|
Current through coils
Roy wrote, "... That is, the coil is capacitively coupled to ground,
and this causes displacement current from the coil to ground." In fact, if there were no such current -- if there were no capacitance from the coil to the world outside the coil -- then the time delay through the coil, calculated from tau = sqrt(L*C), would be zero. It is exactly this current that allows there to be a transmission-line behaviour and a corresponding time delay. That's not to say, however, that a physically very small loading coil with practically no capacitance to ground would not work as a loading coil. It just wouldn't have a transmission line behaviour worth mentioning. It is also exactly this displacement current from a large coil that allows the current at one end of the coil to be substantially different from the current at the other end. Cheers, Tom |
Current through coils
Cecil Moore wrote:
Tom Donaly wrote: Cecil, that's the worst analogy I've ever read in my life. The PSK signals lose phase when they are superposed. The forward and reflected currents lose phase when they are superposed. Looks like a perfect analogy to me. Do you disagree with Gene Fuller? Gene Fuller, W4SZ wrote: 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. Do you disagree with Gene? How can Tom and Roy possibly use a signal whose phase cannot be recovered to measure phase? But Cecil, it can be recovered. See my earlier remarks. 73, Tom Donaly, KA6RUH |
Current through coils
Tom Donaly wrote:
This is just another way of writing 2Acos(kx+d/2)(e^i(wt+d/2). Notice that the part cos(kx+d/2) still contains the phase information? If Cecil were any kind of experimentalist he could easily tease the phase information out of any standing wave on his antenna system. I have previously teased that information from that equation. Perhaps you forgot. It's how to determine the exact phase shift along a thin-wire 1/2WL dipole. I showed how to do that days/weeks/years ago. It's the *phase* of the standing wave current that does not yield any phase information. I have been very careful with that caveat in my statements. cos(kx+d/2) indeed does still contain the phase information. If you will re-read my postings, you will see that I said the *PHASE* term of the reflected current doesn't contain any phase information. FYI, that's the e^i(wt+d/2) term and that part is what Roy used to make his phase measurements which has been my objection for years. It's all archived on Google. It is I, not Roy or Tom, who used the phase information in cos(kx+d/2) to determine phase. When the term containing the phase information is actually used, the delay through the coil is shown to be in the tens of degrees. In the 1/2WL thin-wire dipole, the phase shift between two points is arc-cos(amplitude1) - arc-cos(amplitude2). The only phase information is, as you and Gene Fuller rightly assert, in the amplitude of the standing wave current, NOT in the phase of the standing wave current that Roy measured. If the e^i(wt+d/2) term is used, as Roy and Tom have done, it suffers from the absence of any phase information at all. Gene Fuller said it all days ago: Gene Fuller, W4SZ wrote: 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. i.e. there's no remaining phase information in e^i(wt+d/2) term. The only "phase" remaining is the cos (kz) term, which is really an amplitude description, not a phase. i.e. there is phase information in the cos(k+d/2) term, but that's not the part of the wave that Roy and Tom were using to determine delay through the coil. I have been hoping someone would jump in who understood the math. To summarize: cos(kx+d/2) is proportional to the *amplitude* of the standing wave current. When I used the amplitude of the standing wave current to estimate the phase, the gurus objected. e^i(wt+d/2) is proportional to the phase of the standing wave current and, ironically contains no phase information, just as Gene asserted. Yet, this is what Roy chose to measure in trying to determine the phase shift through a coil and that's the entire problem with his measurements. He was expecting to measure zero phase shift and he did because there was no phase shift information available from his measurement of the e^i(wt+d/2) term. I told Roy a long time ago, in general, how to calculate the phase shift from the cos(kx+d/2 amplitude term but he replied with "gobbledygook" or some such. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
|
Current through coils
K7ITM wrote:
In fact, if there were no such current -- if there were no capacitance from the coil to the world outside the coil -- then the time delay through the coil, calculated from tau = sqrt(L*C), would be zero. It is exactly this current that allows there to be a transmission-line behaviour and a corresponding time delay. Tom, have you read what Dr. Corum had to say about that on page 8 of http://www.ttr.com/corum/index.htm? Here's a partial quote: "The problem has been that many experimenters working self-resonant helices have pursued the concept of coil self- capacitance without really understanding where the notion comes from or why it was ever invoked by engineers." -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
Tom Donaly wrote:
But Cecil, it can be recovered. See my earlier remarks. Yes, it can be recovered and I showed how years ago. Roy and Tom rejected that approach and instead reverted to measuring the phase of the standing wave current which is known to contain zero phase information. Go figure. Seriously, I showed those two how to calculate the phase shift in a 1/2WL thin-wire dipole using an arc-cos function. They responded with a personal attack. It's all on Google. If you will check my past postings on Google, you will find me saying, if the current at the base of the coil is one amp, we can estimate the phase shift through the coil by arc-cos(It) where It is the current out of the top of the coil. That is admittedly a very rough estimate since the coil distorts the current away from a perfect cosine envelope but it is closer than measuring the phase of a signal whose phase is known to be unchanging. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
I've been trying to tell Yuri (and others) that for three years now. 73 Tom Tom. You tell that to the RF ammeters installed on the vertical, W9UCW's pictures on my page! You can mumbo-jumbo all the theory, you can dream of, but reality shows that in the say, quarter wave vertical, with loading coil the current at both ends of the coil is different. Cecil explained the various situation depending where the coil is placed within the radiator and at overall antenna curve. Try this test, no meters necessary (perhaps the aquarium strip thermometer): Take your 80m Hustler antenna with Hustler loading coil and whip. At the resonant frequency put about 600 Watts to it for a while. Stop transmitting and go feel (or read the temperature on the strips) the coil, bottom end and the top end. Same temperature? Temperature is proportional to the current flow (same diameter wire) - warmer end - more current. Then test two: Keep the RF flowing until heat shrink tubing on the coil starts melting. Where does it melt first? Bottom of the coil or nicely uniformly as you claim it should? Then answer Cecil question about his demonstration of different currents at the ends! The rest is on my web page as I mentioned, with pictures. 73 Yuri, www.K3BU.us |
Current through coils
Tom Donaly wrote:
Gene Fuller, W4SZ wrote: 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. I think that if Gene believes that, he should redo his math. Tom, the math equations that you posted supports Gene's assertions 100%. They are essentially identical to the same equations that Gene posted. Many thanks to both of you guys for posting the technical facts. -- 73, Cecil http://www.qsl.net/w5dxp |
Current through coils
K7ITM wrote:
Roy wrote, "... That is, the coil is capacitively coupled to ground, and this causes displacement current from the coil to ground." In fact, if there were no such current -- if there were no capacitance from the coil to the world outside the coil -- then the time delay through the coil, calculated from tau = sqrt(L*C), would be zero. It is exactly this current that allows there to be a transmission-line behaviour and a corresponding time delay. Yes. And this, not the C across the coil, is what should be used for transmission line formulas when treating an inductor as a transmission line. When the ground was removed and replaced by a wire, the transmission line properties of the coil changed dramatically, while the C across the coil didn't change significantly. That's not to say, however, that a physically very small loading coil with practically no capacitance to ground would not work as a loading coil. It just wouldn't have a transmission line behaviour worth mentioning. It is also exactly this displacement current from a large coil that allows the current at one end of the coil to be substantially different from the current at the other end. Yes again, with one slight modification. You'll note from the EZNEC models that the current actually increases some as you go up from the bottom of the inductor. This is the effect noted by King which is due to imperfect coupling between turns. It results in currents at both ends being less than at the center. A transmission line can be represented by a series of L networks with series L and shunt C. You can achieve any desired accuracy by breaking the total L and C into enough L network sections. The requirement for validity is that the length of line represented by each section must be very small relative to a wavelength. For the example coil, a single section is entirely adequate at the 5.89 MHz frequency of analysis. However, at some higher frequency this model won't be adequate, and either more L sections or a distributed model is necessary. If the reasons for this aren't obvious, many texts cover it quite well. No special "traveling wave" analysis is required. I spent several years of my career designing very high speed TDR and sampling circuits, which involved a great deal of modeling. At the tens of GHz equivalent bandwidths of the circuitry, even very small structures such as chip capacitors and short connecting runs often had to be treated as transmission lines. One of the skills important to building an accurate model which would run in a reasonable amount of time, particularly on the much slower machines being used in the earlier part of that period, is determining when a lumped L, pi, or tee model is adequate and when a full-blown transmission line model has to be used(*). My models were used in the development of quite a number of circuits that were successfully produced in large numbers. (*) One of the characteristics of the SPICE programs at the time was that the time step was never longer than the delay of the shortest transmission line in the model. So if you willy-nilly modeled everything as a transmission line, you'd end up with an excruciatingly short time step and consequently unnecessarily long calculation time. Roy Lewallen, W7EL |
| All times are GMT +1. The time now is 05:44 AM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com