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Cecil Moore wrote:
wrote: Stick with Vfwd and Vref; they always work. On the contrary, from the results of Roy's calculations, it appears that Vfwd and Vref cannot be used to predict total forward power and total reflected power in a lossy line system. Of course if you let go of Pfwd and Pref and just used Vfwd and Vref you would quickly learn that you had no interest in Pfwd and Pref and thus your inability to determine them would not cause you much distress. Which is good since it will turn out that there is no value in forward and reverse power on a line with non-real Z0. ....Keith |
It's simple, yet too complicated for you to actually do it and show us.
Over the years, I had the unfortunate experience to occasionally meet an engineer for whom everything was very simple. They could never understand why it took the rest of us so long to solve all those really trivial problems. But after watching these folks in action for a while, I noticed that they were somehow never able to actually produce any hardware that actually worked. When pressed, they were very skilled at working themselves into a spot where they wouldn't ever have to actually deliver a working device, changing groups, projects, or responsibilities whenever they got too close to actually having to deliver. One common ploy was to regard the mere creation of a working model beneath their dignity and an unworthy use of their great knowledge. Surely you're not one of those, are you? Roy Lewallen, W7EL Cecil Moore wrote: Roy Lewallen wrote: The "apparent" violation is entirely in your own mind. You claim that the reflected power is greater than the forward power while at the same time the Poynting vector points toward the load. That's an obvious contradiction which can be remedied simply by collecting like terms. -- 73, Cecil, W5DXP |
In another branch of this thread it has been demonstrated that the
conventional RC formula correctly predicts the reflected voltage for lines with non-real Z0. Convential RC is the general predictor for transmission lines. I disagree with the demonstration. Maybe you are a bit biased? :) Use conventional RC formula for purely real Zo only, which is what we mostly do anyways. The caveat is that for lines with non-real Z0, |rho|^2 can not be used to predict reflected power. |rho|^2 predicts power only for the special case of lines with real Z0. This may be true, but are you saying that a capacitor can reflect an RMS voltage wave that is greater than the one that charges it? Slick |
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Radio913 wrote:
wrote in message ... Its not. Its 3.8 volts, which is entirely consistent with circuit theory and 'classic' rho. Not only is it higher than the incident voltage, it is higher than the source voltage. Ok, now take the capacitor off, and measure the voltage at the end of the inductor. What do you get? Not possible for me. My scope probes are in the order of 15 pf, which is signifcant for this experiment. Afraid to measure it, eh? Go ahead, we can ignore the 15 pF for now, as the load was 100pF. What do you get? You asked for an open. 15pf is about 1000 ohms at 10 Mhz. This is a long way from an open. In any case, I suggest it might be your turn to produce some experimental results. But try using classic rho and revised rho to predict the results for a shorted load (Zl = 0). Only classic rho gives a reasonable result for voltage and current. Classic Rho gives -1, which is a short, and conjugate Rho gives +1j, which is ALSO a short. As I recall, the purpose of rho was to compute the reflected voltage so that net voltage could be computed using: Vnet = Vfwd + Vref Using rho = -1 produces Vref = -Vfwd yielding Vnet = 0 as expected for a short. Using rho = +1j produces Vref = +1j * Vfwd which does not produce Vnet=0. This is not the expected result for a short. Note that the success and failure above are independent of the actual value of Vi. Although when I compute rho using the revised formula for the experiment being considered I obtain rho = 0.885 + j0.464 = 1.0 /_ 27 rho = (Zl-Z0conj)/(Zl+Z0conj) = -(34-j138)/(34+j138) = as above Has a new revised definition for rho suddenly appeared? Hint: What is the center of the Smith Chart when it is normalized to Zo=50+j200? Not really. A rho= -1 means something else with a complex Zo. Hint: What is the center of the Smith Chart when it is normalized to Zo=50+j50? Let's review. Two competing proposals (classic and revised rho) were used to predict the outcome of an experiment. The experiment was performed; the results resoundingly in favour of classic rho. I disagree. You need to answer the "hint" question. I invite you to demonstrate an error in the logic, the evaluation, or the experimental results. Riddles just do not cut it. Plus, you never measured the Vi (incident voltage wave) coming out of the inductor, which is what you would use for the voltage RC. Are you questioning the scientific method or just the results of this experiment? If the former, the discussion should probably move to a different group; if the latter, you are invited to replicate the experiment and demonstrate that an error was made. I question your methods and your calculations. Feel free to point out the flaws. If you can find none, question why you hold so tenaciously to revised rho when it does not work. I will try the experiment when i get the chance. Excellent. There is nothing better than seeing it with your own eyes. Are you saying that a capacitor can reflect a RMS voltage wave that is greater than the incident voltage wave that charges it? That is the result of the theory and of the observation. What more can one ask for? (And since the circuit is resonant, it should not really be a surprise). ....Keith |
Richard Harrison wrote:
If the transmitter is matched to the feedline to deliver maximum power, no reflected power gets through the matching network. This means that all reflected power is re-reflected by the network. But then if there is no matching network, the reflected power must enter the transmitter. Where does it go then? Is this what cooks the final? This is demonstrative of the difficulties that arise when the loose wording promulgated by Bird, et al, is accepted literally. Not to mention the difficulties that arise on lines with complex Z0. Reasonable answers are only obtained once this view of reflected power travelling back along the line is discarded. It is a voltage wave which does the travelling. Once this view of reflected power is discarded, you will be free to study the implementation of your Bird and understand how it computes the average of p(t) = v(t) * i(t) (the real definition of average power on the line), by doing some additions and subtractions of v(t) and i(t) and displaying it on an appropriate scale. Unfortunately for clear understanding, the intermediate results of this mathematical manipularion have been labelled Pfwd and Pref with the result that many believe these actually exist. I'd encourage anyone who doubts to do the derivation and show that when you subtract the Pfwd and Pref displayed by your Bird that all you have done is calculate Pnet = average(v(t) * i(t)) in a round about fashion, so that of course it produces the right answer. But this is no reason to ascribe physical meaning to Pfwd and Pref, especially when it is clear that the whole notion collapses in the general case of lines with complex Z0. Rejecting the notion of Pfwd and Pref does not mean that your Bird will stop being useful. When used to obtain a Pref of 0 it will be functioning perfectly fine as a TLI and when Pref is not 0, you can still obtain Pnet by subtracting Pref from Pfwd. But you will now understand how it really works and will not be misled by false labels. ....Keith |
Richard Harrison wrote:
Bird Technical Series #1 may be enlightning: "By proper combination of the two samples (derived from volts and amps) we obtain an RF voltage proportional to the square root of main line power---." "---the scale which has been marked in watts corresponding to the power being sampled from the main line." IF Z0 is perfectly resistive, or if the line is distortionless, then Pdelivered = Pforward - Preflected = |V+|^2/Z0 - |V-|^2/Z0 is perfectly correct (W.C. Johnson, pages 150-151). The Bird voltmeter readings (scale marked in watts) take into account phase relations between forward and reflected voltage waves and forward and reflected current waves. The formula and the Bird Company assume that low loss line is being used, so that Z0 is nearly resistive and the error due to non-resistive Z0 is small compared to the sum of all other system errors. If Z0 is sufficiently reactive that I have to doubt the usefulness of the Bird meter, then I need to take other action with respect to the lossiness of the coax. My 600 W PA was not intended to be a heating element for the coax. There is an old saying "if you look hard enough you can find fault with anything". Or anyone, including Terman and Bird. Bill W0IYH |
wrote:
Of course if you let go of Pfwd and Pref and just used Vfwd and Vref you would quickly learn that you had no interest in Pfwd and Pref and thus your inability to determine them would not cause you much distress. Uh Keith, radiated *POWER* is what we are trying to get from our antennas. -- 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! =----- |
Roy Lewallen wrote:
It's simple, yet too complicated for you to actually do it and show us. Over the years, I had the unfortunate experience to occasionally meet an engineer for whom everything was very simple. They could never understand why it took the rest of us so long to solve all those really trivial problems. But after watching these folks in action for a while, I noticed that they were somehow never able to actually produce any hardware that actually worked. When pressed, they were very skilled at working themselves into a spot where they wouldn't ever have to actually deliver a working device, changing groups, projects, or responsibilities whenever they got too close to actually having to deliver. One common ploy was to regard the mere creation of a working model beneath their dignity and an unworthy use of their great knowledge. Surely you're not one of those, are you? Nope, I accept your model and point out the conceptual error you made. Probably in grammer school, you were taught to collect like terms. Why are you so dead set against performing that simple necessary math function? You say "the real power that would be calculated for the incident wave alone" is the forward power but Chipman does ***NOT*** say that! In fact, he says there is additional interaction (interference power) that must be taken into account when Z0 is complex. -- 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! =----- |
wrote:
Reasonable answers are only obtained once this view of reflected power travelling back along the line is discarded. It is a voltage wave which does the travelling. Let's say we send a TDR pulse down the line and witness the reflection. Are you saying there's no power (energy) in the reflected pulse? -- 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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