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#21
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Gene Fuller wrote:
Isn't superposition wonderful! 73, Gene W4SZ Yup, it's why I find religion so amusing. tom K0TAR |
#23
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Cecil Moore wrote:
Gene Fuller wrote: Why would anyone try to prove that the basic math of adding sinusoidal functions is incorrect? To the contrary, you are the one who insists that a standing wave and its constituent traveling wave components are somehow different and unique. Actually, it was you who made that assertion and thanks for the opportunity to quote you once again: 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. So standing waves are "somehow different" from traveling waves according to your own assertions. The traveling wave possesses phase characteristics and the standing wave doesn't. Cecil, You keep making the same mistake. Yes, you can analyze traveling waves instead of standing waves if you so choose. However, there is not one bit of additional physical information in the traveling waves that is not in the standing wave. Any "phase characteristic" is simply a function of the mathematical manipulations you use. Perhaps someday you will actually understand superposition, but I won't hold my breath. 73, Gene W4SZ |
#24
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Gene Fuller wrote:
However, there is not one bit of additional physical information in the traveling waves that is not in the standing wave. I agree with you but W8JI and W7EL have rejected the concept that there is any phase information in the standing wave current magnitude. They have rejected any use of the arc-cosine function in calculating that phase. The following graphs show the difference in the standing wave current and the traveling wave current. http://www.qsl.net/w5dxp/travstnd.GIF The standing wave current phase contains zero phase information as you have stated. As you say, all the standing wave current phase information is contained in the magnitude but the arc-cosine function for obtaining that phase information has been rejected by the experts. For the traveling wave, there is phase information contained in the phase, none in the magnitude. Every time you make a technical assertion, you support my argument. Seems your argument is really with the side that rejects the arc- cosine function for obtaining phase information. -- 73, Cecil http://www.qsl.net/w5dxp |
#25
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En/na Roy Lewallen ha escrit:
EA3FYA - Toni wrote: I know that a coax cable does not radiate (if common mode currents properly suppressed) because both conductors are apparently "in the same place" (wouldn't know how to express it in more technical terms). Here's why it doesn't radiate: In a coaxial cable with a solid shield, the differential mode current is entirely inside the shield. Current and fields penetrate only a very small distance from the inner surface of the shield, and no significant amount ever makes it through to the outside. This is assuming that the shield is at least several skin depths thick, which is a good assumption at HF and above. When you say "Current and fields penetrate only a very small distance...", I agree for the current part, but I'm not so sure for the fields part: As I understand it you can not "stop a field" in no way, though you can certainly nullify it with an identical but opposite field. Then the question is whether the two fields (the one from the current flowing in the shield + the one from the current flowing in the inner conductor) nullify at all points in the immediate vicinity of the shield. I certainly believe it but would like to understand why this is so. I guess the mathematical proof would involve assuming the braid is an infinite number of conductors equally spaced around the center conductor, each having it's infinitesimal share of the shield current, and integrating all of their fields at the point of interest (Would probably be able to do so back when I was at university but now it is too strong math for me). Would this be a good approximation of the problem? -- Toni |
#26
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Thanks four your answers.
I was forgetting you normally use coax in a unbalanced configuration where the braid is supposed to be at 0 voltage so only currents matter. Would all this still hold if you used the coax as a _balanced_ transmission line? (unusual but -I think- possible). In this case wouldn't voltages develop on the braid that could capacitively couple to other conductors? (assuming perfect solid shield, ...) -- Toni |
#27
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the voltage on the braid is not zero on the inside, it varies along with the
wave traveling along the inside of the coax. and the currents are exactly balanced inside the coax also, they have to be or it wouldn't work. this notion of balanced vs un-balanced transmission lines is totally confusing to most people, in a proper system, say just with a dummy load on a coax the currents on the shield exactly balance the current on center conductor. so why do we go through all this stuff with bal-uns?? and coax chokes?? the currents are already balanced, so WHY?? come on you gurus out there, explain this one! "Toni" wrote in message ... Thanks four your answers. I was forgetting you normally use coax in a unbalanced configuration where the braid is supposed to be at 0 voltage so only currents matter. Would all this still hold if you used the coax as a _balanced_ transmission line? (unusual but -I think- possible). In this case wouldn't voltages develop on the braid that could capacitively couple to other conductors? (assuming perfect solid shield, ...) -- Toni |
#28
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Dave wrote:
so why do we go through all this stuff with bal-uns?? and coax chokes?? the currents are already balanced, so WHY?? come on you gurus out there, explain this one! Water comes out of a hose whether the hose is leaky or not. So why ever bother patching or replacing a leaky hose? -- 73, Cecil http://www.qsl.net/w5dxp |
#29
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Cecil Moore wrote:
Gene Fuller wrote: However, there is not one bit of additional physical information in the traveling waves that is not in the standing wave. I agree with you but W8JI and W7EL have rejected the concept that there is any phase information in the standing wave current magnitude. They have rejected any use of the arc-cosine function in calculating that phase. The following graphs show the difference in the standing wave current and the traveling wave current. http://www.qsl.net/w5dxp/travstnd.GIF The standing wave current phase contains zero phase information as you have stated. As you say, all the standing wave current phase information is contained in the magnitude but the arc-cosine function for obtaining that phase information has been rejected by the experts. For the traveling wave, there is phase information contained in the phase, none in the magnitude. Every time you make a technical assertion, you support my argument. Seems your argument is really with the side that rejects the arc- cosine function for obtaining phase information. Cecil, You still don't get it. When I said the phase information was gone, I meant it. Any phase information you think you find by looking at the constituent traveling waves is merely an artifact of the math. It has no physical meaning or reality. If there is anything interesting left in the traveling wave analysis, then the standing wave is not the complete representation of the electromagnetic phenomena. This is a different problem. Yes, you can apply modulation, insert directional couplers, look at startup transients, or perform other tricks to get "real" phase information. However, that again becomes a different problem, not the original simple steady-state combination of traveling waves into a standing wave. 73, Gene W4SZ |
#30
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![]() Toni wrote: En/na Roy Lewallen ha escrit: EA3FYA - Toni wrote: I know that a coax cable does not radiate (if common mode currents properly suppressed) because both conductors are apparently "in the same place" (wouldn't know how to express it in more technical terms). Here's why it doesn't radiate: In a coaxial cable with a solid shield, the differential mode current is entirely inside the shield. Current and fields penetrate only a very small distance from the inner surface of the shield, and no significant amount ever makes it through to the outside. This is assuming that the shield is at least several skin depths thick, which is a good assumption at HF and above. When you say "Current and fields penetrate only a very small distance...", I agree for the current part, but I'm not so sure for the fields part: As I understand it you can not "stop a field" in no way, though you can certainly nullify it with an identical but opposite field. You bet you can stop a field. It can be stopped either by reflection, absorption, or a combination of the two. Inside an anechoic chamber, absorbing materials stop internal fields to prevent reflections. A screen room or metallic shield reflects external fields. Then the question is whether the two fields (the one from the current flowing in the shield + the one from the current flowing in the inner conductor) nullify at all points in the immediate vicinity of the shield. I certainly believe it but would like to understand why this is so. Indeed they do. Look up Ampere's Law. If you draw a boundary through the middle of the shield or outside the shield, you'll find that the sum of currents within that boundary is zero. According to the law, that means that no net field penetrates the boundary. Because of the physical symmetry, no net field means no field at all. I guess the mathematical proof would involve assuming the braid is an infinite number of conductors equally spaced around the center conductor, each having it's infinitesimal share of the shield current, and integrating all of their fields at the point of interest (Would probably be able to do so back when I was at university but now it is too strong math for me). Would this be a good approximation of the problem? No, it's not that complicated, but a path or surface integration is required to use Ampere's law. Roy Lewallen, W7EL |
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