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Current through coils
wrote: Your measurements are probably wrong. Not much accuracy is needed to measure a constant phase of zero. Anyone is invited to duplicate those phase measurements. When did you measure that? After we resolve the error in current, we can move on. I rigged up a 6m dipole yesterday with current pickups driving equal lengths of coax. Remember, we are not discussing the accuracy of my magnitude measurements, only of my phase measurements. After we resolve exactly who made the measurement error, we can move on. My measurements agree with Figure 14-4 in Kraus' "Antennas for All Applications", 3rd edition. Question: would it be legal for me to scan that graphic and post it on my web page if I give Kraus full credit? -- 73, Cecil, W5DXP |
Current through coils
Indeed, Ian. Just so.
It's amazing to me that this thread has gone on for so many postings. (And here I am contributing to it! ;-) But I go back to the very first posting in the thread, where Reg said it all. Well, maybe not QUITE all, but close. I'd invite folk to go back and look at that posting. I believe Reg noted that a real physical (single-layer solenoid) coil has: inductance, capacitance (with a radial electric field), wire resistance and radiation resistance. One additional item I'd like to note is turn-to-turn capacitance; Reg may have been thinking of that one too, but I didn't get it explicitly from his posting. Measuring currents doesn't present too much of a challenge if one is careful about it, but measuring voltage is an entirely different matter, since it's in the presence of a time-varying magnetic field if there is any current in the coil. But it is possible to measure the electric field and the magnetic field if one wishes. I have not been following this thread very closely, because I really do NOT expect "the answer" to be any different than what, as Reg noted in that initial posting, is predicted by an ACCURATE model. People who designed travelling wave tubes understood very well the properties of a helix of wire with respect to propagating a wave. The software I've been using for years now to predict single-layer solenoid coil behaviour takes into account all the interesting effects, and will predict quite accurately the first parallel resonance and the first series resonance, the Q, the inductance, and fundamental transmission line characteristics below resonance. I have other resources that let me predict the change in behaviour when a coil is inside a shield. I've never been surprised by any of the results: taken to correct limits, they all join up, as Ian notes, with conventional circuit theory. In fact, even the complex models match conventional circuit theory, just with more elements in the model. I do note that one must be careful about exactly what conventional circuit theory actually says. For example, many people seem to think that Kirchoff's Voltage Law is something like, "voltages around a loop add up to zero." That is an abbreviated statement of the law, and is in general not correct. Cheers, Tom |
Current through coils
Bill Turner wrote:
I've been following this mind-numbing discussion for days now and still don't have the answer to the original question: Is the current the same at both ends of a mobile whip's loading coil? Bill, W6WRT No, it isn't, provided it's a solenoidal coil of reasonable length. I don't think anyone disagrees with this. The disagreements are to why, and what would happen to the current if the coil were made very short. Roy Lewallen, W7EL |
Current through coils
Cecil,
One more time. 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. (This is one more definition of phase to add to the confusion.) Of course, all of this depends on an ideal system with no losses, etc. The real world is not ideal, but your posed problem does not appear to contain any of those nasty realities. You have undoubtedly seen small phase offsets reported in EZNEC for this sort of antenna. Those phase offsets represent the impact of real-world effects, such as radiation and ground effects. The applicability of linear superposition and the assumption of steady-state conditions means that the resulting standing wave contains ALL of the possible information about the system in steady-state mode. Yes, you can divide the problem back into two traveling waves, in the manner that Kraus, Balanis, and the entire world understand. But you won't gain any new information by doing so, because any unique traveling wave information is permanently lost. 73, Gene W4SZ Cecil Moore wrote: [snip] These are not the results predicted by my neighboring ham friend. I'm confused but here are the things I know for sure. 1. The resonant frequency changed when I installed the coil so the coil is not a perfect replacement for the wire. 2. The feedpoint impedance decreased from 60 ohms to 45 ohms. Since 45 ohms is closer to 50 ohms than is 60 ohms, I'm not too interested in knowing why.. 3. The current at 'x' increased from 0.92 amp at 0 deg in the wire dipole to 1.1 amp at 0 deg in the loaded dipole. The phase didn't change. 4.The current at 'y' increased from 0.38 amps at 0 deg in the wire dipole to 0.6 amp at 0 deg in the loaded dipole. The phase didn't change. 5. No matter where I measure the current in either system, the phase always comes up zero degrees between any two points from tip to tip anywhere on either dipole no matter how far apart are the measurement points. My neighboring ham friend said the number of degrees in the coil had to be the number of degrees in the wire and indeed, both are measured to be zero degrees, but I wonder if that's really what he had in mind when he said the delay would be equal. Zero equals zero, but what does that mean for me? The change in feedpoint impedance and the different current magnitudes don't much bother me but I am really bothered by those phase measurements. The dipole is 180 degrees long and the current should be changing phase, at least on the wire if not through the coil. I need some expert to explain how those phase measurements on the wire are possible on both antennas. I know my phase measurements are correct but why are they always zero degrees? And since they are always zero degrees, what information are they providing? -- 73, Cecil, W5DXP |
Current through coils
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Current through coils
"Roy Lewallen" wrote :
No, it isn't, provided it's a solenoidal coil of reasonable length. I don't think anyone disagrees with this. The disagreements are to why, and what would happen to the current if the coil were made very short. The phase shift measurements are being made using a signal with unchanging phase and thus incapable of providing a phase shift. The disagreement is what would valid phase measurements actually look like if a signal capable of changing phase was used to make the phase measurements. -- 73, Cecil, W5DXP .. |
Current through coils
Cecil Moore wrote: I rigged up a 6m dipole yesterday with current pickups driving equal lengths of coax. Remember, we are not discussing the accuracy of my magnitude measurements, only of my phase measurements. What was the indicator? What was the coupling device? On six meters, it would take a darned small probe and indicator to not greatly perturb the system. If I was going to test something like this, I'd use a small indicator hanging from the antenna and do it on a low frequency. So, tell us about the probe and indicator. 73 Tom |
Current through coils
"Gene Fuller" wrote:
Phase is gone. Kaput. Vanished. Cannot be recovered. Never to be seen again. So how can a signal, devoid of phase, be used to measure the phase shift through a loading coil? The only "phase" remaining is the cos (kz) term, which is really an amplitude description, not a phase. How can one make a phase measurement using only the amplitude of a signal? -- 73, Cecil, W5DXP |
Current through coils
"Roy Lewallen" wrote: The measurement looks good to me. The phase is exactly what EZNEC predicts -- constant along the wire. The ratio in magnitudes we'd expect depends on the positions along the wire, not just the spacing. We are not talking about the magnitude measurements right now. We are talking about the phase measurements. What good does it do to use a signal, whose phase is fixed, to measure the phase shift through a coil or through a wire. The phase is constant along the wire with or without the presence of a coil. Why then is a phase shift of zero on both sides of the coil surprising? And of what importance is that measurement? -- 73, Cecil, W5DXP |
Current through coils
wrote
Cecil Moore wrote: I rigged up a 6m dipole yesterday with current pickups driving equal lengths of coax. Remember, we are not discussing the accuracy of my magnitude measurements, only of my phase measurements. What was the indicator? What was the coupling device? I have an assortment of toroids of various materials from Amidon. I'm at work right now and I don't remember if I used 43 or F material. On six meters, it would take a darned small probe and indicator to not greatly perturb the system. They are small toroids. I chose 6m because the dipole area was physically small. If I was going to test something like this, I'd use a small indicator hanging from the antenna and do it on a low frequency. Please feel free to make that measurement. W7EL just reported that EZNEC agrees with my phase measurements. So does Kraus. So, tell us about the probe and indicator. Similar to the ones W7EL used. They were calibrated within one turn of each other. The signals at the ends of the coax lines were calibrated for equality in magnitude and phase. Magnitudes are a relative measurement but phase was not. I ran the experiment two ways. One was Lissajous figures on my 100 MHz Leader. The other was putting the two samples in opposite phase to each other, i.e. phasor subtraction. For small angles, the angle is equal to the sine of the angle so the addition of two coherent sine waves yields an amplitude proportional to the phase difference when the phase difference is small. The phase difference was so small it was virtually undetectable. -- 73, Cecil, W5DXP |
Current through coils
Cecil Moore wrote:
"Gene Fuller" wrote: Phase is gone. Kaput. Vanished. Cannot be recovered. Never to be seen again. So how can a signal, devoid of phase, be used to measure the phase shift through a loading coil? The only "phase" remaining is the cos (kz) term, which is really an amplitude description, not a phase. How can one make a phase measurement using only the amplitude of a signal? -- 73, Cecil, W5DXP Cecil, The phase is uniformly zero, so the phase shift is also zero. Your messages seem to imply that there is some sort of characteristic "phase shift" in a loading coil. Ain't so. In the example of a standing wave antenna the phase shift is zero, both experimentally and theoretically. (Approximate. Real world conditions might cause small non-zero shifts.) If you place this same loading coil in a traveling wave antenna you can undoubtedly measure some sort of phase shift. (Exact amount left as an exercise for the student.) Bottom line: Any characteristic, such as phase, that explicitly depends on the wave nature of a signal needs to be referenced to that condition, not some arbitrary setup. 73, Gene W4SZ |
Current through coils
Gene Fuller wrote:
Cecil, The phase is uniformly zero, so the phase shift is also zero. Your messages seem to imply that there is some sort of characteristic "phase shift" in a loading coil. Ain't so. In the example of a standing wave antenna the phase shift is zero, both experimentally and theoretically. (Approximate. Real world conditions might cause small non-zero shifts.) If you place this same loading coil in a traveling wave antenna you can undoubtedly measure some sort of phase shift. (Exact amount left as an exercise for the student.) Bottom line: Any characteristic, such as phase, that explicitly depends on the wave nature of a signal needs to be referenced to that condition, not some arbitrary setup. It's likely that quite a number of people don't realize that there's no phase shift of current or voltage along a short or open circuited lossless transmission line -- except for, like on an antenna, periodic polarity reversals. Roy Lewallen, W7EL |
Current through coils
"Gene Fuller" wrote:.. Your messages seem to imply that there is some sort of characteristic "phase shift" in a loading coil. Ain't so. Thanks, Gene. What was implied is what has been reported as fact by others - that the zero phase shift on both ends of a loading coil measures the delay through the coil to be close to zero. In the example of a standing wave antenna the phase shift is zero, both experimentally and theoretically. (Approximate. Real world conditions might cause small non-zero shifts.) Please note that the phase shift in the wire is also zero. That the phase shift is measured to be zero in a coil or a wire in a standing wave environment is not of any practical importance whatsoever. If you place this same loading coil in a traveling wave antenna you can undoubtedly measure some sort of phase shift. (Exact amount left as an exercise for the student.) That was the discussion involving Dr. Corum's papers and the VF of large RF inductors, like a 75m bugcatcher coil. -- 73, Cecil, W5DXP |
Current through coils
"Roy Lewallen" wrote: It's likely that quite a number of people don't realize that there's no phase shift of current or voltage along a short or open circuited lossless transmission line -- except for, like on an antenna, periodic polarity reversals. If we have 45 degrees of transmission line and measure no phase shift at each end of that 45 degrees, does that mean the transmission line is really zero degrees long? If we have an unknown number of degrees of coil and measure no phase on each side of the coil, does that mean the coil is really zero degrees long? -- 73, Cecil, W5DXP |
Current through coils
Cecil,
You're a sly one, but not quite sly enough. 8-) 8-) What is the meaning of "delay" in a standing wave antenna? Delay, like phase, depends on the environment. The measurement results reported by W8JI a few days ago stand on their own. He described the setup and measurements adequately. If you choose to make extrapolations to another environment, have at it. Just don't expect anyone else to automatically agree with your extrapolations. As I recall, this three-year saga started with consideration of a loaded mobile antenna, which I believe would be considered a standing wave antenna. Have you since equipped your steed with a Beverage or rhombic? Are phase shifts and delays now important? 73, Gene W4SZ Cecil Moore wrote: "Gene Fuller" wrote:.. Your messages seem to imply that there is some sort of characteristic "phase shift" in a loading coil. Ain't so. Thanks, Gene. What was implied is what has been reported as fact by others - that the zero phase shift on both ends of a loading coil measures the delay through the coil to be close to zero. In the example of a standing wave antenna the phase shift is zero, both experimentally and theoretically. (Approximate. Real world conditions might cause small non-zero shifts.) Please note that the phase shift in the wire is also zero. That the phase shift is measured to be zero in a coil or a wire in a standing wave environment is not of any practical importance whatsoever. If you place this same loading coil in a traveling wave antenna you can undoubtedly measure some sort of phase shift. (Exact amount left as an exercise for the student.) That was the discussion involving Dr. Corum's papers and the VF of large RF inductors, like a 75m bugcatcher coil. -- 73, Cecil, W5DXP |
Current through coils
Cecil Moore wrote: What was the indicator? What was the coupling device? I have an assortment of toroids of various materials from Amidon. I'm at work right now and I don't remember if I used 43 or F material. How did you construct the coupling transformer? How did you make it immune to electric fields? On six meters, it would take a darned small probe and indicator to not greatly perturb the system. They are small toroids. I chose 6m because the dipole area was physically small. They may be small toroids, but what do they connect to? What is the common mode impedance of the indicator at 50MHz? If I was going to test something like this, I'd use a small indicator hanging from the antenna and do it on a low frequency. Please feel free to make that measurement. W7EL just reported that EZNEC agrees with my phase measurements. So does Kraus. You may not have been measuring anything like you think, in particular voltage. Just because EZnec agrees with the phase measurements it doesn't mean the current measurement was even remotely close to being correct. So, tell us about the probe and indicator. Similar to the ones W7EL used. They were calibrated within one turn of each other. The signals at the ends of the coax lines were calibrated for equality in magnitude and phase. Magnitudes are a relative measurement but phase was not. I ran the experiment two ways. I you hung coaxial lines off a current transformer on a 6-meter dipole down to some test instrument, you wasted a lot of time. There isn't any possible way you measured an unperturbed system. You'd better reconfigure as a monopole, and do it on a lower frequency. One was Lissajous figures on my 100 MHz Leader. The other was putting the two samples in opposite phase to each other, i.e. phasor subtraction. For small angles, the angle is equal to the sine of the angle so the addition of two coherent sine waves yields an amplitude proportional to the phase difference when the phase difference is small. The phase difference was so small it was virtually undetectable. I wouldn't trust that system at all. Even with the equipment I have and having build hunderds of sampling systems, I'd never attempt a measurement like you just made. You brought a pair of long coaxial lines that are grounded at the far end up to within an inch of the antenna, hung what amounts to being a capacitor between the high voltage high impedance side of the antenna to ground, and presumed to measure voltage and current on six meters! You can get away with a toroid on 80 or 40 meters if the test gear and lines are all near ground potential along with the point of the antenna you are measuring, but it is really off the wall to assume you can hang a toroid on a dipole past a loading coil, have that toroid connect to a cable hanging in air down to some test gear, and not severely perturb the system. Worse yet, the test was done on 50MHz! I used a very small floating probe with NO earth path and the indicator right on the probe. I had to do that to not perturb the system on 40 and 80 meters. I can't imagine trying to do what I did on six meters, I'd have needed an entire metering system the size of a half-dollar or smaller. Making a measurement at the base of a vertical on a low frequency like Roy did allows the person making the measurement to get away with lots of things, but I can tell you right now I'd NEVER be able to measure anything in a short mobile antenna if I ran cables up to the probes, in particular if I made the test on six meters! 73 Tom |
Current through coils
Gene Fuller wrote:
What is the meaning of "delay" in a standing wave antenna? Same as in a traveling wave antenna - the length of time it takes a traveling wave signal to make it through a coil or a wire. The lumped-circuit model assumes that delay is equal to zero even for traveling wave antennas. Delay, like phase, depends on the environment. I defined what I meant by "delay" through a coil a few days ago. It was the delay experienced by a traveling wave flowing through a coil or 1/2 the delay experienced by a traveling wave making a round trip to the end of a coil and back based on the self-resonant frequency. That's what the velocity factor calculations were all about. Does the 0.66 velocity factor disappear when RG-8 is used as a stub? Then neither does the 0.0175 coil velocity factor disappear when it is used in a standing wave environment. The cos(kz)*cos(wt) nature of the standing wave current prohibits that standing wave current from being used to determine the velocity factor of a coil or of a wire. The lumped-circuit model assumes the velocity factor through any and every coil to be *greater than unity*. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Cecil Moore wrote:
I officially withdraw my measurements as evidence in this debate and instead substitute the EZNEC results, provided by W7EL, as evidence. What does it mean that EZNEC agrees with my possibly flawed phase measurements? What does it mean that EZNEC agrees with my argument and disagrees with yours? How does one use a signal with unchanging phase to measure the phase shift through a coil or wire? Forgive me if I'm wrong, but seems to me, you have claimed to have done exactly that. Please explain how you did that so we can judge whether your measurements were also flawed. I expected you would want to discuss the problems of making a measurement in the high impedance part of an antenna with a toroid around the wire and a coaxial cable running down to a 100MHz scope on 50Mhz. I see now you really only want to spew, and have no desire to learn anything. I took the time to carefully outline what Roy's measurements and mine would be significantly more reliable, and I see you disregarded that also. I'm very disappointed in your reaction to the effort I made to help you understand measurement techniques. 73 Tom |
Current through coils
wrote:
I took the time to carefully outline what Roy's measurements and mine would be significantly more reliable, and I see you disregarded that also. You demanded that I defend my measurements so I removed them as evidence. Now I am requesting that you defend the use of a signal without phase to measure phase through a coil. That's a very simple request. If one refuses such a simple request to defend one's methods, what is one to think? I gave up on my measurements rather than defend them. If you don't defend yours, are you automatically giving up on them? If no, one might then wonder why you require me to defend my measurements while you refuse to defend yours. Gene says standing wave current doesn't carry any phase information. I concur. Roy says EZNEC agrees with my possibly flawed measurements. I concur. Since EZNEC disagrees with your conclusions about your measurements, and agrees with my conclusion about your measurements, could your conclusions possibly be flawed? I'm sure many readers would be interested in a detailed explanation of exactly how to use a signal with unchanging phase to measure the phase shift through a coil. I certainly would be more interested in that explanation than a boring tutorial on measurement techniques. Heck, even the IEEE would be interested in such a unique technique and it might even be patentable. You could start by explaining the center graphic in the following: http://www.qsl.net/w5dxp/3freq.gif One can't help but notice your absolute silence on that subject. I'm very disappointed in your reaction to the effort I made to help you understand measurement techniques. humor Reminds me of a T-shirt I saw. It read, "I'm from the government. I'm here 'to help you'." /humor Logical diversions are very transparent - they even have names. That one is called "diverting the issue". The issue is not my measurements since I have withdrawn them as evidence. Seems that automatically makes your measurements the subject of the discussion. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Cecil Moore wrote: wrote: I took the time to carefully outline what Roy's measurements and mine would be significantly more reliable, and I see you disregarded that also. You demanded that I defend my measurements so I removed them as evidence. Now I am requesting that you defend the use of a signal without phase to measure phase through a coil. That's a very simple request. If one refuses such a simple request to defend one's methods, what is one to think? I didn't demand anything. I asked what equipment and frequency you used. When you told me, it became very obvious you could have made a much more reliable measurement. I'm suprised you wouldn't want to learn more about measurements. I don't see anything Roy said that disagrees with what I measured, so that's a non-issue. I take it you don't want to discuss how to make better measurements? 73 Tom |
Current through coils
Cecil Moore wrote: You could start by explaining the center graphic in the following: http://www.qsl.net/w5dxp/3freq.gif I don't understand what it is and how you "constructed" it. Maybe you can explain. Did you build one and verify the results? I've never seen a coil loaded antenna work on three exactly even harmonics without special manipulation of lengths. That would make an interesting antenna if you could just make it any length and any coil and have it work on harmonics. 73 Tom |
Current through coils
Cecil Moore wrote:
wrote: Just because EZnec agrees with the phase measurements it doesn't mean the current measurement was even remotely close to being correct. I officially withdraw my measurements as evidence in this debate and instead substitute the EZNEC results, provided by W7EL, as evidence. What does it mean that EZNEC agrees with my possibly flawed phase measurements? What does it mean that EZNEC agrees with my argument and disagrees with yours? How does one use a signal with unchanging phase to measure the phase shift through a coil or wire? Forgive me if I'm wrong, but seems to me, you have claimed to have done exactly that. Please explain how you did that so we can judge whether your measurements were also flawed. Doesn't the change in distance between a pair of current nodes (or voltage nodes) in the standing wave pattern on a straight conductor that straddle an inserted inductor show its effective electrical length? I would think that as long as a fairly pure standing wave could be arranged with the inductor inside it, this method would allow the effective electrical length to be measured at arbitrary frequencies that are not at all related to the coil's self resonant frequency. It seems that this is nearly what you are demonstrating with your EZNEC examples. Electrical length (propagation distance) is collapsing into the inductor. |
Current through coils
Cecil,
I have to admit I am mostly lost about the meaning in your post below. However, nothing in my comments was intended in any way as support or denial of the measurements presented by Tom, W8JI. They look proper to me, but I am not an expert on such measurements. This entire thread is quite bizarre in that all sorts of special cases are being debated (Tesla coils????) while the simplest basic level of standing wave behavior is overlooked and even challenged. I suppose that's typical for RRAA. 73, Gene W4SZ Cecil Moore wrote: wrote: I took the time to carefully outline what Roy's measurements and mine would be significantly more reliable, and I see you disregarded that also. You demanded that I defend my measurements so I removed them as evidence. Now I am requesting that you defend the use of a signal without phase to measure phase through a coil. That's a very simple request. If one refuses such a simple request to defend one's methods, what is one to think? I gave up on my measurements rather than defend them. If you don't defend yours, are you automatically giving up on them? If no, one might then wonder why you require me to defend my measurements while you refuse to defend yours. Gene says standing wave current doesn't carry any phase information. I concur. Roy says EZNEC agrees with my possibly flawed measurements. I concur. Since EZNEC disagrees with your conclusions about your measurements, and agrees with my conclusion about your measurements, could your conclusions possibly be flawed? I'm sure many readers would be interested in a detailed explanation of exactly how to use a signal with unchanging phase to measure the phase shift through a coil. I certainly would be more interested in that explanation than a boring tutorial on measurement techniques. Heck, even the IEEE would be interested in such a unique technique and it might even be patentable. You could start by explaining the center graphic in the following: http://www.qsl.net/w5dxp/3freq.gif One can't help but notice your absolute silence on that subject. I'm very disappointed in your reaction to the effort I made to help you understand measurement techniques. humor Reminds me of a T-shirt I saw. It read, "I'm from the government. I'm here 'to help you'." /humor Logical diversions are very transparent - they even have names. That one is called "diverting the issue". The issue is not my measurements since I have withdrawn them as evidence. Seems that automatically makes your measurements the subject of the discussion. |
Current through coils
wrote:
I'm suprised you wouldn't want to learn more about measurements. I'm surprised you wouldn't want to share your engineering knowledge, e.g. is it really possible to use a phaseless signal to measure phase shift? If so, please enlighten us. I can't figure out how to do it. Since you apparently have figured it out, please share your knowledge with us. I don't see anything Roy said that disagrees with what I measured, so that's a non-issue. Roy and I have previously agreed with your measurements. It's your conclusions about those measurements that he apparently doesn't seem to understand and neither do I. I'm certainly not speaking for Roy, but when he points out to you that my measurements agree with EZNEC, one wonders what that means in reality. I take it you don't want to discuss how to make better measurements? I take it you don't want to share you knowledge of how to measure phase using a phaseless signal? Such a feat is extremely more important than any measurement discussion. Please just explain from a technical standpoint how a signal without phase can be used to measure a phase shift through a coil. That's an extremely simple request and would be extremely useful to the entire group. While you are at it, please explain the EZNEC results in the middle graphic at: http://www.qsl.net/w5dxp/3freq.gif. the request for which you seem to have forgotten about. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
wrote:
Cecil Moore wrote: You could start by explaining the center graphic in the following: http://www.qsl.net/w5dxp/3freq.gif I don't understand what it is and how you "constructed" it. Maybe you can explain. I simulated a typical vertical base-loaded coil system using the helical coil feature of EZNEC. I found the resonant frequency and displayed the results in the left graphic. I then left everything else alone while I multiplied the resonant frequency by 2 and displayed the results in the middle graphic. I then multiplied the resonant frequency 3 and displayed the results in the right graphic. It's a no-brainer. Have you never done such with EZNEC? Would someone, somewhere, please explain the ~0.2 amps at the bottom of the coil in the middle configuration Vs the ~2.0 amps at the top of the coil? Is EZNEC reporting bogus results? -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
John Popelish wrote:
It seems that this is nearly what you are demonstrating with your EZNEC examples. Electrical length (propagation distance) is collapsing into the inductor. Please explain how that could be possible with constant magnitude and phase of the currents through the coil. The magnitude and phase is absolutely constant according to the presuppositions of the lumped-circuit model. How could it possibly collapse? -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Gene Fuller wrote:
However, nothing in my comments was intended in any way as support or denial of the measurements presented by Tom, W8JI. They look proper to me, but I am not an expert on such measurements. How about his conclusions about those measurements? Maybe you can help him out. How does one measure the phase shift through a coil using a signal that doesn't support phase? If there is a way, I and others would certainly like to know about it. So far, I confess that I haven't been able to figure it out. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Cecil Moore wrote:
Reg Edwards wrote: A 100 turn coil, 10 inches long, 2 inches in diameter, has an inductance of 102 microhenrys, a Q of aproximately 380 at F = 1.9 MHz, and the self-resonant frequency is 12.0 MHz. I'll bet the measured self-resonant frequency would be lower if mounted as a base-loading coil on my pickup. Seems the VF of the coil is 0.041 based on 10" being 1/4WL at 12 MHz. Assuming that VF holds down to 1.9 MHz we can calculate the electrical length of the coil on 1.9 MHz which will be the same as the phase shift through the coil. So I get about ~14 degrees of phase shift through that coil at 1.9 MHz assuming the self-resonant frequency really is 12 MHz at the spot where the coil is mounted. If the coil were used on 3.8 MHz, the phase shift would be ~28 degrees. But my 75m bugcatcher coil shows to be self-resonant at 6.6 MHz while sitting there on my pickup being driven by an MFJ-259B. It is 6.5" long. When 6.5" is 1/4WL at 6.6 MHz, the VF = 0.0145, considerably lower than the coil above and operating much closer to its self- resonant frequency. A length of 6.5" coil with a VF of 0.145 on 4 MHz is ~55 degrees of phase shift. And indeed the net current at the top of the coil drops to about 2/3 of what it is at the bottom. Ok, I have had a thought. And I had to go back to where everyone, starting with Cecil, was talking about or responding to a constant delay through the coil. Picking a nice round number, say 55 degrees, I would then need 35 degrees of whip above that coil to make a quarter wave resonant antenna, correct? So, it should work just as well, using Cecil's reasoning, if I displace that coil to another position. He did measure the coil as a standalone device which causes a fixed delay, correct? Ok, so now I move that coil up the antenna, not much, say 2 degrees. Now I have 2 degrees below the coil, and 33 above it. It will still be resonant, right? Now I move it another 2, and another and another, until it it at the top, with no stinger. With the reasoning I have heard from Cecil, it will always be resonant. tom K0TAR |
Current through coils
Cecil Moore wrote:
wrote: Cecil Moore wrote: You could start by explaining the center graphic in the following: http://www.qsl.net/w5dxp/3freq.gif I don't understand what it is and how you "constructed" it. Maybe you can explain. I simulated a typical vertical base-loaded coil system using the helical coil feature of EZNEC. I found the resonant frequency and displayed the results in the left graphic. I then left everything else alone while I multiplied the resonant frequency by 2 and displayed the results in the middle graphic. I then multiplied the resonant frequency 3 and displayed the results in the right graphic. It's a no-brainer. Have you never done such with EZNEC? Would someone, somewhere, please explain the ~0.2 amps at the bottom of the coil in the middle configuration Vs the ~2.0 amps at the top of the coil? Is EZNEC reporting bogus results? It looks like various magnitudes that you would find at 2 points along a standing wave, with various fractions of the wave in the inductor as frequency changes. In spite of hitting these various magnitude values, there are still only two phases, 0 or 180 anywhere outside the coil. In some cases, the standing wave goes through a node, inside the coil and reverses phase from one end of the coil to the other. |
Current through coils
Cecil Moore wrote:
John Popelish wrote: It seems that this is nearly what you are demonstrating with your EZNEC examples. Electrical length (propagation distance) is collapsing into the inductor. Please explain how that could be possible with constant magnitude and phase of the currents through the coil. The magnitude and phase is absolutely constant according to the presuppositions of the lumped-circuit model. How could it possibly collapse? Have I claimed that the lumped model strictly applies? ;-) |
Current through coils
Tom Ring wrote:
Picking a nice round number, say 55 degrees, I would then need 35 degrees of whip above that coil to make a quarter wave resonant antenna, correct? No, that is a myth spread by some people as a strawman argument. Please don't support that strawman. The requirement for resonance at the feedpoint is that the phasor sum of the forward and reflected voltages be in phase with the phasor sum of the forward and reflected currents. Therefore, 90 degrees is *NOT* required in the round trip. Indeed, the round trip for the voltage doesn't have to be the same number of degrees as the round trip for the current. That is a misconception spawned by the lumped-circuit model where everything is perfect and waves travel faster than the speed of light. The real world is not so perfect. Given that the resonant feedpoint impedance equals (Vfor+Vref)/(Ifor+Iref) isn't it obvious that the individual components are not required to have the same phase? For instance, Vfor could be at +50 degrees, Vref could be at -40 degrees, Ifor could be at +20 degrees, Iref could be at -10 degrees, and the feedpoint impedance would still be resistive. The coil distorts the heck out of the phase relationships between the voltages and the currents. Why is it surprising that the result is unpredictable and needs an antenna analyzer to find the resonant frequency? Factor in that the lowest 50 ohm SWR may not be at the purely resistive point and what do you have? -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
John Popelish wrote:
It looks like various magnitudes that you would find at 2 points along a standing wave, with various fractions of the wave in the inductor as frequency changes. In spite of hitting these various magnitude values, there are still only two phases, 0 or 180 anywhere outside the coil. In some cases, the standing wave goes through a node, inside the coil and reverses phase from one end of the coil to the other. Exactly! Now please explain how the nS delay through a coil could possibly be measured using a signal that abruptly shifts fixed phase by 180 degrees every 180 degrees. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
John Popelish wrote:
Cecil Moore wrote: Please explain how that could be possible with constant magnitude and phase of the currents through the coil. The magnitude and phase is absolutely constant according to the presuppositions of the lumped-circuit model. How could it possibly collapse? Have I claimed that the lumped model strictly applies? ;-) No, but someone else has. ;-) -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Wanna bet?
The phase shift along the coil plus the phase shift along antenna conductors does NOT add up 90 degrees when the antenna is 1/4-wave resonant. It is not anywhere near to it. |
Current through coils
Cecil Moore wrote:
John Popelish wrote: It looks like various magnitudes that you would find at 2 points along a standing wave, with various fractions of the wave in the inductor as frequency changes. In spite of hitting these various magnitude values, there are still only two phases, 0 or 180 anywhere outside the coil. In some cases, the standing wave goes through a node, inside the coil and reverses phase from one end of the coil to the other. Exactly! Now please explain how the nS delay through a coil could possibly be measured using a signal that abruptly shifts fixed phase by 180 degrees every 180 degrees. The only way I can see to do it is to go outside the coil and look at how the standing current nodes move. Standing wave phase is with respect to position, not time. |
Current through coils
Cecil Moore wrote:
John Popelish wrote: Cecil Moore wrote: Please explain how that could be possible with constant magnitude and phase of the currents through the coil. The magnitude and phase is absolutely constant according to the presuppositions of the lumped-circuit model. How could it possibly collapse? Have I claimed that the lumped model strictly applies? ;-) No, but someone else has. ;-) You trying to get a fight started? "Did you hear what he said about your wife?" |
Current through coils
Reg Edwards wrote:
Wanna bet? The phase shift along the coil plus the phase shift along antenna conductors does NOT add up 90 degrees when the antenna is 1/4-wave resonant. It is not anywhere near to it. The first problem is that the phase shift along the antenna is about zero. Having the phase shift along the antenna and coil add to 90 degrees would require a 90 degree shift of current phase across the coil, regardless of the size of the coil or the antenna. Only the inductor in an EH antenna drive system has that magical property. I assume you have to empty the internal coulomb bucket periodically, but I'm sure that's proprietary. Pluck a guitar string and watch it oscillate. Notice that all parts of the string reach their maximum and minimum excursions at the same time. There's no delay from one point to another. Same thing happens on a (standing wave) antenna or a shorted or open transmission line, and for the same reason. Roy Lewallen, W7EL |
Current through coils
wrote:
Cecil Moore wrote: wrote: I'm suprised you wouldn't want to learn more about measurements. I'm surprised you wouldn't want to share your engineering knowledge, e.g. is it really possible to use a phaseless signal to measure phase shift? If so, please enlighten us. I can't figure out how to do it. Since you apparently have figured it out, please share your knowledge with us. I think you are the one with a problem, not me. I don't know what a "phaseless" signal is. Does it come from a phaseless signal generator through phaseless transmission lines?? I don't see anything Roy said that disagrees with what I measured, so that's a non-issue. Roy and I have previously agreed with your measurements. It's your conclusions about those measurements that he apparently doesn't seem to understand and neither do I. I'm certainly not speaking for Roy, but when he points out to you that my measurements agree with EZNEC, one wonders what that means in reality. I haven't seen Roy question anything and he hasn't told me he doesn't understand anything. Can you requote his post where he said that, or should I take your word for that along with the phaseless signal?? "Phaseless signals". What an imaginative creation. If you measure zero phase angle between two points, you *have* measured the phase shift between them, and it's zero. Sounds like what Cecil is looking for is a way to measure a non-zero phase shift when the phase shift is in fact zero, to make it fit his theory. Sorry, it's zero. Period. If that doesn't fit the theory, then the theory is faulty. I said that Cecil's phase measurements agree with EZNEC (and generally accepted theory) -- there should be almost no phase shift in the current along the wire. I also said that I couldn't say whether his amplitude measurements are correct, since Cecil didn't say (at least in the quoted posting I saw) what the positions along the wire were, just their distance from each other. But it's a trivial thing to model with EZNEC, so anyone can easily see and judge for himself. With an antenna that simple, any disagreement between EZNEC and measurements is almost certain to be due to either measurement error or failure to make the antenna like the model, e.g., prevent common mode feedline currents. Roy Lewallen, W7EL |
Current through coils
wrote:
I think you are the one with a problem, not me. I don't know what a "phaseless" signal is. Does it come from a phaseless signal generator through phaseless transmission lines?? I apologize, but I actually have to plead complete ignorance on this subject. You seem to be the expert on using phaseless signals to measure phase. But I'm willing to learn and it is certainly considerably more interesting than a tutorial on measurements. I haven't seen Roy question anything and he hasn't told me he doesn't understand anything. Can you requote his post where he said that, or should I take your word for that along with the phaseless signal?? Please stop asking me to do your research for you. I'm a very busy person anchoring the bass section in my Methodist Choir's Easter music and performing a very demanding solo. Have you any idea how difficult it is for a Homo Sapien to hit low-low C? Roy said about my measurements: "The measurement looks good to me. The phase is exactly what EZNEC predicts -- constant along the wire. The ratio in magnitudes we'd expect depends on the positions along the wire, not just the spacing. Roy Lewallen, W7EL" That's why I withdrew my "flawed" measurements and offered Roy's "exactly" observation instead. I'm thinking back and it seems to me that Roy reported his measurements without drawing strange conclusions about them. But you can lay all the objections to waste simply by explaining how to measure phase using a source signal that doesn't change phase. You have asserted that you have done it. Please tell us how. -- 73, Cecil http://www.qsl.net/w5dxp |
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