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#31
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Thanks.
So between equations 8.29 and 8.30 the author calculates VSWR without any source-related terms -- as every other textbook author does. Roy Lewallen, W7EL Wes Stewart wrote: On Mon, 29 Nov 2004 12:57:16 -0800, Roy Lewallen wrote: |I offer a third third response. | |On p. 175, Chipman states: | |"Equation (8.27) demonstrates explicitly that the shape of a standing |wave pattern representing |V(d)| as a function of d on a transmission |line is in no way affected by the quantities Vs, Zs and [rho]s at the |source." | |And equation 8.29 on p. 176, the calculation of reflection coefficient, |contains no source-dependent terms. I'm sure that somewhere in the book, |the author derives SWR in terms of reflection coefficient. Indeed he does---on the next page. http://users.triconet.org/wesandlind...manPage177.pdf Equation (8.30) 1 + |rho| VSWR ------------- 1 - |rho| |
#32
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On Mon, 29 Nov 2004 12:57:16 -0800, Roy Lewallen
wrote: |I offer a third third response. | |On p. 175, Chipman states: | |"Equation (8.27) demonstrates explicitly that the shape of a standing |wave pattern representing |V(d)| as a function of d on a transmission |line is in no way affected by the quantities Vs, Zs and [rho]s at the |source." | |And equation 8.29 on p. 176, the calculation of reflection coefficient, |contains no source-dependent terms. I'm sure that somewhere in the book, |the author derives SWR in terms of reflection coefficient. | |These are the facts: | |1. The SWR, positions of the standing waves, reflection coefficient seen |looking into the line, impedance seen looking into the line, and dB line |loss are independent of source impedance. |2. The actual amount of power delivered to a line for a given Thevenin |source voltage will, of course, depend on the source impedance, just as |it would if the source were directly connected to a load. Therefore, the |absolute amount of power dissipated in the load depends on source |impedance. The dB line loss, however, doesn't. Also, the length of time |the line requires to reach equilibrium after initially turning on the |source depends on the source impedance. | |These can be found, explicitly stated and/or in easily interpreted |equation form, in a host of references. | |I see nothing in the text Wes has kindly posted which contradicts these |facts, and I'm sure there's nothing elsewhere in the text that does. | |I often have a hard time understanding Richard's postings, so it's |possible that he's not disagreeing with the statements I've made, |either. If so, I apologize for the misinterpretation. In an earlier post to this thread, Richard stated: |"This is yet another of my references that attend to my recent, short |thread on the nature of power determination error, and mismatched |loads AND sources. In fact ALL of these references I've offered |explicitly describe that the source MUST be matched for ANY of these |equations about transmission lines bandied about to accurately offer |true answers. The naive presumptions that Source Z is immaterial to |the outcome of analysis is quite widespread here." I almost demurred, much as Roy did, because this statement is not universal, but I held off because I believe (and hope) that Richard is talking only about *power* measurement errors. Wes |
#33
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On Mon, 29 Nov 2004 12:57:16 -0800, Roy Lewallen
wrote: |I offer a third third response. | |On p. 175, Chipman states: | |"Equation (8.27) demonstrates explicitly that the shape of a standing |wave pattern representing |V(d)| as a function of d on a transmission |line is in no way affected by the quantities Vs, Zs and [rho]s at the |source." | |And equation 8.29 on p. 176, the calculation of reflection coefficient, |contains no source-dependent terms. I'm sure that somewhere in the book, |the author derives SWR in terms of reflection coefficient. Indeed he does---on the next page. http://users.triconet.org/wesandlind...manPage177.pdf Equation (8.30) 1 + |rho| VSWR ------------- 1 - |rho| |
#34
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Once again, it's not clear to me just what you're trying to prove.
Do you disagree with either of the two numbered statements made in my posting? If so, which part(s) of which one(s) -- I'm sure I can demonstrate their correctness. If not, we probably don't disagree. Roy Lewallen, W7EL Richard Clark wrote: On Mon, 29 Nov 2004 12:57:16 -0800, Roy Lewallen wrote: I'm sure that somewhere in the book, the author derives SWR in terms of reflection coefficient. Hi Roy, No doubt. As I am not immediately interested in the reductionist art of SWR, then the remainder seemed of little relevance to line losses and the contribution of source Z to power determination error. You continue, in part quote of a part quote: representing |V(d)| as a function of d which given you offer no more comment upon it, gives me the impression you are unaware of what function d is. The point of the matter is that this very equation you chose is examined in isolation, by you, but is returned to on several occasions by Chipman where he quite "explicitly" exhibits how V(d) ranges wildly for situations where both ends of the line are terminated by forced mismatches. This is a uncommon technique for determining SWR (still not my point, but nonetheless an obvious example). And yes, I realize I often have a hard time understanding Richard's postings and I often grieve over this on long winter evenings. Roy, you are too coy by half. ;-) However, your aside into SWR shape and the focus on reductions to typical applications (source matches line) does leave a dilemma because there is now conflict between your isolated quote of Chipman and the demonstration of EZNEC as reported by my late, short lived thread. I would offer that EZNEC fully supports Chipman's other comments on this same quoted material you drew from him, and goes to the matter I offered of an uncommon technique for SWR determination. My EZNEC reports are also supported by bench results, and other sources also recited here. All this seems to leave you on the outside looking in. As it broaches upon topics that you have long cautioned me that discussion would not "change your mind," I doubt this will go any further. 73's Richard Clark, KB7QHC |
#35
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Roy Lewallen wrote:
I see nothing in the text Wes has kindly posted which contradicts these facts, and I'm sure there's nothing elsewhere in the text that does. None of us are perfect. All of us (except Jim Kelley :-) will admit to being human, i.e. capable of making a mistake. The Z(s) of the source, no doubt, has an effect on the power sourced by the source. But the "power sourced by the source" has no effect on SWR, which is independent of source impedance. Given a steady-state forward power, nothing else depends upon source impedance. If a one ohm source is capable of supplying the same voltage as a one megohm source, the steady-state results will be identical. Given any source with any source impedance, there exists a forward power. Given any forward power, the source impedance during steady- state is completely irrelevant. -- 73, Cecil http://www.qsl.net/w5dxp |
#36
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On Mon, 29 Nov 2004 12:57:16 -0800, Roy Lewallen
wrote: I'm sure that somewhere in the book, the author derives SWR in terms of reflection coefficient. Hi Roy, No doubt. As I am not immediately interested in the reductionist art of SWR, then the remainder seemed of little relevance to line losses and the contribution of source Z to power determination error. You continue, in part quote of a part quote: representing |V(d)| as a function of d which given you offer no more comment upon it, gives me the impression you are unaware of what function d is. The point of the matter is that this very equation you chose is examined in isolation, by you, but is returned to on several occasions by Chipman where he quite "explicitly" exhibits how V(d) ranges wildly for situations where both ends of the line are terminated by forced mismatches. This is a uncommon technique for determining SWR (still not my point, but nonetheless an obvious example). And yes, I realize I often have a hard time understanding Richard's postings and I often grieve over this on long winter evenings. Roy, you are too coy by half. ;-) However, your aside into SWR shape and the focus on reductions to typical applications (source matches line) does leave a dilemma because there is now conflict between your isolated quote of Chipman and the demonstration of EZNEC as reported by my late, short lived thread. I would offer that EZNEC fully supports Chipman's other comments on this same quoted material you drew from him, and goes to the matter I offered of an uncommon technique for SWR determination. My EZNEC reports are also supported by bench results, and other sources also recited here. All this seems to leave you on the outside looking in. As it broaches upon topics that you have long cautioned me that discussion would not "change your mind," I doubt this will go any further. 73's Richard Clark, KB7QHC |
#37
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On Mon, 29 Nov 2004 15:14:42 -0800, Roy Lewallen
wrote: Once again, it's not clear to me just what you're trying to prove. Hi Roy, Troubles me too.... In the interim I have recognized where I went off the deep end in the thread I have alluded to. The attempt there was to reproduce, in EZNEC, results obtained at the bench and offered through various references which demonstrate Mismatch Uncertainty. Although the model of the transmission line was certainly of the highest quality, EZNEC lacks the capacity to separate forward and reverse waves for analysis. In that regard I mistakenly attributed the results I obtained to my bench results. Situation is that with a SWR meter moving along a line mismatched at both ends, there is a distinct variation in the computed Power. When I did this at the bench, I could evidence about a 30% variation which was consistent with theory and clearly exhibits the contribution of Source Z when it is other than 50 Ohms. When I recently attempted to model this in EZNEC, I again saw the wild fluctuation of power that seduced me with its complementary results into thinking I had achieved the same results. When I revisited those results (EZNEC ones that is), all I had done was prove the mismatch through the abstraction of the power reported at my moving test load. When I reverse engineered the voltages from the known R, it became obvious that the VSWR corresponded to every expectation - or was close enough given the degree of resolution I had available with 20 test points distributed along the line. Chipman describes this in his late chapters - one of which Wes has provided. However, this does nothing to detract from Chipman's work that includes the FULL treatment of all variables that lead to the common usages. Such treatments include the Source with full characterizations and goes on to discuss the R of the Source and not just its Z. That so many texts disregard this level of examination does not deny its importance to issues that go beyond SWR. Those lesser texts presume Source Z unlike Chipman, and its significance is lost to the student who hasn't been grounded in the fundamentals. And thus we arrive at the topic at hand and listed in the Subject Line: Additional Line Losses Due to SWR. In that regard, the Source Z is entirely an active player and any additional mismatch that it presents to the system eventually finds additional loss (caloric) injected into it. Chipman's work in that regard is complete enough to offer Bob the framework to render a complete solution and to explain how and why this additional loss appears. It may not bear on his measurements directly (the measure of a stub's Q as it eventually turns out), but it is related closely enough to provide tangible leads. 73's Richard Clark, KB7QHC |
#38
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Richard Clark wrote:
Chipman's work in that regard is complete enough to offer Bob the framework to render a complete solution and to explain how and why this additional loss appears. It may not bear on his measurements directly (the measure of a stub's Q as it eventually turns out), but it is related closely enough to provide tangible leads. The fact remains that a transmission line/antenna system has the same characteristics whether one watt, 100 watts, or 1000 watts are input to it, i.e. the efficiency and SWR of the antenna system does not depend upon the source impedance. -- 73, Cecil http://www.qsl.net/w5dxp |
#39
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On Mon, 29 Nov 2004 22:18:56 -0600, Cecil Moore
wrote: the efficiency and SWR of the antenna system does not depend upon the source impedance. Life's like a box of chocolates, hmmm? Get off the bench. |
#40
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Roy, W7EL wrote:
"I`m sure that somewhere in the book the author (Chipman) derives SWR in terms of reflection coefficient." I`ve read, here I believe, that Chipman`s book is one of the "Schaum`s Outline" series. It would need to derive SWR in terms of reflection coefficient to be complete. Attenuation of a uniform transmission line is a function of the loss per unit length and the total length. A line with SWR has higher voltage and current in spots than a matched line. Thus it has higher loss. Analysis is complicated in lossy lines due to decline in SWR back from the reflection point. If SWR at the reflection point is less than 2:1, added loss due to SWR is hardly detectable. Such a line is considered perfect. The ARRL Antenna Book has graphs relating SWR to fopward and reflected power readings as given by a Bird wattmeter. The "Antenna Book" also has a graph of additional loss due to standing waves versus the SWR at the load for values between SWR=1.5 and SWR=20. I don`t have a dog in this fight. There is nothing particularly strange about solutions to equations which describe particular relations on a transmission line. The functions are not erratic but continuous and predictable. Selection of the right formulas is all that`s needed to get the right answer from the right data. Best regards, Richard Harrison, KB5WZI |
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