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Cecil Moore wrote:
wrote: May I suggest that if you had read the posting to which I responded and the rest of my response you would have found exactly the example you are looking for: the forward voltage and current on a transmission line when a standing wave is present (and the reflected as well). How did the standing wave get there in the first place? *POWER* from the generator. You simply cannot have standing waves without a power source, a forward wave, and a reflected wave. You are asking us to completely ignore the cause of standing waves. It is not obvious to me how you extrapolate my postings to these outrageous assertions. For sure there is power from the generator. It is needed to charge the line and to provide whatever power is consumed in the load and line losses. When a standing wave is present, for sure there is a forward and reflected voltage and current wave. After all it is called a voltage standing wave. But these voltage and current forward and reflected waves do not have power. They are exactly the same as the voltages computed using superposition in circuit analysis, they are superposed in exactly the same way to find the resultant voltage, and it is illegal, except in special cases, to assume that these constituent voltage terms represent power. May I suggest, for clearer understanding, that just for a few moments (say 30 minutes), you set aside RF and consider how a line is charged by a step function. Do the voltage and current reflection diagrams. And then consider the energy flow just in front and just following the voltage step as it propagates down the line and back and down and back... Take the time to do this for the following cases... - matched generator - line terminated in Z0 - line open - line shorted After the line has charged consider what happens when the generator voltage is set back to 0. Do it all again for a mismatched generator. Then for a charged open termination line, consider what happens when a load of Z0 is applied. And then when the load is removed. For each of these cases determine how the voltage fronts propagate, the energy flow in front of and following the step, the resulting energy distribution on the line and whether this energy is stored in the capacitance or inductance or H field or E field. Because of the step function excitation, none of these computations are difficult. With this example it is easy to see when energy is flowing and when it is not, and contrast this to the energy flows computed using the forward and reflected voltages. Well maybe the above is more than 30 minutes, but there is much to be learned from a thorough understanding of the behaviour with this simple excitation. Now replace step excitation with sinusoidal; the principles are the same, but the computations are more complex and the resulting voltage and energy distributions on the line are more interesting. But the fundamentals are the same. The above thought experiment was the one that made clear to me the fallacy of assigning power to the forward and reflected voltage waves. So there is some risk for you doing this thought experiment; the results may conflict with some of your deeply held beliefs. It is a risk worth taking. ....Keith |
"Ian White, G3SEK" wrote:
Keith wrote: [...] in general (using my definition of general) [...] in general (using my definition of general) [...] in general (using my definition of general) [...] in the general case (using my definition of general) You don't have to be defensive about this, Keith - you're on solid ground. In this discussion, where we're trying to be scientific, you are using the word in the correct *scientific* sense, meaning "in all cases." Cecil is using it in a different and looser sense, meaning "in common cases." "General" is just another of those words like "theory" where the scientific usage and regular conversational usage are almost completely opposite. Yes, indeed, 'general' is one of those words with multiple mutually inconsistent meanings. My dictionary gives seven definitions for the adjective form and they alternate between meanings which 'include all' and those which 'include most'. ....Keith |
wrote:
But these voltage and current forward and reflected waves do not have power. The source puts out power. If that energy doesn't go into the forward and reflected waves, where does it go? I am not going to do any of your thought experiments until you stop ignoring the questions I previously asked you about mine. 100W source---one second long 291.4 ohm lossless line----50 ohm load During steady-state, the transmission line contains 300 joules that have not made it to the load. Where did that 300 joules go? -- 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! =----- |
Cecil More wrote:
wrote: Cecil Moore wrote: power = volts * amps = (Vfwd + Vref) * (Ifwd + Iref) = Vf*If + Vf*Ir + Vr*If + Vr*Ir Seems you've lost a couple of terms in there. As with any other math model, the negligible secondary terms are dropped. If the phase angle above is close to 90 degrees, two terms drop out. A Bird wattmeter assumes that two of the phase angles are close to 90 degrees. If they are not, you are using the wrong instrument for your measurements, a common problem. So again, it works in the specific, but not in the general case. Does this mean that forward and reverse waves only have power in the special case of low-loss feedlines? It means that if you use a 50 ohm Bird wattmeter in any environment except a 50 ohm one, you are using the wrong instrument. The answer appears orthogonal to the question. With my model, incident and reflected VOLTAGE waves and CURRENT waves do exist. This is in common, I think, with most authors on the subject. And this all works fine since superposition holds for voltage and current. It certainly doesn't work with your assertion that reflections don't exist. Go back 3 sentences and re-read "reflected VOLTAGE waves and CURRENT waves do exist". I think you were quoting from something I wrote. Your math model would have us believe that a power source pumps energy into voltage and current waves only to have that energy disappear from the universe for a time that is convenient for your math model. Of course, that very energy is magically created once again at the load. We've heard this bad magician's trick before. "Close your eyes while I make this elephant disappear." I am unsure how you extrapolate this from my writings. To recap. Everything with the incident and reflected wave model works as long as you stick to voltage and current waves. It is only when extended to include power (as done by Bird and others), that the model starts to deteriorate. For a resistive 50 ohm environment, the model does not deteriorate. The Bird is designed for such an environment. There doesn't exist any real-world instrument that will measure anything and everything. Every real-world measuring instrument has limitations. If you attempt to use a Bird in a 100-j100 ohm environment, that is your problem, not Bird's. So you are agreed: The reflected POWER model does not work in general. So to get the right answers in the general case (using my definition of general), compute your forward and reverse voltages and currents. Use superposition to derive the resultant voltages and currents at any point on the line and then use p(t) = v(t) * i(t) to compute the power, which you may then average if you desire. We've already been down this road but here it is, once again. Consider a one second long lossless feedline with an SWR of 5.83:1 and a Z0-matched source of 100 watts. After steady-state is reached, the feedline contains 300 joules of energy which cannot stand still. A Bird wattmeter reads 200w forward and 100w reflected. Your power calculation gives 100 watts everywhere implying that there are only 100 joules in transit in the feedline. No. You have made an incorrect implication. 100 watts flowing says nothing about how much energy is stored. What happened to the other 200 joules pumped into the system by the source during the transient state? Hint: there is 100 joules in the reflected waves and 100 joules in the re-reflected waves. The Bird is correct. Your experiment appears to be incompletely or incorrectly specified. If the source is Z0 matched to the transmission line, how did you get a re-reflected wave at the source? ....Keith |
Newton may not have been EXACTLY right but he had a damned near-enough
practical approximation. What was his approximation for the orbit of Mercury? :-) ====================== In MY thermometer mercury does not rotate - it just goes up and down. According to Bush it's something to do with apple trees. --- Reg |
And then consider the energy
flow just in front and just following the voltage step as it propagates down the line and back and down and back... ======================= Kelvin had trouble with voltage steps when trying to predict signalling speed on the first Atlantic telegraph cable. They went ahead, chartered the Great Eastern steamship, steamed West and laid the thing anyway. Shortly afterwards it broke. Kelvin was created a Lord for his un-finished services and had a bridge named after him. But it was indeed a difficult problem in that day and age. Twenty years later, hard-of-hearing Heaviside invented a brand new branch of mathematics, the Operational Calculus, to solve that particular, and a great number of other problems. For HIS services to mankind, as a revolutionary, he was derided by the pompous Establishment whose members resorted only to plagiarised text books. Heaviside only had a Layer named after him. To their own credit, it was American communications engineers who eventually acknowledged his genius. But then, Americans always did have sympathetic feelings towards revolutionaries. What amazes me is the amount of trouble some modern American engineers still appear to suffer from on the subject of propagation of an electric current along a pair of wires. Wires have been around a long time now. Instead of thinking in terms of frequency and waves why not do as Oliver did and try time and waveshape. It worked for him. Exactly what, where and when is being reflected becomes clear. And if anybody enjoys playing with numbers just replace 'j-omega' with 's' (It was 'p' when I first played with it.) Incidentally, the concept of wire-gauges originated in my home city, Birmingham, England. Faraday was familiar with it. It was internationally known as the BWG. The Americans, just to be different, changed theirs to AWG. Now (nearly) everybody has gone metric. ;o) ---- Reg, G4FGQ |
Reg, G4FGQ wrote:
"Instead of thinking in terms of frequency and waves why not do as Oliver did and try time and waveshape?" The digital revolution is well underway and Reg has the right idea. Radio is the domain of frequency and waves however. Terman`s first words: "Electrical energy that has escaped into free space exists in the form of electromagnetic waves." are still true. Step functions lost interest with the demise of telegraph, but ones and zeros are back bigger than ever. Best regards, Richard Harrison, KB5WZI |
On Tue, 23 Sep 2003 11:15:42 +0100, "Ian White, G3SEK"
wrote: Eschew sesquipedalianism! Gesundheit. |
Richard Clark wrote:
What is the V/I for a 1 degree rise? Same as before, Richard, dissipative. This is a binary measurement. The possible values are dissipative and non-dissipative. -- 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! =----- |
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