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Current through coils
On Fri, 10 Mar 2006 17:04:54 GMT, "Cecil Moore"
wrote: the RF current wave :-) |
Current through coils
Quoted from an e-mail exchange I am having:
However, I'd like you to reconsider your position concerning inductances in series with a line that has both forward and reverse currents flowing, as in short mobile antennas. As a result of two currents from the same source flowing in opposite directions, a standing wave is inevitable, hence different values of current at different points along the wire in the inductor. That is incorrect for the conditions we are outlining, and it is misleading Cecil. It has him lost in a world of reflections. You have gone outside the limits of the model by assuming, incorrectly, the inductor has no or little flux linkage from end-to-end and has large stray capacitance to the outside world compared to load impedance. The conductor used to build a inductor does not have current slowly winding its way along that path. There is no virtually no difference in phase delay in current at each end of a relatively compact inductor. It is very easy to measure that. It also have very little group delay compared to the group delay one would expect from a transmission line or antenna the same length. I know that because I have measured it hundreds of times. I have repeated a url below that Cecil posted on the rraa. The material in that url agrees with my position, and specifically states that circuit analysis is invalid when the model contains distributed currents, and admonishes that anyone who disbelieves this has forgotten the warning about the situation given in sophmore EE courses. The Tesla coil, by definition of how it works, violates all boundaries of the examples myself and others are giving Cecil. It does not apply to the discussion at all. The Tesla coil is intentionally of exceptionally long form factor. It has virtually an open circuit at the end, and is by operation self-resonant at the operating frequency. It has a very large amount of distributed capacitance compared to termination impedance, since the termination is an open. It is not operated at a fraction of self-resonance as people SHOULD know a good mobile loading coil is. It has no bearing at all on the discussion, any more than it would if I started measuring the plate choke from an AL1200 amplifier at the self-resonant frequency with an open termination, or a loading coil for a 75 meter antenna at the self-resonant frequency. Everyone (except Cecil) has been very careful to give the boundaries and describe the effects. The Tesla coil does not fit the boundaries described, and the secondary inductor in the Tesla coil behaves nothing like an inductor operated well below self resonance. http://www.ttr.com/corum/index.htm The very first paragraph of that reference should have been a red flag that it does not apply to this discussion. Here is what it says: "Can one model the physical operation of a Tesla coil appropriately with only lumped-element circuits? If not, why not? It was pointed out long ago that, at its operating frequency, a Tesla coil is NOT a lumped-element induction coil. Forget the quest for "many turns of fine wire". In fact, a Tesla coil has more in common with a cavity resonator than it does with a conventional inductor." The key words they use, and they even drew attention to the words by a type style change, "at its operating frequency, a Tesla coil is NOT a lumped-element induction coil". They were very clear about that, and go on to describe how it does behave like a normal induction coil. Everyone in the conversation has been very careful to clearly establish the boundary conditions that the behavior we are talking about is significantly below self-resonance, an inductor of compact form factor, and an inductor of good design. I can't understand why anyone would attempt to reference an article that, in the very opening, states the inductor is operating at self-resonance! I can't understand why anyone would reference an article that violates the boundaries of termination impedance outlined in the discussion, where it has been stated over and over again the inductor must be terminated in an impedance that is low compared to leakage impedances. I can't imagine anyone using a lossy Tesla coil as an antenna or part of an antenna system. Please read the opening paragraphs of the article you reference. 73 Tom |
Current through coils
On Fri, 10 Mar 2006 17:04:54 GMT, "Cecil Moore"
wrote: wrote: What Cecil needs to do is bias the coil with a DC bias current that safely exceeds the peak RF current. Then he would have RF current flowing in only one direction. I should have said: have the RF current wave flowing only in the forward direction.Wes knew what I meant. I did? |
Current through coils
Cecil Moore wrote:
wrote: What Cecil needs to do is bias the coil with a DC bias current that safely exceeds the peak RF current. Then he would have RF current flowing in only one direction. I should have said: have the RF current wave flowing only in the forward direction.Wes knew what I meant. I think a lot of the contention on this subject is based on little more than such multiple meanings for common term, "current". When you talk about current flowing, you seem to be thinking of current waves traveling along a conductor. Others seem to be saying "current" and thinking of charge movement. I think that only the second is technically correct (current is the movement of charge, not the traveling wave or standing wave pattern in that movement), but I think I understand what you are picturing. We switch to the short hand concept of calling a pattern of current changes a current every time we make an AC current measurement and refer to it as a non zero value, as we do with amperes (RMS). Standing waves involve no net wave travel in either direction, though anywhere except at the current nodes, charge is certainly moving back and forth along the conductor, during a cycle. Thus, there certainly are instantaneous currents in both directions (depending on location and instant) along any conductor sustaining a standing wave, everywhere, except at the current nodes. And everywhere, except at voltage nodes, half way between current nodes, charge is piling up (as electrons move toward every other current node) and spreading out as electrons moves away from the remaining current nodes) creating voltage changes. Traveling waves have a very similar charge movement as that which takes place half way between the nodes and peaks of the standing wave pattern. But there are no nodes or peaks, so the current swings between the same 2 values, everywhere along the conductor. Charge arrives from one direction, instead of from both directions and leaves in the other direction, each half cycle. Every other half cycle, the directions of arrival and departure reverse, even though the wave always moves in one direction. I am talking about conduction in wire, not EM waves in space, here. This current (movement of charge in either a standing wave or a traveling wave) creates H field and the changes in that H field can be monitored with a current transformer. But at any single point, current measured with a current transformer has no way of knowing if the current changes seen are the result of a standing wave or a traveling wave. In both cases, charge is seen to be moving in alternating directions. But if you slide the current transformer along the conductor, the current magnitude will vary if standing waves are responsible for the current, and remain, essentially constant, if traveling waves are its source. I apologize for stating the painfully obvious, but when basic terminology is the cause of misunderstanding, it sometimes helps to back up a step in the direction of a more basic view to uncover the origin of the misunderstanding. |
Current through coils
wrote :
That is incorrect for the conditions we are outlining, and it is misleading Cecil. It has him lost in a world of reflections. What is causing the misleading part is: THE LUMPED-CIRCUIT MODEL FAILS IN THE PRESENCE OF STANDING WAVES! There is no virtually no difference in phase delay in current at each end of a relatively compact inductor. Is a 75m bugcatcher coil a "relatively compact indictor"? If you say yes, you are stuck with its measured delay. If you say no, then we are not discussing the typical amateur radio mobile loading coil. Of course, one turn on a toroid is going to exhibit the characteristics you are presenting. But that is not a typical bugcatcher coil either. The Tesla coil, by definition of how it works, violates all boundaries of the examples myself and others are giving Cecil. It does not apply to the discussion at all. False: A 75m bugcatcher coil used as a 1/4WL resonator on 9-10 MHz meets the minimum requirements for a Tesla coil. It uses 1/6 wavelength of wire on 75m. I'll bet it would certainly arc at a kilowatt. The typical minimum Tesla system is a coil with a top hat sphere. It looks a lot like your 160m mobile antenna. :-) It is not operated at a fraction of self-resonance as people SHOULD know a good mobile loading coil is. A 75m bugcatcher coil is operating close enough to its self-resonant frequency that the self-resonant effects are certainly present. A 75m bugcatcher coil can be considered to be a lumped circuit impedance at 60 Hz but certainly not at 4000000 Hz. In fact, that is the whole question. At what frequency can the lumped circuit model be validly used on a 75m bugcatcher coil? I'm willing to bet that frequency is lower than 1000000 Hz. It has no bearing at all on the discussion, ... Wishful thinking on your part. .. In fact, a Tesla coil has more in common with a cavity resonator than it does with a conventional inductor." A 75m bugcatcher coil has more in common with a cavity resonator than it does with your lumped circuit inductance. "at its operating frequency, a Tesla coil is NOT a lumped-element induction coil". Neither is a 75m bugcatcher coil. Everyone in the conversation has been very careful to clearly establish the boundary conditions that the behavior we are talking about is significantly below self-resonance, an inductor of compact form factor, and an inductor of good design. A 75m bugcatcher coil used on 4 MHz is NOT significantly below the self-resonant frequency of 9-10 MHz. THE LUMPED-CIRCUIT MODEL FAILS IN A STANDING WAVE ENVIRONMENT! In the face of that simple technical fact, all other discussion is moot. Anyone wishing to validly model a 75m bugcatcher coil used on a mobile antenna is forced to choose a model that does not presuppose faster than light wave travel through a 75m bugcatcher coil. It's as simple as that. Tom, with a straight face, I want you to assert that the RF waves on a 75m bugcatcher mobile antenna are traveling faster than the speed of light. If it takes 125 nanoseconds for the forward current wave to make it from the end of the antenna and back to the feedpoint, then the lumped-circuit model yields invalid results. TDR anyone? -- 73, Cecil, W5DXP |
Current through coils
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Current through coils
Cecil Moore wrote: A 75m bugcatcher coil used on 4 MHz is NOT significantly below the self-resonant frequency of 9-10 MHz. Yes it is, but no so far as to have perfectly equal currents at each end an zero phase shift in current. It is in the neither land between a Tesla coil (which is still nothing like my mobile antenna, but at least getting closer) and a idealized lumped component. THE LUMPED-CIRCUIT MODEL FAILS IN A STANDING WAVE ENVIRONMENT! In the face of that simple technical fact, all other discussion is moot. Anyone wishing to validly model a 75m bugcatcher coil used on a mobile antenna is forced to choose a model that does not presuppose faster than light wave travel through a 75m bugcatcher coil. It's as simple as that. Nonsense. You are ignoring the coupling mechanisim inside the inductor. Tom, with a straight face, I want you to assert that the RF waves on a 75m bugcatcher mobile antenna are traveling faster than the speed of light. If it takes 125 nanoseconds for the forward current wave to make it from the end of the antenna and back to the feedpoint, then the lumped-circuit model yields invalid results. TDR anyone? They are not travelling faster than light. What you (and the one or two others who seem to agree with you) repeatedly ignore or forget is magnetic flux couples one turn to another. A real inductor is always someplace between the two extremes of something like a radial mode helice (helically loaded whip) and an ideal lumped component. Since you have taken the path of totally forgetting or ignoring flux coupling, you are reaching incorrect conclusions. Using the Tesla coil model is a good example. Everyone is freely admitting there is *some* transmission line effect going on. There is some distrbuted component (a series of inductors shunted by capacitors) going on. Everyone (except you) is being careful to qualify remarks by specifying the inductor is operating well below self-resonance. If you weren't so pig-headed you could look at the measured data at: http://www.w8ji.com/mobile_antenna_c...ts_at_w8ji.htm ....and see that as inductors move towards self-resonance they do begin to display characteristics of transmission lines. It's too bad in three years you have claimed others made a measurement error, when in fact the error is in thinking all of the current in a loading coil slowly winds its way around turn by turn and the magnetic field linking turns does not cause charges in other turns to move long before current traveleing at light speed would wind through the copper path. Until you stop, put the beer away, and think about this a while you'll continue to butt your head up against people who KNOW how inductors behave. 73 Tom |
Current through coils
Wes Stewart wrote:
"Cecil Moore" wrote: Wes knew what I meant. I did? Hopefully anyone with an IQ above 80 knew that. :-) -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
John Popelish wrote:
Any discussion of inductors and waves needs to either select an example inductor for discussion, e.g., 75m bugcatcher mobile loading coil. or remain general enough to cover anything that might be called an inductor. e.g., Maxwell's equations. Certainly not a lumped-circuit model that presupposes that EM waves travel through anything and everything faster than the speed of light. Or else, endless and pointless arguments will ensue. Yep, notice how the inductors that Tom is talking keep getting smaller and smaller with time. Pretty soon they will indeed be point-sized inductors. :-) -- 73, Cecil http://www.qsl.net/w5dxp |
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