they are indistinguishable), they can both be described with phasor notation. The difference between a traveling wave and a standing wave is how the phasor representing the current at one point differs from the phasor representing the current at a neighboring point. For traveling waves, the phasor of a neighboring point has the same amplitude but a different phase shift (passes through zero at a different time). For standing waves, the phasor of a neighboring point has the same phase shift, but a different amplitude, unless the neighboring point is on the other side of a current node. Then it has the opposite phase. But at any point along both standing waves and traveling waves, there certainly is a phasor that represents the current at that point. You need to get past this misconception that standing waves are not current and are not describable by phasors. I think your concepts are correct in lots of ways and recently improving, but this is a recurring snag that keeps detouring your adversaries into straw men that you offer them on a platter. |
Current through coils
Richard Clark wrote: On Wed, 22 Mar 2006 13:08:37 GMT, Cecil Moore wrote: False! Gene Fuller, Tom Donaly, and Roy Lewallen have all made postings that support my side of the argument against yours. There's very little about Cecil's theories that I agree with. He's made many statements which are completely false as I've pointed out numerous times, and his conclusions are largely based on misconceptions and faulty understanding of basic principles. Roy Lewallen, W7EL |
Current through coils
Roy Lewallen wrote:
(snip) I think the problem Cecil is having with it is that the currents on an antenna behave in a manner that's similar to an open circuited transmission line, which results in the phase angle of the total current -- which can be represented as a phasor -- being the same at every point along the line. The total current isn't current which has "stopped flowing". He is so close, but still has a couple misconceptions blocking him. |
Current through coils
John Popelish wrote:
Since, in both standing waves and traveling waves, current at a point, changes magnitude and sign in exactly the same way (at a point, they are indistinguishable), they can both be described with phasor notation. Limiting oneself to a point measurement is handicapping onself. When the equation for standing wave current is compared to the equation for traveling wave current, the real differences are obvious. For standing waves, the phasor of a neighboring point has the same phase shift, ... Exactly! Therefore, it cannot be used to measure the phase shift through a coil or even through a wire. But at any point along both standing waves and traveling waves, there certainly is a phasor that represents the current at that point. For the standing wave current it is a phasor that doesn't rotate all up and down the wire. You have to admit, that's a weird phasor. It's more akin to DC than anything else. You need to get past this misconception that standing waves are not current and are not describable by phasors. Standing waves current is the superposition of two essentially equal currents traveling in opposite directions. If it was equal DC currents traveling in opposite directions, what would the net current be? -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Ian White GM3SEK wrote:
All the questions you ask other people are rooted in your own misconceptions. In other words, the questions are rigged so that they cannot be answered except by agreeing with you. Cecil, W5DXP wrote: No, Ian, my questions are rigged so they cannot be answered except by agreeing with the laws of physics and gurus cannot afford to show their ignorance of the laws of physics. That leaves them between a rock and a hard place as far as answering my questions are concerned. That's the only reason for the "Silence of the Gurus". -- Esteemed gentlemen: I am more of an engineer, inventor rather than scientist, analytic, cosine kind of guy. But I will elevate myself on an occasion to find the root of the problem and proof if needed. Let me try to put the things in perspective, from real life facts, to this "scientwific" debate about the "problem". 1. First I was smacked by the fact that on my 80m Hustler resonator, I re-shrinked, (burned) the insulation on the coil from the bottom up, while operating contest, mobile from Toronto, while spending few last days with my dear mother. I was running about 600W to regular (200W) 80m coil with heavier duty transmitting, during the contest, it was no surprise that it melted the heat shrink tubing covering the turns. Most people make occasional transmission and do not fry the coil even with more that what it is rated for. On 40m and up I rewound other coils with heavy wire or tubing and extended whips. I would not see the problem there. I appeared to me strange that the bottom of the coil was fried. Normally I would have expected either "uniform frying" , or perhaps more on the top due to the raising, accumulating heat. It kind of bothered me, because it was against the conventional "wisdom" or what I knew. 2. When the topic came up, Barry, W9UCW, knew about the effect and done some real life, real antennas, real RF ammeters measurements and found out that indeed, the RF current at both ends of a typical loading coil (quarter wave resonant, coil loaded vertical antenna) is different. He described it, offered some photos and data about the measurements. Please see http://www.k3bu.us/loadingcoils.htm In his example he showed that 40m typical, loaded vertical, had about 40% less current at the top than at the bottom. That made me happy, it made sense and explained what was happening with my Hustler. So I got some insight into the workings of the loaded, shortened radiator or element. Reality agreed with engineering measurements, experiment, that anyone can reproduce and verify. 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. The loading coil, as it turned out, had significant contribution to that current distribution that it got my attention. Knowing the effect, we can play with different position of the loading coil within the radiator and experiment with various ways of top loading, to maximize the current flow within the desired portion of the radiator, to maximize the area under the cosine current curve - to maximize the efficiency under given restriction for the particular antenna design. Here comes the compromise between loading inductance (coil, stub) vs. capacitive hat (T, L, disk) loading. 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. So far with me? If not, ask the questions or tell me where I am wrong. 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. 4. Cecil, W5DXP came to the rescue by explaining and modeling the case and supporting K3BU/W9UCW case and reality. We were happy to have that support, shining more light on the scientific side of the effect and effort to correlate the crude observations, real life measurements with theory, modeling and analytical end of it. 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). 6. Cecil, W5DXP started to dig deeper into the effect, showed that if we model the coil as a loading stub, EZNEC shows the difference in the current. When modeled as a solenoid, it also shows the difference. When the loading is inserted as a simple L (zero physical size) inductance, then EZNEC "proves" that current is the same. So for the same inductance, of different physical model (same electrical) properties EZNEC "proves" that each camp is "right". The only problem is, which case represents the reality and should be taken seriously. To prove the point further, Cecil showed that positioning the coil in various position within the radiator, one can aggravate the difference in current at the ends from one extreme of being equal, to the other extreme of being max at one end and zero at the other, depending on the size of inductor, position within the antenna and frequency applied. To me it makes sense, explains the "workings" of the effect, puts some numbers on it and shows how to model it, apply it and UNDERSTAND it. That's the way IT IS - the reality. Anyone can verify it, do the experiment (otherwise, please prove us wrong and where) and now - even model it in EZNEC when using proper definition of the model coil (solenoid or loading stub of equivalent inductance). 7. No, Cecil, didn't do the stupid mobile antenna trick with quarter wave extension. He used it as an example to show how one can play with the model to see the different situations and show that current can be from equal to extreme (max to zero) across the loading coil. In the EZNEC you can vary the position of the coil (solenoid or stub), length of the radiator and frequency to see the current value and distribution across the coil/radiator. Thanks to Cecil (and "solenoid" Roy), we have now improved way of modeling the effect, understanding it and better way of applying to antenna design. Thanks Cecil, you have golden patience, persistency and are a great defender of the TRUTH! 8. But Noooo! W8JI camp and non-believers can not accept that (reality). They cling to the "explanations" why it can't be so, because of "purple electrons, phasor eating extremists" would not allow it to be so. They are dancing around the facts and examples of why it is not so, not answering latest W5DXP questions, but the main thing - not verifying the reality with proper experiment or measurement (not important?). They are trying to prove that reality isn't, Tesla and others are fools, because of what??? Pride? Can't be wrong? Emperor has equal current clothes? 9. If anyone is trying to argue the case that (eventually) current is (or close to) equal, by arguing that sticking the coil at the base or where there is virtually no difference because of the position of the coil and current distribution in the radiator, forget it. We are not arguing that (and we agree with THAT special case). We are arguing with general statement, as displayed on W8JI pages and argued by the "guru followers" that the "RF current across the antenna loading coil IS (always) the same" (or close, losses, bla, bla) in the TYPICAL loaded or mobile antenna. NOT in the special case where it indeed can be. THAT WAS the subject of the discussion and problem that started all of this. Not current in ANY coil, in ANY circuit. We are not arguing minute diffrences due to radiation, resistance, capacitance surface effect, etc., we are arguing the difference of a 40 - 60 % of current drop across a typical loading coil in a typical mobile or loaded antenna. 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). 11. According to K3BU camp, the same happens as above, except the current across the coil drops at the top of the coil to typical 60 - 40 % (for 80 - 40m) and then tapers to zero at the tip. Again, (I hope no dispute over this) efficiency of the antenna is proportional to the area under the current distribution curve across the radiator with the loading coil. So according to W8JI camp, the loaded radiator would have better efficiency than it really has, by about 60 - 40 % of the above the coil current curve area. 12. To me this is significant and worth exploring, because knowing the effect will allow me to better design, model and optimize antenna systems with loaded elements. The difference in the amount of current in the remaining portion of the antenna is not negligible and the effect will magnify itself when designing multielement loaded antennas. If the model shows wrong amounts and distribution of the current, the results will be off and real antenna will not perform as modeled. Those ARE the currents we consider in modeling, show their distribution off and calculate the antenna parameters with (right Roy?) So, to some the whole argument may be insignificant, to me it is, it should be known, properly considered and applied in the antenna modeling and design, otherwise it could cause inaccuracies and wrong results. 13. So where are we now? We have the effect, we have the measurements, we have explanation, we have the recipe how to model it properly, we have some results. Others can duplicate and verify it. (And answer the frickin Cecil's questions!!!!) Then we have W8JI camp that insists that it ain't so, twisting and dancing trying to "prove" that it just can't be so. Using "arguments" of their own, often contradicting themselves (ooops) that it just couldn't be, because, because, because.... Well, IT IS, if your egos like it or not. It is almost amusing to read the thread and arguments trying to defy the reality by "scinetwific proof". Cecil's arguments and question are selectively ignored and not argued, while new twists and detours are brought in. 14. So if the W8JI camp can not swallow reality, so be it, eventually live with giant egg on the face. I am extremely grateful to W9UCW, W5DXP and others in "our" camp for their contribution to the discussion, explanations, shedding their light and wisdom on the subject (and patience, and persistence). I have learned more about the loaded antennas and will use it especially in the design of receiving arrays for the new Tesla Radioclub salt water marsh 175 acre antenna farm (www.TeslaRadio.org) 15. One more time, friendly advise to Tom, W8JI: If you strongly defend posting the truth on Internet, live by it. It takes big guy to admit being wrong, learn and give credit where it is due! Ridiculing someone being wrong, especially when he is right is not in the spirit of ham radio and decency. Playing guru, engineer and pontificating might go well on TV show but in real life there are consequences. Arguing on a wrong side, then after realizing being wrong, going QRT for a while and then emerging as a guru and portraying the subject as own invention, without giving credit to "arguer" smells with plagiarism. I am saying this here, because I was few times publicly ridiculed by Tom while being right, as he later confirmed by eventually propagating "my gospel" as his own. We should be engaging in civil discussions and arguing about the topics, so we can all learn and be better. If this whole exercise will contribute to that, then it was worth it. If not, then keep on truckin'. 16. To the rest of the readers, I though that by now we all would be on the same "camp reality", but looks like there is still a rocky road ahead. I think we will get better results and go to radio heaven for sticking to the "current in loading coils" truth. I promised comprehensive article on the subject, but life keeps throwing more important problems in my path, but I will try hard to do it soon. We shall summarize our camp's wisdom and hope for convincing case for the unbelievers. I tried to portray the picture of reality and significance of the subject in hope of bringing the refusniks aboard. You all can be the judges of what good it was. We could look at some other "gospels" at Tom's web site, but that is another story when subject comes up. (All in the name of dispelling the myths and old waives tales as Tom likes to do.) I don't like to pick a fight, but I don't stand by when there is a significant myth or wrongo being propagated, I will defend the truth with my last breath (if it is worth it :-) I think 'nuf said, we all did our best to conway the reality, the readers can draw their own conclusions and if important, verify this and that to see who is RIGHT and if they can benefit from the findings. Good luck and I rest my "piece of coil". 73 Yuri Blanarovich, www.K3BU.us |
Current through coils
Cecil Moore wrote:
John Popelish wrote: Since, in both standing waves and traveling waves, current at a point, changes magnitude and sign in exactly the same way (at a point, they are indistinguishable), they can both be described with phasor notation. Limiting oneself to a point measurement is handicapping onself. When the equation for standing wave current is compared to the equation for traveling wave current, the real differences are obvious. I was just making sure we were using the same definitions for things like current and phasors. You are jumping ahead. :-) For standing waves, the phasor of a neighboring point has the same phase shift, ... Exactly! Therefore, it cannot be used to measure the phase shift through a coil or even through a wire. I agree, unless you use phase measurement to hunt for the location of the current nodes that have moved as a result of adding the coil. Finding a phase reversal at opposite ends of the coil, for instance, implies that an odd number of nodes reside in the coil. But at any point along both standing waves and traveling waves, there certainly is a phasor that represents the current at that point. For the standing wave current it is a phasor that doesn't rotate all up and down the wire. A phasor rotates at the reference frequency, and with a phase angle that represents the angular difference between the value in question and the reference cycle. Pick a point on the conductor, and if it carries either a standing or traveling wave (or any combination of traveling waves at the reference frequency), the current at that point is describable as a phasor (having a specific magnitude, and a specific phase with respect to the reference cycle). You have to admit, that's a weird phasor. It's more akin to DC than anything else. This is your mental block. A phasor describes the activity at a point, not whether that activity is a result of an energy wave moving past in one direction, the other, or some combination of those. You need to get past this misconception that standing waves are not current and are not describable by phasors. Standing waves current is the superposition of two essentially equal currents traveling in opposite directions. No. Currents do not travel. Current is the movement of charge past a point. Cyclic current is a sloshing back and forth of charge at some frequency. If you want to picture that process with respect to time, you can refer to it as a cycle or wave, but it is a wave on a scope trace or time graph, not a physical wave of something moving along a wire. The physical wave is charge slushing back and forth along the wire. Both traveling energy waves and combinations of them (standing waves, for example) involve energy traveling in various directions, but the current does not travel. It occurs at a point, as charge moves back and forth past that point. When you can separate the concept of current from the concept of energy waves, you might see this snap into focus. I am not trying to be the guru, here. My earlier posts confused these same concepts, when I mentioned current waves traveling in various directions. I was mistaken, and have seen my error, and am trying to get you to see it, also. I should have been speaking of charge waves that produce current. Correct thinking requires correct speaking. You cannot be sloppy with words and have (let alone express) clear thoughts. This thread has done a lot to help me clear up both my words and thoughts, and I thank you for that. I am not absolutely sure that I have eliminated all mistakes from this way of talking about the process under discussion, so I may have to make some more corrections. That is the reason I am watching this thread. If it was equal DC currents traveling in opposite directions, what would the net current be? Their algebraic sum, same as for non equal currents. Same for any combination of currents that result from charge being shoved back and forth by passing energy waves. Instantaneously the current is the algebraic sum of all components passing through that point. If the components are AC at the same frequency, the sum will be some resultant instantaneous current that varies with that same frequency. I think I agree with just about every conclusion you are making about treating coils as slow wave transmission lines. The nits I am picking is in the language you are using to describe these effects to justify those conclusions. I think terminology is at the root of most of the disagreements in this thread. |
Current through coils
Roy Lewallen wrote:
The total current ("standing wave current" in Cecil's parlance) certainly does have an associated phase angle, and its phasor certainly does rotate. The standing wave current phasor in a 1/2WL thin-wire dipole *DOES NOT ROTATE* and is fixed *CONSTANT AT ZERO DEGREES*. Please reference Figure 14-2 in Kraus' "Antennas for All Applications", 3rd edition, page 464. It shows *ZERO* phase shift in the standing wave current from tip to tip in the 1/2WL thin-wire antenna. It is obvious from the standing wave current equation that the phase angle doesn't change with position. This one misconception is what has got you and Tom totally confused. Please correct your misconception. It's the forward current phasor and reflected current phasor that does the rotating. Since they are rotating in opposite directions, the phasor sum of those two phasors is essentially *CONSTANTLY ZERO ACCORDING TO Kraus*. (By "phase" I mean time phase.) A sinusoidal traveling current wave can be expressed as a phasor whose value is a function of position. When you add a forward traveling wave to a reverse traveling wave, you're adding two phasors. The result is a phasor whose value is the vector sum of those two phasors. This is the total current. It has magnitude and phase like any other phasor, and the same rotational speed as its components. A common manifestation of this is the standing wave pattern along a transmission line. Roy, those two phasors that get added are *ROTATING IN OPPOSITE DIRECTIONS!* The phase of the sum of those two phasors is *CONSTANT*! If you had read: http://www.qsl.net/w5dxp/current.htm you would know that already. That's the mistake that you and Tom have been making for years and you are still making the same arrogant mistake. Don't you believe what Gene Fuller posted? ************************************************** ****************** Regarding the cos(kz)*cos(wt) term in a standing wave: Gene Fuller, W4SZ wrote: In a standing wave antenna problem, such as the one you describe, there is no remaining phase information. Any specific phase characteristics of the traveling waves died out when the startup transients died out. Phase is gone. Kaput. Vanished. Cannot be recovered. Never to be seen again. The only "phase" remaining is the cos (kz) term, which is really an amplitude description, not a phase. The so-called "phase reversal" in longer antennas is not really about phase either. It is merely a representation of the periodic sign reversal seen in a cosine function. ************************************************** ******************* In a transmission line with two current waves traveling in opposite directions, the phase of the total wave changes with position along the line. No, it does NOT! at least not by more than a few degrees. In Func(kx)*Func(wt), the phase is divorced from position on the line. If you don't believe me, would you believe EZNEC? When I told Tom that I had measured unchanging phase all along a dipole, you said EZNEC shows the same thing. Only in the special case where the two waves are equal in amplitude (i.e., when the line is lossless and open or shorted at the end) does the phase of the total current -- the sum of the forward and reverse traveling waves -- turn out to be the same at all points along the line. This can be easily seen from the very well known equations describing wave behavior on transmission lines. That's true. Now take a look at Kraus' diagram referenced above. Kraus assumes the forward current and the reflected current on a 1/2WL thin- wire dipole are *EQUAL* in magnitude. The same assumption is approximately true for a 75m mobile antenna. There's no special "standing wave current" that's a "misnomer" or which is a phasor which "doesn't rotate". The total current is indeed a phasor -- its rotation speed is the rotational frequency, 2 * pi * f, just like the traveling wave components from which it can be made by simple addition. Sorry, Roy, you are just wrong on that one. Please dust off your old math book. Func(kx)*Func(wt) DOESN'T ROTATE with 'x'. For any 'x', it just stands there exchanging energy between the E-field and H-field. I think the problem Cecil is having ... Methinks you had better do something about that beam in your own eye rather than worry about the speck in mine. ... with it is that the currents on an antenna behave in a manner that's similar to an open circuited transmission line, which results in the phase angle of the total current -- which can be represented as a phasor -- being the same at every point along the line. That's not a problem. That's exactly what Kraus and EZNEC both say. You are the one having a problem with it and you really need to correct your misconceptions before the discussion can progress. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Roy Lewallen wrote:
There's very little about Cecil's theories that I agree with. But you did say EZNEC agreed with the phase measurements I reported for my dipole, i.e. so close to zero I couldn't tell any difference from zero. This leaves you in the unenviable position of disagreeing with (EZNEC and me). -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
John Popelish wrote:
He is so close, but still has a couple misconceptions blocking him. If those are technical misconceptions, please let's discuss them. Last time, when I rewrote my posting to make the meaning clearer, you agreed with it. Perhaps, this is also another example of my poor writing style. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Cecil Moore wrote:
John Popelish wrote: He is so close, but still has a couple misconceptions blocking him. If those are technical misconceptions, please let's discuss them. Last time, when I rewrote my posting to make the meaning clearer, you agreed with it. Perhaps, this is also another example of my poor writing style. Or another mistake on my part. I am more interested in clearing up the later. As far as what is happening in a loading coil, I am in general agreement with you and EZNEC. It is only the details of the discussion of the cause that I am still tripping over. It may be you or I may have an untied shoe. |
Current through coils
John Popelish wrote:
Cecil Moore wrote: Exactly! Therefore, [standing wave phase] cannot be used to measure the phase shift through a coil or even through a wire. I agree, unless you use phase measurement to hunt for the location of the current nodes that have moved as a result of adding the coil. Finding a phase reversal at opposite ends of the coil, for instance, implies that an odd number of nodes reside in the coil. John, I didn't say the amplitude couldn't be used to determine phase. The current nodes are associated wiht amplitudes, not phase. A phasor rotates at the reference frequency, and with a phase angle that represents the angular difference between the value in question and the reference cycle. Pick a point on the conductor, and if it carries either a standing or traveling wave (or any combination of traveling waves at the reference frequency), the current at that point is describable as a phasor (having a specific magnitude, and a specific phase with respect to the reference cycle). Yes, but the standing wave phasor doesn't change phase with position. The traveling wave phasors change phase with position. That's a big difference. No. Currents do not travel. Current is the movement of charge past a point. So current doesn't flow and all the references to "current flow" are wrong? If so, your task is a lot bigger than mine. May I suggest a new thread titled, "Current Doesn't Flow". -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
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. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
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. In conjunction with shunt C, it can be analyzed as a transmission line, but only in conjunction with shunt C. Remove the shunt C and it ceases looking like a transmission line. 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. 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. Roy Lewallen, W7EL |
Current through coils
Roy Lewallen wrote:
A coil itself isn't a slow wave transmission line. In conjunction with shunt C, it can be analyzed as a transmission line, but only in conjunction with shunt C. A 75m bugcatcher has its own shunt C called "distributed capacitance". It's what causes the self-resonant frequency of my 75m bugcatcher coil to be only 60% higher than the 4 MHz operating frequency. Remove the shunt C and it ceases looking like a transmission line. That's true *only* for a lumped-circuit inductance. It is NOT true for a 75m bugcatcher which has it very own distributed capacitance built in. It is *IMPOSSIBLE* to remove the distributed shunt capacitance from a 75m bugcatcher coil. 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. That may be true but please tell us how to remove the ground from a 75m mobile bugcatcher mobile antenna installation. 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. Sorry Roy, Dr. Corum disagrees with your statement. You really should read the details of the Dr. Corum web page references that I posted. His test for the validity of his helix equations is: 5*N*D^2/lamda(0) = 1 where N is number of turns, D is diameter, and lamda(0) is the self-resonant frequency. That value for my 75m bugcatcher coil is 0.4 so his equation for velocity factor is valid. The velocity factor for my 75m bugcatcher coil calculates out to be 0.0175. Now that's what I call a "slow wave" coil. But I have offered all these references weeks ago. Are you too arrogant to even have read them? (Another rhetorical question) -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Cecil Moore wrote:
John Popelish wrote: Cecil Moore wrote: I agree, unless you use phase measurement to hunt for the location of the current nodes that have moved as a result of adding the coil. Finding a phase reversal at opposite ends of the coil, for instance, implies that an odd number of nodes reside in the coil. John, I didn't say the amplitude couldn't be used to determine phase. The current nodes are associated wiht amplitudes, not phase. 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? A phasor rotates at the reference frequency, and with a phase angle that represents the angular difference between the value in question and the reference cycle. Pick a point on the conductor, and if it carries either a standing or traveling wave (or any combination of traveling waves at the reference frequency), the current at that point is describable as a phasor (having a specific magnitude, and a specific phase with respect to the reference cycle). Yes, but the standing wave phasor doesn't change phase with position. The traveling wave phasors change phase with position. That's a big difference. 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? No. Currents do not travel. Current is the movement of charge past a point. So current doesn't flow and all the references to "current flow" are wrong? Afraid so. The concept of current already includes the concept of flow. Current is charge flow. Current flow is charge flow flow?? If so, your task is a lot bigger than mine. May I suggest a new thread titled, "Current Doesn't Flow". I wonder how long that thread would "flow". |
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
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. This is because the inductor does not replace a certain "electrical degrees" and have a cosine current drop related to those degrees. Sorry, Tom, Dr. Corum has proven you wrong on that score. A 1/2WL thin-wire dipole has a perfect cosine curve. Other structures deviate away from that perfect cosine curve. The coil certainly deviates away from that perfect cosine curve. But if you look at the current waveforms at: http://www.k6mhe.com/n7ws/Loaded%20antennas.htm figure 3, you can still see the outline of that cosine curve. 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. You can take the same loading coil and move it around in a standing wave environment and obtain any current distribution including current flowing into both ends of the coil at once. It is the idea that the loading coil drops a certain current because of "electrical degrees" that is so untrue. Sorry, Tom, that's just an old wives' tale of yours. Please respond to my new thread, "Silence of the Gurus". -- 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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