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#291
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I apologize. I read and was referring to the same quote, and interpreted it to mean that the first measurement was made with the coil at the base of the antenna. So where was it -- 78" from the bottom? Roy Lewallen, W7EL Yes, mast 78" - coil - 38" top whip we keep saying, looking at typical mobile antenna with loading coil about 2/3 up the quarter wave radiator. The lower the frequency, more loading, more pronounced effect. Caution, using toroid current transformers with scope leads would detune the antenna setup and introduce errors. You can get away with this at the base, but any stray capacitance up the radiator will detune it and skew the results. Need to use thermal RF current ammeters or current probe with detector and small meter together, no wires. Yuri, K3BU |
#292
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I'm sorry, I didn't catch the step where you got from cos(18 degrees) = 0.951 to 2.5%. Roy Lewallen, W7EL Just (in case) speculating that because of reflected wave either 5 or 2.5% reduction. I have not done the measurements yet. Yuri |
#293
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Thanks for the clarification.
I'm not entirely convinced that the ammeter is the best idea. There are enough internal wires and coils to introduce a real possibility of error when in close proximity to an inductor. It shouldn't be as much of a problem with a toroid, but I'm still a little leery. I agree it would be difficult to do the measurements well with a scope anywhere but at the base of the antenna. Current probes and a detecting meter might be ok, but you'd have to take a lot of care to avoid making an unintentional loop which would couple to the inductor, and you'd have to calibrate the potentially nonlinear detector. Phase information would be lacking, too. I'm waiting for Cecil's response, since by his theory, as I understand it, we should be able to get a decent phase shift through an inductor at the base of an antenna providing the antenna is significantly longer than a quarter wavelength. And if I understand your theory, we should be able to see a full 30% change in magnitude and 45 degree change in phase in the current through a base mounted inductor, if it's loading a 45 degree radiatior to resonance. Am I correct? I could measure that with the same setup but with an antenna removed from the mount. And 30% and 45 degrees should be much easier to resolve with any accuracy than the 2.5 or 5 percent you predict for the setup I did measure. Incidentally, I take it that your prediction for the setup I did measure includes an 18 degree phase shift of current from input to output of the inductor? Roy Lewallen, W7EL Yuri Blanarovich wrote: I apologize. I read and was referring to the same quote, and interpreted it to mean that the first measurement was made with the coil at the base of the antenna. So where was it -- 78" from the bottom? Roy Lewallen, W7EL Yes, mast 78" - coil - 38" top whip we keep saying, looking at typical mobile antenna with loading coil about 2/3 up the quarter wave radiator. The lower the frequency, more loading, more pronounced effect. Caution, using toroid current transformers with scope leads would detune the antenna setup and introduce errors. You can get away with this at the base, but any stray capacitance up the radiator will detune it and skew the results. Need to use thermal RF current ammeters or current probe with detector and small meter together, no wires. Yuri, K3BU |
#294
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Incidentally, I take it that your prediction for the setup I did measure
includes an 18 degree phase shift of current from input to output of the inductor? Roy Lewallen, W7EL Yes, I used Cecil estimate/calculation and taking cos 18 = 0.951056516 which is 4.8943483% Yuri |
#295
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Roy Lewallen wrote:
So now you're saying that any coil at the base of a short vertical antenna, regardless of its value, will have equal currents at the input and output? No, I didn't say that. I wish you would read what I say. If the coil is a low reactance (not many degrees) and the current maximum point is inside the coil, the two currents will tend to be equal. Ok, suppose I make the measurement at, say, 10 MHz, where the coil is no longer at the current maximum. Tell you what. I'll set up a 33 foot wire vertical, to eliminate the difficulty of the mounting arrangement. I'll furnish you the base impedance at 10 MHz, and even let you choose the inductor value. Be sure and choose a value that will clearly illustrate your point. Using the fine education you received from Balanis et al, calculate the current into and out of the inductor (phase and magnitude), and I'll set it up and measure it. Since it is a fair amount of work on my part, though, I'd like to do a dry run first, using, say, the base impedance predicted by EZNEC. Then, after you've shown us how you make the calculations, I'll build the antenna and do the measurement. I'd hate to go to the considerable trouble of setting it up and find that you somehow aren't able to do the calculation. I can't do the calculation because I don't know the attenuation factor. Do you think my inability to do the calculation proves anything about what's happening in reality at the antenna? You guys need to turn loose of the concept that what happens or doesn't happen on a piece of paper dictates reality. I can describe a base-loaded configuration that will demonstrate the principle. Take a 75m bugcatcher coil, one of the 6"x6" models, and choose a stinger that resonants the antenna in the 75m-80m band. Then measure the in and out currents at a frequency a little below resonance. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#296
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Yuri Blanarovich wrote:
If the feedpoint current was at 0 deg of the radiator length, and coil replaces 18 deg of wire, the cos 18 deg = 0.951 which should make difference, drop in the coil current 5% (or half, 2.5 deg?) Providing current maximum is exactly at the bottom end of the coil. It wasn't. The coil made the antenna too long so the current maximum was inside the coil. But this points up a measurement problem. I doubt that these measurements are 5% accurate. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#297
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Ok. So far, we have your calculation that the output current should be
5% smaller, and 18 degrees shifted in phase (lagging, I presume) from the input; and Cecil's, that the output current should equal the current, both in phase and magnitude. I don't know if Richard is going to do the calculation or not, so I'll wait a little longer. Anyone else like to hazard a prediction? Roy Lewallen, W7EL Yuri Blanarovich wrote: Incidentally, I take it that your prediction for the setup I did measure includes an 18 degree phase shift of current from input to output of the inductor? Roy Lewallen, W7EL Yes, I used Cecil estimate/calculation and taking cos 18 = 0.951056516 which is 4.8943483% Yuri |
#298
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Cecil Moore wrote:
Roy Lewallen wrote: . . . So in the past, you've predicted no difference, something like 20 or 45 degrees phase shift, or an indeterminate amount. It's good to see you've settled on one figure. There are three possibilities listed earlier. What happens with a coil depends upon where it is located. Please read that over and over until it soaks in. That's the problem. The more times I read what you've posted, the more confused I've gotten. My inductor was placed at the antenna base because I could measure the currents there with reasonable accuracy. Yep, you are looking for your keys under the streetlight because the light is better there than it is where you really lost the keys. You have a unique talent for turning an honest effort at being truthful and accurate into an insult, as you did with Ian. On his web site, Yuri quoted W9UCW as measuring the currents at the ends of a toroid mounted at the base of the antenna as being 100 mA at the bottom and 79 at the top. You must, then, believe these measurements to be in error. If the toroid is not mounted at a current maximum point, i.e. if the feedpoint impedance is slightly capacitive, then those figures could be accurate. I didn't pay any attention to them. Could be his coil causes a larger phase shift than your coil. You making your antenna too long ensured that the current maximum point would fall inside the coil. Whether you realize it or not, you are biasing the outcome of your experiment to agree with your pre-conceived (sacred cow) notions. This is precisely why I've given you the opportunity to choose the inductor for the 10 MHz test. You choose it so that it will best illustrate what you say is true. Shucks, I even encourage you to do the experiments yourself. . . . Roy Lewallen, W7EL |
#299
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Cecil Moore wrote:
Roy Lewallen wrote: So now you're saying that any coil at the base of a short vertical antenna, regardless of its value, will have equal currents at the input and output? No, I didn't say that. I wish you would read what I say. If the coil is a low reactance (not many degrees) and the current maximum point is inside the coil, the two currents will tend to be equal. I did read what you said. You said that it wouldn't exhibit a phase shift if placed at a current maximum. The current at the base of a short vertical antenna is at its maximum there. So now if you're saying that it *won't* exhibit a phase shift if placed at the base of a short antenna, let's try this. Suppose I remount my antenna to eliminate the shunting effect of the mounting, and do my measurements at 3.8 MHz as before. Suppose the base input Z is, say, 35 -j380. You choose any inductor value you'd like, that will best illustrate your method, and tell me what output to input current ratio to expect. Ok, suppose I make the measurement at, say, 10 MHz, where the coil is no longer at the current maximum. Tell you what. I'll set up a 33 foot wire vertical, to eliminate the difficulty of the mounting arrangement. I'll furnish you the base impedance at 10 MHz, and even let you choose the inductor value. Be sure and choose a value that will clearly illustrate your point. Using the fine education you received from Balanis et al, calculate the current into and out of the inductor (phase and magnitude), and I'll set it up and measure it. Since it is a fair amount of work on my part, though, I'd like to do a dry run first, using, say, the base impedance predicted by EZNEC. Then, after you've shown us how you make the calculations, I'll build the antenna and do the measurement. I'd hate to go to the considerable trouble of setting it up and find that you somehow aren't able to do the calculation. I can't do the calculation because I don't know the attenuation factor. What "attenuation factor" is it you need? Is it something that can be measured? If not, how about an equation or prediction with the "attenuation factor" as a variable? We can estimate a probable range of values, then see if the measurement results are within them. Do you think my inability to do the calculation proves anything about what's happening in reality at the antenna? You guys need to turn loose of the concept that what happens or doesn't happen on a piece of paper dictates reality. I hope to demonstrate what constitutes reality by theoretical analysis and by measurement. Where I come from, that counts much more than arm-waving, insulting, and vague explanations. Ultimately, each of the readers of these exchanges will decide what to believe, and I'm sure you will have convinced some. I can describe a base-loaded configuration that will demonstrate the principle. Take a 75m bugcatcher coil, one of the 6"x6" models, and choose a stinger that resonants the antenna in the 75m-80m band. Then measure the in and out currents at a frequency a little below resonance. I have no disagreement that a "bugcatcher" coil, or any coil of physically significant size, will exhibit a phase shift and magnitude change of current from one end to the other. Where we disagree is that you believe that a physically very small inductor will also exhibit this. I don't. I'm proposing a test which will show, with reasonable certainty, which viewpoint is correct. I fully expect every test I make to bring forth a flurry of objections. So I'm giving you the opportunity to choose the inductor which will best illustrate your point of view. I want to limit the parameters of the test to conditions I think I can measure with reasonable accuracy. With the equipment I've got, that pretty much limits me to doing measurements at the antenna base. But I think (although I'm still not sure) that you're now saying that there should be a substantial current difference between the input and output of a small inductor at the base of an antenna, if the antenna and inductor are properly chosen. So, you choose. And if you won't make the measurement, I will. Roy Lewallen, W7EL |
#300
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The absolute accuracy of the measurement isn't important. All that
matters is the accuracy of the ratio of currents at input and output, which is a lot easier to get with reasonable accuracy. What I'm looking for now, however, is your recommendation for a test which will clearly show the current ratio you claim will happen, of such a magnitude that the result will be clear even in the presence of a few percent error. Based on my measurements of currents with both probes on the same lead, and averaging results with probes reversed, I think I can measure the ratio within about 2% at ratios near 1, and resolve phase shifts of a few degrees. If you can come up with a test that'll produce 30% amplitude change and 45 degrees phase shift, I guarantee I tell whether the result is closer to that or to the zero amplitude change and zero phase shift I predict. Roy Lewallen, W7EL Cecil Moore wrote: Yuri Blanarovich wrote: If the feedpoint current was at 0 deg of the radiator length, and coil replaces 18 deg of wire, the cos 18 deg = 0.951 which should make difference, drop in the coil current 5% (or half, 2.5 deg?) Providing current maximum is exactly at the bottom end of the coil. It wasn't. The coil made the antenna too long so the current maximum was inside the coil. But this points up a measurement problem. I doubt that these measurements are 5% accurate. |
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