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Current through coils
Ian White GM3SEK wrote: Such probes are routinely used for RFI, RF hazards and screened-room measurements, where connecting wires would disturb the fields or act as pathways for RF leakage. They do have a disadvantage that might be relevant to this discussion: because the probe head has to be self-powered, and has to include some kind of encoder and optical transmitter as well as the normal current transformer, the battery and extra area of PC board will increase the probe's self-capacitance. There are two advantages to a fiberoptic coupled probe. One advantage of fiber optic coupling from probes to indicator is the coupling leads from sensor to indicating instument do not have a good direct path to earth or equipment like conventional probes. I actually built a form of this for one of the measurements Ceci rejected. http://www.w8ji.com/building_a_current_meter.htm In this case the information, current, is conveyed by light through air directly to my eye. For the purposes of this discussion, however, the real advantage is different. Since it is unlikely anyone disagreeing has a fiber-optic coupled probe (the fiber optical cable simply replaces the wire between the sensor and the indicator or sensor information processing system) it is unlikely anyone can prove Cecil wrong. This all seems logical to me, because Cecil has asked for measurements. The pattern has been after he gets measurement results and finds they disagree with his theory, he has to blame the difference on something. The most logical thing any person can do when they repeatidly accept results of measurements made by multiple people using multiple methods is to come up with a measurement no one can make. For example? Most people understand a current transformer measures current. The original debate was K3BU and W9UCW made a statement current is high only in the first few turns of a loading inductor, and thus loading inductor Q did not matter for efficiency of an antenna. I proposed antenna losses were swamped out by ground losses in a vehicle, and because of very high ground losses the effects of coil resistance were diluted. I measured the inductor and found as quite logically anyone would expect that current ratio depended on the ratio of stray C from the coil to load C at the open end of the coil. Yuri K3BU argued the coil replaced a certain number of degrees electrical height, and I disagreed. I said a 20-degree long antenna with a loading coil did NOT have 70 degrees of antenna wound up in the coil. Most people experienced in systems like this from an engineering standpoint agreed with me. Somewhere about that time Cecil brought reflected waves into the discussion. After a series of "what happens if" Cecil wanted measurements. When they were made, he and Yuri announced the measurements proved their points. When the person making the measurements corrected those misstatements and pointed out the measurements didn't support their claims, the only logical course was to discredit the measurements and ask for new ones. When new measurements again disagreed with the concept of huge current or phase delay of current that was tied to degrees the coil replaces, the only course was to reject those measurements. So here we are today, two or three years later, still trying to find a measurement that will agree with what Cecil and Yuri proposed or for another person of reasonable engineering experience to agree with the notion the coil behaves as a coiled up antenna or transmission line rather than behaving more like a lumped component in a small heavily loaded mobile antenna. Since dozens of hours of measurements acceptable to most people were rejected, the only solution would be to require a measurement with instrumentation no one has. This way Cecil can say no one can prove him wrong, and that allows him to continue to demand others agree with him. In my opinion, the real advantage of optically coupled probes in this thread is no one is likely to have them. 73 Tom |
Current through coils
Reg Edwards wrote: From basic transmission line theory, the velocity of propagation along a coil is estimated by - V = 1 / Sqrt( L * C ) metres per second, So Reg, for a fixed installation, why would L and C change much with frequency, like from 16 nS at 16 MHz to 3 nS at 4 MHz? If we took it down to 1 MHz, would the delay go below 3 nS? -- 73, Cecil http://www.qsl.net/w5dxp ========================================== Sorry Cec, I havn't the foggiest idea. ---- Reg. =========================================== On second thoughts, since L and C are functions of a coil's physical dimensions it must be something else which is changing with frequency. ---- Reg. |
Current through coils
wrote:
Before I comment on your posting below, I think you can prove to yourself that your measurements are flawed. You measured 3 nS delay through your coil at 4 MHz. Now perform the same measurement at the self-resonant frequency. The delay through the coil is known to be 15.6 nS at the self-resonant frequency. If your delay measurement isn't 15.6 nS, then there is something wrong with your methods. Better yet, measure the delay at 1, 2, 4, 8, &16 MHz and report the results. ... it is unlikely anyone can prove Cecil wrong. That's because in order to prove me wrong, you have to prove yourself right. You simply haven't done that because you refuse to engage me at a technical level. You have ignored my technical questions and refused to discuss the technical details. Many readers have noticed that, wonder why, and have commented on it in emails to me. This all seems logical to me, because Cecil has asked for measurements. The pattern has been after he gets measurement results and finds they disagree with his theory, he has to blame the difference on something. Tom, your measurements agree perfectly with my theory. You are measuring standing wave currrent. That standing wave current magnitude is pictured in every good book on antennas. Kraus also shows the phase which, for a thin wire dipole, is fixed at zero from tip to tip on the antenna. It is understandable why you measured zero standing wave current phase shift through the coil. THE STANDING WAVE CURRENT PHASE SHIFT IS ZERO WHETHER THE COIL IS IN THE CIRCUIT OR NOT! Since the phase of the standing wave current is fixed and unchanging whether the coil is in the circuit or not, why do you think measuring that unchanging phase around a coil proves anything? I proposed antenna losses were swamped out by ground losses in a vehicle, and because of very high ground losses the effects of coil resistance were diluted. I agree with that and have never argued otherwise. I measured the inductor and found as quite logically anyone would expect that current ratio depended on the ratio of stray C from the coil to load C at the open end of the coil. Yuri K3BU argued the coil replaced a certain number of degrees electrical height, and I disagreed. The following reports a 10-20 degree phase shift through most coils. http://lists.contesting.com/archives.../msg00540.html Most people experienced in systems like this from an engineering standpoint agreed with me. Somewhere about that time Cecil brought reflected waves into the discussion. Those "most people" don't understand forward and reflected waves on a standing-wave antenna. You have proven by your postings here that you do not understand forward and reflected waves on a standing- wave antenna like a 75m bugcatcher mobile antenna. Worse yet, you refuse to discuss the antenna at a technical level and have simply sandbagged your misconceptions. I remember when you were using the lumped inductance feature of EZNEC to try to prove your point, certainly an invalid proof. When we started this thread, it was obvious that you didn't know the standing wave current phase is fixed near zero degrees so measuring it is futile. After a series of "what happens if" Cecil wanted measurements. When they were made, he and Yuri announced the measurements proved their points. Yes, they did prove that the current at the ends of the coil were NOT equal. You said they were. I said they were not. Out of all of your and Roy's measured results, the current was equal in only the case of the small toroidal coil and that's because it was located at a standing wave current maximum (loop). When the person making the measurements corrected those misstatements and pointed out the measurements didn't support their claims, the only logical course was to discredit the measurements and ask for new ones. You sure have selective memory, Tom. I fully accepted your standing- wave current measurements. But standing-wave current measurements cannot be used to measure the traveling-wave delay through a coil. That should be obvious to everyone by now. The delay through the coil causes a phase shift in the forward wave and the reflected wave, not in the standing wave. THE PHASE OF THE STANDING WAVE CURRENT IS KNOWN NOT TO CHANGE AND THAT'S EXACTLY WHAT YOU MEASURED, VIRTUALLY NO SHIFT. Kraus agrees. Figure 14-2 of "Antennas For All Applications", 3rd edition shows a graph of the phase of the standing wave current. That phase is zero tip-to-tip for a thin-wire 1/2WL dipole. When new measurements again disagreed with the concept of huge current or phase delay of current that was tied to degrees the coil replaces, the only course was to reject those measurements. THOSE MEASUREMENTS WERE NOT REJECTED! They were accepted as perfectly valid measurements of standing wave current. Those characteristics are pictured in Kraus and your measurements agree perfectly with them. Your argument is a strawman. The fact is that a standing wave measurement CANNOT yield the current delay through the coil any more than it can yield the current delay through a wire. YOU CANNOT MEASURE THE DELAY THROUGH THE COIL USING CURRENT KNOWN NOT TO CHANGE PHASE! So here we are today, two or three years later, still trying to find a measurement that will agree with what Cecil and Yuri proposed or for another person of reasonable engineering experience to agree with the notion the coil behaves as a coiled up antenna or transmission line rather than behaving more like a lumped component in a small heavily loaded mobile antenna. This is not about you or me or Yuri. It is about getting down to the truth. Yet you rave on and on about personalities. Why don't you discuss technical issues instead of personalities? There is a phase shift in the forward current through the loading coil. There is a phase shift in the reflected current through the loading coil. Those phasors are rotating in opposite directions so the net phase is fixed. You can measure standing wave current phase in thousands of experiments from now to kingdom come and you will not be measuring the phase shift of the forward and reflected current through the coil. Your measurements, so far, are meaningless. You have NEVER measured the delay through the coil. I guess I'm going to have to draw you some pictures and post them on my web page. Since dozens of hours of measurements acceptable to most people were rejected, the only solution would be to require a measurement with instrumentation no one has. I fully accept your standing wave current measurements, Tom, but standing wave current measurements will not yield the information that we are after. We need to know the phase shift in the forward and reflected currents through the coil. Standing wave measurements simply will not yield that information. Self-resonance measurements will yield that information. The delay through a coil that is self-resonant on 16 MHz is 15.6 nS. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Reg Edwards wrote:
Having started it, I havn't been taking much notice of this long-winded thread. Its all too clever for poor little me! ;o) Just curious, Reg, are you familiar with phasors used to represent traveling waves where the phasor has a rotation about the origin proportional to the frequency? Are you familiar with the phasor addition of two of those waves traveling in opposite directions forming standing waves? -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Reg Edwards wrote:
On second thoughts, since L and C are functions of a coil's physical dimensions it must be something else which is changing with frequency. Or maybe nothing is changing appreciably over relatively small frequency excursions. Maybe the measurements are not measuring what someone thinks they are measuring. The only experiment so far that has actually measured the delay through a coil is the self-resonant frequency measurement. Tom's and Roy's results are perfectly consistent with the measurement of standing wave current whose phase is known to be constant and unchanging. That measurement yields no new information. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Cecil, W5DXP wrote:
"Kraus agrees." Pity the fool that argues with Terman or Kraus! In Kraus` Figure 14-2 of the 3rd edition of "Antennas", the 1/2-wave is resonant and shows no phase shift from end to end. In Figure 14-4, phase is shown to make an abrupt phase transition at a point 1/2-wave back from the open circuit at the tip of the antenna. This is predictable from the behavior of an open-circuited transmission line as shown by Terman in Fig. 4-7 in his 1955 edition. Kraus` Figure 23-21 shows how a self-resonant coil can replace a short-circuited 1/4-wave stub in a phase-reversing trap. If you don`t have the 3rd edition of "Antennas", get it. Cecil wrote: "I am not asking for fiber optic measurements." Very likely they aren`t necessary. I`ve measured currents along antennas draging a sampling loop along them with a rope. A transit determined the position and its telescope made the r-f ammeter in the loop readable. Surely a loop and its ammeter can be small enough not to upset the measurements if you use enough power and have a low enough frequency. As Richard Clark might say: "We don`t need no stinkin` fiber optics." Best regards, Richard Harrison, KB5WZI |
Current through coils
Cecil Moore wrote:
John Popelish wrote: ... I see no reason to assume the transmission line method (delay independent of frequency) strictly applies. It might, but it would take more than you saying so to assure me that it is a fact. Assume the environment of the coil is fixed like the variable stinger measurement I reported earlier. Besides the frequency term, the phase constant depends upon L, C, R, and G as does the Z0 equation. Why would the L, C, R, and G change appreciably over a relatively narrow frequency range as in my bugcatcher coil measurements going from 6.7 MHz to 3.0 MHz? We are not talking about L, C, R, or any other inherent property changing with frequency. We are talking about the delay of a current wave in a single direction (anybody have a pair of directional coupler current probes?) through a complex component that has several different mechanisms that contribute to the total current passing through it. It is the vector sum (superposition) of those current components that is in question. Over a narrow frequency range, it is conceivable to me, that the phase (delay) of that sum might shift, dramatically, though any component of that sum might change its magnitude only slightly (no faster than in proportion to the frequency), and the phase of that component might change not at all. And I didn't mean to imply that the delay is "independent" of frequency, just that it is not nearly as frequency dependent as Tom's measurements would suggest. If Tom made his measurements from 1 MHz to 16 MHz, what do you think the curve would look like? Freq 1 2 4 8 16 MHz Delay ___ ___ 3 ___ 16 nS That looks non-linear to me. How about you? Definitely nonlinear, just like impedance is very nonlinear as the frequency passes through any resonance. This is why I am suspicious of a measurement made at resonance, being extrapolated to non resonant conditions. |
Current through coils
John Popelish wrote:
We are not talking about L, C, R, or any other inherent property changing with frequency. The velocity factor of the coil is based on those quantities and can be calculated. The velocity factor of a transmission line is based on those quantities and can be calculated. Freq 1 2 4 8 16 MHz Delay ___ ___ 3 ___ 16 nS That looks non-linear to me. How about you? Definitely nonlinear, just like impedance is very nonlinear as the frequency passes through any resonance. Care to fill in the blanks above? This is why I am suspicious of a measurement made at resonance, being extrapolated to non resonant conditions. Self-resonance is simply where the round trip delay through the coil puts the forward and reflected voltages and the forward and reflected currents either at zero degrees or 180 degrees. That's what happens at an open-ended 1/4WL stub. That's also what happens at the feedpoint of a resonant standing wave antenna like a 75m mobile bugcatcher antenna. Resonant mobile antennas are "self-resonant antenna systems". -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
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