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Current through coils
Richard Clark wrote:
First, several years ago, came the shocking observation that the current into a coil is not the same as the current out of it. Somewhere along the debate, this practical measurement was then expressed to be in conflict with Kirchhoff's theories. However, Kirchhoff's current law is for currents into and out of the same point intersection, not component. The association with a point is found in that the "lumped" inductance is a dimensionless load. The association with Kirchhoff was strained to fit the load to then condemn the load instead of simply rejecting that failed model and using the correct one. So much has been said in this debate - and this is at least the third or fourth re-make of the whole show - that I honestly cannot remember if the exact words that Richard reports were ever used. If they were, then they were excessively condensed, skipping some essential steps in the explanation. Both sides of the debate have often been guilty of skipping details that seemed "obvious" (at least to their way of thinking) in order to get to their main point. So please let me try to respond to Richard's criticism above. Since I don't want to skip anything this time, this is going to take a little longer. If there's anything that someone doesn't agree with, please comment... but please read the whole thing first. Many of the problems with this debate are because people start to throw in comments before finding out where the original poster is heading. This destroys any kind of connected thinking, and reduces the "debate" into a series of disconnected nit-picks. The main electrical property of the thing we call a "coil" or "inductor" is - obviously - inductance. But a real-life coil has many other properties as well, and these complicate the picture. If we're going to understand loading coils at all, we first need to strip away all the complications, and understand what loading by pure inductance would do. Then we can put back the complications and see what difference they make. If we want to understand real-life loading coils, it's absolutely vital to understand which parts of the coil's behaviour are due to its inductance, and which parts are due to other things. Please have patience about this. If we cannot even agree what pure inductance does, then this debate will run forever... From the beginning, then: "Lumped" inductance is another name for the pure electrical property of inductance, applied at a single point in a circuit. It has none of the complications of a real-life coil: no physical size, no distributed self-capacitance, and no external electric or magnetic fields. Its only connections with the antenna are through its two terminals. Lumped inductance is just inductance and nothing else. Unlike capacitance, inductance has NO ability to store charge. If you push an electron into one terminal of a pure inductance, one electron must instantaneously pop out from the other terminal. If there was any delay in this process, it would mean that charge is being stored somewhere... and then we'd no longer be talking about pure inductance [1]. The inability to store charge means there can be no difference between the instantaneous currents at the two terminals of a lumped/pure inductance. Any difference in amplitude or phase at a given instant would mean that charge is being stored or borrowed from some other time in the RF cycle... which inductance cannot do. There is some kind of difference in phase and amplitude in the voltage between its two terminals, but not in the current. Therefore any difference in currents between the two ends of a real-life coil are NOT due to its inductance. They come from those OTHER properties that make a real-life coil more complicated. But let's stay with loading by pure lumped inductance for a little longer, and look at a centre-loaded whip. The loading inductance is responsible for almost all the features of the voltage and current profiles along the antenna. Starting at the bottom (the feedpoint), voltages are low and currents are high, so the feedpoint impedance is low. Going up the lower part of the whip, the magnitudes of the voltage and current remain almost constant until we meet the loading inductance. As we have seen, if the whip is loaded by pure inductance only, there is no change in current between the two terminals of the inductance - but there's a big step increase in voltage. At the upper terminal, the current is the same but the voltage is very high, so we're into a much higher-impedance environment. As we go further up towards the top of the whip, current magnitude has to taper off to zero at the very top. This also means that the voltage magnitude has to increase even more as we approach the top of the whip. Single-point loading by pure inductance has thus created almost all the major features that we see in a practical centre-loaded whip - particularly the big step change in voltage across the loading coil. What we don't see in a practical antenna are exactly equal current magnitudes and zero phase shift between the terminals of a real-life loading coil - but that is ONLY because a real-life coil is not a pure inductance. The harder we try to reach that ideal (by winding the coil on a high-permeability toroidal core which confines the external fields and allows the whole thing to become very small), the closer the currents at the bottom of the coil come to being equal. Solid theory and accurate measurements come together to support each other. The only gap between theory and practice is due to our inability to construct a pure inductance that has no other complicating properties. If we can agree about pure inductive loading, we all have a firm place to stand. Then we can then put back those "other" complicating properties of a real-life loading coil, and see what difference they make. [1] This principle of "conservation of charge" is also the underlying principle of Kirchhoff's current law. If you connect three ordinary wires together, the current flowing into the junction from one wire must be exactly and instantaneously balanced by the currents flowing in or out on the other two wires. If this was not so, there would have to be some means of adding, storing or losing electrons at the junction... which contradicts our initial assumption of three simple wires with no special properties. It is not strictly accurate to say that Kirchhoff's current law applies to pure inductance, but the underlying principle of "conservation of charge" does apply. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Current through coils
Ian White GM3SEK wrote:
(snip everything but context) From the beginning, then: "Lumped" inductance is another name for the pure electrical property of inductance, applied at a single point in a circuit. It has none of the complications of a real-life coil: no physical size, no distributed self-capacitance, and no external electric or magnetic fields. Its only connections with the antenna are through its two terminals. Lumped inductance is just inductance and nothing else. (snip) Single-point loading by pure inductance has thus created almost all the major features that we see in a practical centre-loaded whip - particularly the big step change in voltage across the loading coil. (snip) If we can agree about pure inductive loading, we all have a firm place to stand. Then we can then put back those "other" complicating properties of a real-life loading coil, and see what difference they make. I see nothing to quibble over yet. :-) |
Current through coils
Patterns of current over length is another subject. But you keep saying
that there is something different about current in a standing wave. There isn't. Do you really think that func(kx)*func(wt) is the same thing as func(kx +/- wt)? If so, time to dust off the old math books. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Roy Lewallen wrote:
The point I'm trying to make is that you don't need any particular physical size or any particular length of wire to make something that behaves like a transmission line to any degree of accuracy. Are you admitting that a 75m bugcatcher behaves like a transmission line? -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
EVERYTHING has Inductance, Capacitance and Resistance, and therefore
behaves as a transmission line. ---- Reg, G4FGQ |
Current through coils
Cecil Moore wrote:
John Popelish wrote: Cecil Moore wrote: John Popelish wrote: If the sensor sits at a single point and sees an AC current, you have no way, from this one measurement, if this current is the result of a standing wave (two oppositely traveling equal waves adding), or a single traveling wave, or any combination of traveling waves of different amplitudes. You know only the net current at that point. But if one it smart enough to slide the sensor up and down the wire and note the phase is fixed and unchanging, one knows he is dealing with a standing wave. Another point, entirely. My point is that current has a point definition, and standing wave current is certainly indistinguishable from traveling wave current, at a point. Current is current. Patterns of current over length is another subject. But you keep saying that there is something different about current in a standing wave. There isn't. Do you really think that func(kx)*func(wt) is the same thing as func(kx +/- wt)? If so, time to dust off the old math books. ( I restored some context) func(kx)*func(wt) describes the instantaneous current if you pick a point along dimension, x, and a moment in time, t. It is a map of the pattern of current over these two dimensions. func(kx +/- wt) describes a different pattern of the instantaneous current if you pick a point along dimension, x, and a moment in time, t. If you put a tiny current transformer around some point of the conductors in question, (pick an x) and watch the pattern of current through time (without comparing the phase to any reference) you will see a sinusoidal current variation for both the standing and traveling wave cases. The amplitude will vary in a different way, over x, for the traveling and standing wave cases. If you include comparing the phase of sinusoidal current cycle you see, to a reference phase, that will also vary in a different way over x, for the traveling and standing wave cases. But regardless, at a point (any particular x) the pattern of current variation as time passes, will be a sinusoid, in either case. There is no difference in kind of current you would measure. The pattern of how this sinusoidal current varies in both phase and magnitude is very different in the two cases (standing and traveling waves), but you need both a phase reference and multiple locations to see the differences. The the definition of the word "current", in simplest form, is, the rate of charge movement past a point at some moment in time. An extension of this instantaneous and point definition might include a sinusoidal cyclic variation through time, by adding a frequency, phase and amplitude, to specify a common pattern of current over time, but still at a point. Adding in an additional function of position allows the extension of this definition of current over time to also include spacial variation of the time dependent pattern. But if you say the words "the current is different", and don't include a lot of additional verbiage to indicate that you are talking about the two dimensional pattern of the variation of current over time and location, some people are going to misunderstand you and argue based on picturing another definition of what might be legitimately meant by the word, "current". I made it clear what definition I was using for the word "current" (the instantaneous point definition) and you are arguing with me, while using some different definition. I realize that I am being pedantic, here, and stating the painfully obvious. But if your goal is to have other minds synchronize with the abstract thoughts rippling through your mind, you have to be pedantic. If you are just using this topic to argue, because you enjoy argument, then never mind. |
Current through coils
John Popelish wrote:
The pattern of how this sinusoidal current varies in both phase and magnitude is very different in the two cases (standing and traveling waves), but you need both a phase reference and multiple locations to see the differences. Exactly! And the multiple locations are available for us to measure. Since you like handicaps so much, how about just plucking out your eyeballs and chopping off your hands? :-) -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
John P. wrote, among other things,
"The pattern of how this sinusoidal current varies in both phase and magnitude is very different in the two cases (standing and traveling waves), but you need both a phase reference and multiple locations to see the differences. " Do you really need the phase reference? Traditionally (since the beginning of measuring them, and sometimes still today), standing waves on a uniform transmission line have been measured by finding a point of minimum amplitude (as measured by voltage, or alternatively by current) and a point of maximum amplitude, with no reference to phase. In fact, SWR was reasonably defined as the ratio of max/min amplitudes. If you know that the wave you're observing is a sinusoid and you have min and max amplitudes along the line, then you can resolve the wave into two travelling-wave amplitudes; you won't know which is which but you will know the two amplitudes. If there is but one source in the system, it's reasonable to think that the higher amplitude travelling wave was the one coming from the direction of that source. In fact, you don't even need to find the minimum and the maximum points. Again, given sinusoidal excitation and a uniform line, some small set of points with accurate amplitude measurement at each will suffice, since they will uniquely determine the amplitudes of the two waves and the line attenuation. You would have to know the spacing of the points and that they were dense enough that there is not a spacial aliasing problem (points distributed over more than 1/4 wavelength...). It's common to think of a standing wave as the result of two travelling waves, one in each direction, but another way to think of a standing wave pattern is as a pure standing wave plus a pure travelling wave. The minimum-amplitude represents the amplitude of the travelling-wave portion. The difference between max and min represents the amplitude of the standing wave portion. For some folk, it's enlightening to see an animation of the waves for several different values of SWR. Cheers, Tom |
Current through coils
K7ITM wrote:
John P. wrote, among other things, "The pattern of how this sinusoidal current varies in both phase and magnitude is very different in the two cases (standing and traveling waves), but you need both a phase reference and multiple locations to see the differences. " Do you really need the phase reference? Traditionally (since the beginning of measuring them, and sometimes still today), standing waves on a uniform transmission line have been measured by finding a point of minimum amplitude (as measured by voltage, or alternatively by current) and a point of maximum amplitude, with no reference to phase. In fact, SWR was reasonably defined as the ratio of max/min amplitudes. (snip) What I was trying to say is that to completely see (measure) all the differences between the current pattern in a standing wave versus a traveling wave (or any combination of traveling waves of different magnitudes in opposite directions, with or without losses, especially when there are discontinuities in the conductor, like loading coils) those observations would include phase versus position. In many practical cases, you can infer what you need to know about the two traveling waves by just taking amplitude measurements, as you suggest. |
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