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#191
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I'm not sure why anyone would think that you can treat an antenna, or a
loading coil of significant length, as a lumped element and expect to get anything resembling accurate results. Who in the world is proposing such a thing? Or is this something to be attributed to the "old wives" and "gurus", so we can then show how much smarter we are by pointing out how stupid it is? Gee whiz, golly, yes, representing an antenna as a two terminal black box with zero size presents a problem. And no, you can't put a box around anything having any length and expect the current in to equal the current out. And why should this be surprising to anyone? Yes, a solenoid produces a local (near) field in the direction of its axis. The far field that remains depends on the size and aspect ratio of the solenoid. Hence, we have solenoidal antennas that radiate primarily axially and those which radiate primarily radially. It's not clear to me how this bears on the topic. Jim Kelley wrote: Jim Kelley wrote: What if you draw a two terminal black box around the middle few feet of a 1/4 wave vertical? What makes the sum of the currents at both ends become equal to zero? Sorry to be obtuse, Roy. The point is only that antenna circuits obviously present a problem to the assumption that such two terminal black boxes will necessarily have equal currents at both terminals. A solenoid should produce a field in the direction of its axis, should it not? 73, Jim AC6XG |
#192
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Roy Lewallen wrote:
Gee whiz, golly, yes, representing an antenna as a two terminal black box with zero size presents a problem. And no, you can't put a box around anything having any length and expect the current in to equal the current out. And why should this be surprising to anyone? The wire comprising an inductor has length. The inductor radiates. The inductor has two terminals with different currents at each. What was it you said about Coulombs again? 73, Jim AC6XG |
#193
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Sigh.
I give up. It's time for me to get back to work. Have fun, folks. Roy Lewallen, W7EL Jim Kelley wrote: Roy Lewallen wrote: Gee whiz, golly, yes, representing an antenna as a two terminal black box with zero size presents a problem. And no, you can't put a box around anything having any length and expect the current in to equal the current out. And why should this be surprising to anyone? The wire comprising an inductor has length. The inductor radiates. The inductor has two terminals with different currents at each. What was it you said about Coulombs again? 73, Jim AC6XG |
#194
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No, I will make one more comment. After a bit of reflection, I think
this might be at the core of some people's problem in envisioning a lumped inductor. When a current flows into an inductor, it doesn't go round and round and round the turns, taking its time to get to the other end. An inductor wound with 100 feet of wire behaves nothing like a 100 foot wire. Why? It's because when the current begins flowing, it creates a magnetic field. This field couples to, or links with, the other turns. The portion of the field from one turn that links with the others is the measurable quantity called the coefficient of coupling. For a good HF toroid, it's commonly 99% or better; solenoids are lower, and vary with aspect ratio. The field from the input turn creates a voltage all along the wire in the other turns which, in turn, produce an output current (presuming there's a load to sustain current flow). Consequently, the current at the input appears nearly instantaneously at the output. Those who are physics oriented can have lots of fun, I'm sure, debating just how long it takes. The field travels at near the speed of light, but the ability of the current to change rapidly is limited by other factors. So please flush your minds of the image of current whirling around the coil, turn by turn, wending its way from one end to the other. It doesn't work at all like that. The coupling of fields from turn to turn or region to region is what brings about the property of inductance in the first place. Radiation is another issue, and provides a path for current, via displacement current, to free space. (I can see it now in Weekly World News: WORLD FILLING WITH COULOMBS! DISASTER LOOMS!) For a component to fit the lumped element model, radiation has to be negligible. And, for the same reason, it can't be allowed to interact with external fields as a receiver, either. This is very fundamental stuff. You can find a lot more about the topic in any elementary circuit analysis or physics text. If you don't believe what you read there, just killfile my postings -- you won't believe me, either, and reading what I post will be a waste of time for both of us. Real inductors, of course, are neither zero length nor do they have a perfect coefficient of coupling. And they do radiate. The essence of engineering is to understand the principles well enough to realize which imperfections are important enough to affect the outcome in a particular situation. We simplify the problem by putting aside the inconsequential effects, but don't oversimplify by ignoring factors that are important for the job at hand. Those who insist on using only the simplest model for all applications will often get invalid results. And those who use only the most complex model for all applications (as is often done in computer circuit modeling), often lose track of what's really going on -- they become good analysts but poor designers. I've seen people capable of only those approaches struggle, and fail, to become competent design engineers. And with that, I'm outta here. Hope my postings have been helpful. Roy Lewallen wrote: Sigh. I give up. It's time for me to get back to work. Have fun, folks. Roy Lewallen, W7EL Jim Kelley wrote: Roy Lewallen wrote: Gee whiz, golly, yes, representing an antenna as a two terminal black box with zero size presents a problem. And no, you can't put a box around anything having any length and expect the current in to equal the current out. And why should this be surprising to anyone? The wire comprising an inductor has length. The inductor radiates. The inductor has two terminals with different currents at each. What was it you said about Coulombs again? 73, Jim AC6XG |
#195
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Roy Lewallen wrote:
no, you can't put a box around anything having any length and expect the current in to equal the current out. And why should this be surprising to anyone? Possibly because radio amateurs are not taught well about what a "lumped component" is. All lumped components are defined as having zero (or negligible) physical dimensions relative to the wavelength at the operating frequency. Similarly, lumped networks are defined as having zero (or negligible) lead lengths relative to the operating wavelength. In this idealized case, the current into and out of the two terminals of a lumped inductor is always exactly the same. This remains true even if that component is embedded into an antenna where current variations along the length of the conductor do exist. Because all practical components and networks have some finite physical size, lumped-component behaviour is never absolutely perfect. In principle, any real component must also show some "antenna-like" behaviour, which does allow some variation of current between its terminals... but in practice this effect is usually very small indeed. For example, for physically small components the lumped-component approximation works well in circuit simulations at frequencies up to several GHz. (You may have to simulate each component as a small network in order to account accurately for self-capacitance, self-inductance and loss resistance, but these are still networks of idealized lumped components.) Yes, a solenoid produces a local (near) field in the direction of its axis. The far field that remains depends on the size and aspect ratio of the solenoid. Hence, we have solenoidal antennas that radiate primarily axially and those which radiate primarily radially. It's not clear to me how this bears on the topic. Quite a lot, I think. At one extreme, a loading coil may be so small that it behaves as a near-perfect lumped inductor. Such an inductor will not radiate, and will have almost zero difference in current between its two terminals. Those two properties - lack of radiation and no difference in terminal currents - are locked together. At the other extreme, you may have a long, skinny loading coil that has significant antenna-like properties, radiating at right-angles to its length like a "rubber-duck" (more formally known as a normal-mode helix). In this case the coil does form part of the radiating structure, so you do expect to see a variation in current along the length of the coil, and hence a difference between the currents at its two ends. Once again, the two properties of radiation and current variation are locked together. This brings us back to the question of practical loading coils, and how much radiation (and therefore current variation along the length) we can expect. I haven't ever tried to work it out, but my guess is that a fairly short "square" coil that has been optimized for high Q is not going to radiate much, and that we therefore shouldn't expect a large difference in current between its two ends. Let's see now... a 3.5MHz loading coil that is as much as 10 inches long would scale down to 0.010 inches at 3.5GHz... at that frequency I'd expect to be able to model such a tiny inductor very accurately as a small network of lumped components with no radiating properties. On the other hand, people who mistakenly believe that even an ideal lumped inductor can have a difference between the currents at its two terminals are rather unlikely to be convinced. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek |
#196
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On Tue, 04 Nov 2003 15:37:11 -0800, Roy Lewallen
wrote: [lots of good stuff snipped] | |And with that, I'm outta here. Hope my postings have been helpful. Thanks, Roy. I'm surprised you stuck around this long. Your posts are always helpful. 73 Wes N7WS |
#197
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Roy,
Let me first apologize for having put the burr under your saddle. This is not my intent. The intent is to try to stimulate some additional thinking on the subject, and perhaps apply some other things we also know about these devices. Your contributions here are obviously invaluable to the entire group. Of course they are helpful. Having recognized that, I hope there are no subjects which are off limits to debate - or opinions that are deemed beyond reproach. Yuri has introduced an interesting subject; one that appears to have two disparate points of view. At this point, in my view, the side asserting that there is no difference in current from one end of an inductor to the other hasn't defended its position as well as the other side. The point you make for the torroid is well taken I think. Flux from this type of coil is well confined to the core of the inductor. But torroidal coils are by design, a unique case. I don't think the same case can be made for the helix, or a loopstick type coil for example. These coils do radiate quite well along their axis if nowhere else, and might therefore be expected to behave in a fashion not unlike other radiators, i.e. impedance and hence, current, would vary with position. Since air core coils are ubiquitous in antenna construction, I don't think it's unreasonable to discuss their performance, and consider the findings Yuri has presented as being both reasonable and viable. 73, Jim AC6XG Roy Lewallen wrote: No, I will make one more comment. After a bit of reflection, I think this might be at the core of some people's problem in envisioning a lumped inductor. When a current flows into an inductor, it doesn't go round and round and round the turns, taking its time to get to the other end. An inductor wound with 100 feet of wire behaves nothing like a 100 foot wire. Why? It's because when the current begins flowing, it creates a magnetic field. This field couples to, or links with, the other turns. The portion of the field from one turn that links with the others is the measurable quantity called the coefficient of coupling. For a good HF toroid, it's commonly 99% or better; solenoids are lower, and vary with aspect ratio. The field from the input turn creates a voltage all along the wire in the other turns which, in turn, produce an output current (presuming there's a load to sustain current flow). Consequently, the current at the input appears nearly instantaneously at the output. Those who are physics oriented can have lots of fun, I'm sure, debating just how long it takes. The field travels at near the speed of light, but the ability of the current to change rapidly is limited by other factors. So please flush your minds of the image of current whirling around the coil, turn by turn, wending its way from one end to the other. It doesn't work at all like that. The coupling of fields from turn to turn or region to region is what brings about the property of inductance in the first place. Radiation is another issue, and provides a path for current, via displacement current, to free space. (I can see it now in Weekly World News: WORLD FILLING WITH COULOMBS! DISASTER LOOMS!) For a component to fit the lumped element model, radiation has to be negligible. And, for the same reason, it can't be allowed to interact with external fields as a receiver, either. This is very fundamental stuff. You can find a lot more about the topic in any elementary circuit analysis or physics text. If you don't believe what you read there, just killfile my postings -- you won't believe me, either, and reading what I post will be a waste of time for both of us. Real inductors, of course, are neither zero length nor do they have a perfect coefficient of coupling. And they do radiate. The essence of engineering is to understand the principles well enough to realize which imperfections are important enough to affect the outcome in a particular situation. We simplify the problem by putting aside the inconsequential effects, but don't oversimplify by ignoring factors that are important for the job at hand. Those who insist on using only the simplest model for all applications will often get invalid results. And those who use only the most complex model for all applications (as is often done in computer circuit modeling), often lose track of what's really going on -- they become good analysts but poor designers. I've seen people capable of only those approaches struggle, and fail, to become competent design engineers. And with that, I'm outta here. Hope my postings have been helpful. Roy Lewallen wrote: Sigh. I give up. It's time for me to get back to work. Have fun, folks. Roy Lewallen, W7EL Jim Kelley wrote: Roy Lewallen wrote: Gee whiz, golly, yes, representing an antenna as a two terminal black box with zero size presents a problem. And no, you can't put a box around anything having any length and expect the current in to equal the current out. And why should this be surprising to anyone? The wire comprising an inductor has length. The inductor radiates. The inductor has two terminals with different currents at each. What was it you said about Coulombs again? 73, Jim AC6XG |
#198
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Thanks for the very cogent and informative posting, as yours always are.
One thing you said really rang a bell, and made me consider something I'd never thought much about before. If a coil is radiating significantly, the Q will of course be necessarily poor due to the energy "lost" by radiation. Yet this "loss" won't be detrimental to the antenna performance. Tom Rauch has pointed out that loading inductor loss is very often insignificant compared to ground loss in a typical HF mobile system. Maybe the presumed "loss" of some coils is not as bad as it appears in other antenna applications, either. Roy Lewallen, W7EL Ian White, G3SEK wrote: . . . This brings us back to the question of practical loading coils, and how much radiation (and therefore current variation along the length) we can expect. I haven't ever tried to work it out, but my guess is that a fairly short "square" coil that has been optimized for high Q is not going to radiate much, and that we therefore shouldn't expect a large difference in current between its two ends. . . . |
#199
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Thanks for your down to earth input Roy
I haven't entered this thread because I was very confused how the group was dealing with inductance since what is important to me is the field around it that permits effective, efficient coupling of different circuits. When looking for a lossless system ,coupling by either capacitance or inductance is necessary together with the all important HIGH Q. And if one were to dwell on the wire used as a missing part of a radiator alone and disregarding the ebb and flow of the enclosing field just blows my mind. Interesting that you spoke of lumped loads and distributed loads in terms of modeling, many people here would learn a lot by starting of with a T or pi type matching system e.t.c. all of which use lumped loads, and then manipulate these same lumped loads into distributed loads in multiple coupled circuits in a form of many coupled distributed load ircuits to produce a radiator that can maintain a constant input impedance. After playing with such circuits to form a combination radiating lossless circuit( reverse of complex circuit resolving) the idea of missing radiator lengths would quickly disappear, as it becomes noticable that the energy field equates to the actual length of the inductance and not to a slinky style stretch. There again, as a total amateur with respect to electrical thing a me jigs I could be adding to the mental riots of those who are partaking in this never ending gymnastics.If so I will now sneak quietly out of this conference room before Tom arrives and put everybody in their place as only he can do. Regards Art Roy Lewallen wrote in message ... No, I will make one more comment. After a bit of reflection, I think this might be at the core of some people's problem in envisioning a lumped inductor. When a current flows into an inductor, it doesn't go round and round and round the turns, taking its time to get to the other end. An inductor wound with 100 feet of wire behaves nothing like a 100 foot wire. Why? It's because when the current begins flowing, it creates a magnetic field. This field couples to, or links with, the other turns. The portion of the field from one turn that links with the others is the measurable quantity called the coefficient of coupling. For a good HF toroid, it's commonly 99% or better; solenoids are lower, and vary with aspect ratio. The field from the input turn creates a voltage all along the wire in the other turns which, in turn, produce an output current (presuming there's a load to sustain current flow). Consequently, the current at the input appears nearly instantaneously at the output. Those who are physics oriented can have lots of fun, I'm sure, debating just how long it takes. The field travels at near the speed of light, but the ability of the current to change rapidly is limited by other factors. So please flush your minds of the image of current whirling around the coil, turn by turn, wending its way from one end to the other. It doesn't work at all like that. The coupling of fields from turn to turn or region to region is what brings about the property of inductance in the first place. Radiation is another issue, and provides a path for current, via displacement current, to free space. (I can see it now in Weekly World News: WORLD FILLING WITH COULOMBS! DISASTER LOOMS!) For a component to fit the lumped element model, radiation has to be negligible. And, for the same reason, it can't be allowed to interact with external fields as a receiver, either. This is very fundamental stuff. You can find a lot more about the topic in any elementary circuit analysis or physics text. If you don't believe what you read there, just killfile my postings -- you won't believe me, either, and reading what I post will be a waste of time for both of us. Real inductors, of course, are neither zero length nor do they have a perfect coefficient of coupling. And they do radiate. The essence of engineering is to understand the principles well enough to realize which imperfections are important enough to affect the outcome in a particular situation. We simplify the problem by putting aside the inconsequential effects, but don't oversimplify by ignoring factors that are important for the job at hand. Those who insist on using only the simplest model for all applications will often get invalid results. And those who use only the most complex model for all applications (as is often done in computer circuit modeling), often lose track of what's really going on -- they become good analysts but poor designers. I've seen people capable of only those approaches struggle, and fail, to become competent design engineers. And with that, I'm outta here. Hope my postings have been helpful. Roy Lewallen wrote: Sigh. I give up. It's time for me to get back to work. Have fun, folks. Roy Lewallen, W7EL Jim Kelley wrote: Roy Lewallen wrote: Gee whiz, golly, yes, representing an antenna as a two terminal black box with zero size presents a problem. And no, you can't put a box around anything having any length and expect the current in to equal the current out. And why should this be surprising to anyone? The wire comprising an inductor has length. The inductor radiates. The inductor has two terminals with different currents at each. What was it you said about Coulombs again? 73, Jim AC6XG |
#200
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If reactance can be seen as a "{missing" part
of a radiator how should we view what a capacitor represents? Grin Art Cecil Moore wrote in message ... w4jle wrote: Current through a coil in an antenna. If we feed an antenna at the current point, the current decreases as the voltage increases along the antenna element from feed point to end.. That being said, a coil replacing a segment of an antenna (in order to physically shorten it) will exhibit the same properties (relating to currents) as the segment it replaced. Yep, if the feedpoint impedances are the same and both are lossless, that has to be true. Here's a repeat of a diagram I drew earlier. -----y----------x-----FP-----x----------y----- 1/2WL dipole -----coil-----FP-----coil----- loaded dipole Assume the physical length of the loaded dipole is 1/4WL. Each coil replaces the section between 'x' and 'y'. The currents at 'x' and 'y' are quite different, being 1/8WL apart. Consider an 8 foot center-loaded 75m mobile antenna. 87% of the electrical length of the antenna is in the coil. |
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