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I admit to being hazy about what you choose to call "forward power" and
"reflected power". So that's probably why I don't understand the meaning of "the reflected power can be greater than the forward power into a passive load." Does this mean that both "reflected power" and "forward power" flow into a load? Then, does the "reflected power" flow back out, to go back down the transmission line, or does it contribute to the load dissipation? Roy Lewallen, W7EL Cecil Moore wrote: I made it over to the Texas A&M library today and read Chipman. What seems to have been left out of the discussion are the following quotes from Chipman: "The conclusion is somewhat surprising, though inescapable, that a transmission line can be terminated with a reflection coefficient whose magnitude is as great as 2.41 without there being any implication that the power level of the reflected wave is greater than that of the incident wave." Chipman never said the reflected power can be greater than the forward power into a passive load. In fact, he says if X0/R0 is less than or equal to one, it is impossible for the reflected power to exceed the forward power. |
My analysis shows that fP - rP can have a negative value for some
load impedances when Z0 is complex. Please pay careful attention, though, to the definitions of these terms. As I used them, fP = Re{fE * fIc} and rP = Re{rE * rIc} where I'm using subscript c to denote complex conjugate to avoid confusion with "*" for multiplication, and E and I magnitudes are RMS. Although I didn't prove it in my analysis, the total net average power, which includes a third average power term, into a passive load of course can't be negative. In past comments about my analysis you've chosen to define "forward power" and "reverse power" differently. So when you speak of them, be sure to tell us exactly what they consist of in your discussion. Formulas would be best, as I've given above and in my analysis for my meanings of the terms. Roy Lewallen, W7EL Cecil Moore wrote: Richard Clark wrote: wrote: Chipman never said the reflected power can be greater than the forward power into a passive load. You are the only one to just have suggested he did. Because of a death in the family, I entered the discussion late, but I thought that was what Roy was asserting using his calculations, that fP - rP was a negative value. |
Richard Clark wrote:
And none of these individuals has yet to respond to simple but necessary observations by Chipman of the requirement of the Source Z. Do you join that throng? Where does Source Z appear in equation 7.34? -- 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:
I admit to being hazy about what you choose to call "forward power" and "reflected power". So that's probably why I don't understand the meaning of "the reflected power can be greater than the forward power into a passive load." Does this mean that both "reflected power" and "forward power" flow into a load? Then, does the "reflected power" flow back out, to go back down the transmission line, or does it contribute to the load dissipation? Forward power is all the power incident upon the load. Reflected power is all the power flowing away from the load. If the load is passive, the power flowing away from the load cannot be greater than the power flowing toward the load. Chipman says there is no "implication that the power level of the reflected wave is greater than that of the incident wave". He goes on to say the apparent increase is just a resonance effect. In Dr. Best's QEX article he shows how V1 + V2 = Vtot but P1 + P2 usually doesn't equal Ptot. Chipman's equation 7.34 seems to be of the same ilk. Dr. Best's interference term, 2*Sqrt(P1*P2)cos(theta), has to be added to the power equation to make it balance. I suspect that Chipman's term, 2*X0/R0*Im(rho(z)) is simply that necessary interference term. On page 137, Chipman says: "The question arises as to whether the transmission line equations predict a reflected wave at the termination having a higher power level than the wave incident on the termination, in violation of the principle of conservation of energy". He goes on to say that if |X0/R0|=1 then reflected power cannot be greater than incident power. And |X0/R0|=1 is one of the boundary conditions for the lossy transmission line. -- 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:
My analysis shows that fP - rP can have a negative value for some load impedances when Z0 is complex. Please pay careful attention, though, to the definitions of these terms. As I used them, fP = Re{fE * fIc} and rP = Re{rE * rIc} where I'm using subscript c to denote complex conjugate to avoid confusion with "*" for multiplication, and E and I magnitudes are RMS. Although I didn't prove it in my analysis, the total net average power, which includes a third average power term, into a passive load of course can't be negative. Chipman's third term in equation 7.34 certainly resembles Dr. Best's third term in his equation 12. Dr. Best's third term is known to be an interference term. It's a good bet that Chipman's third term is also an interference term. Incidentally, if that third term is flowing toward the load (as it has to be) it is forward power. So fP is obviously not all the forward power. -- 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! =----- |
On Tue, 16 Sep 2003 22:18:09 -0500, Cecil Moore
wrote: Richard Clark wrote: And none of these individuals has yet to respond to simple but necessary observations by Chipman of the requirement of the Source Z. Do you join that throng? Where does Source Z appear in equation 7.34? OK, so you are part of the throng. Another Evelyn Wood speedreader. 73's Richard Clark, KB7QHC |
On Tue, 16 Sep 2003 23:07:40 -0500, Cecil Moore
wrote: Richard Clark wrote: OK, so you are part of the throng. Another Evelyn Wood speedreader. On what page in Chipman does the Source Z appear relevant? You referenced equation 7.34. What do you see appearing in the illustration on the page opposite? (Simply one of several obvious statements most skim past.) 73's Richard Clark, KB7QHC |
Cecil Moore wrote:
Forward power is all the power incident upon the load. Reflected power is all the power flowing away from the load. If the load is passive, the power flowing away from the load cannot be greater than the power flowing toward the load. These are nice words, but unless there is a way to compute and measure these forward and reflected powers, they won't serve much purpose. As you are fond of pointing out, there are only two directions on a transmission line, so if a third term is needed to make the powers balance, is this third term a forward power or a reflected power or is it apportioned between forward and reflected in some unstated way? Without a clear definition of forward and reflected powers it is pretty much a useless concept. Can anyone provide a clear definition of forward and reflected powers? Is there a method for measuring forward and reflected powers? And do remember, we are discussing the GENERAL case here which includes transmission lines with complex impedances. The (somewhat limited) utility of the concept of forward and reflected powers is understood for the special case of transmission lines with real impedances. ....Keith |
Roy, W7EL wrote:
"Then does the "reflected power" flow back out, to go back down the transmission line, or does it contribute to the load dissipation?" It is a matter of time. Eventually, the reflected power is consumed by the load, though some of it requires several attempts to make it through the load. The load impedance opposes alternating current and thus the power it can produce. Reflected power is a misfit in the load. It does not conform to the E / I ratio of the load. There is a surplus E or I when there is a mismatch. This surplus E or I is reflected initiating the reflected wave traveling back toward the transmitter in the Zo of the line. E / I of the reflected wave must be the same as Zo enforces on the incident wave. Zo is usually a resistance (Ro) in useful lines. Zo is nearly the sq. rt. of L/C in useful lines because their series resistance and shunt conductance are insignificant. In such lines, Ro is lossless. 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. So, the incident power consists of the transmitter output plus the re-reflected power. When this combo hits the load, the same percentage is absorbed by the load as that extracted when the first power out of the transmitter arrived at the load. The difference is that the incident power is now greater as it has grown by the amount of the reflected power. The re-reflected power is coming around again for another go at the load. Best regards, Richard Harrison, KB5WZI |
Keith wrote:
"And do remember, we are discussing the GENERAL case here which includes transmission lines with complex impedances." This is a radio amateur antenna newsgroup. The lines used with these antennas is a special case in which losses in feedlines are usually so low that Zo does not differ from Ro. Great simplification results from a surge impedance which equals sq. rt. L/C. Many still question what happens in the simplified equivalent circuit of a lossless line. Once the simplified line action is understood it will be rewarding to discuss the general case. Keith has repeatedly demenstrated a desire to treat r-f feedlines as d-c wires. That won`t work. Best regards, Richard Harrison, KB5WZI |
On Wed, 17 Sep 2003 06:17:54 -0500, Cecil Moore
wrote: Richard Clark wrote: wrote: On what page in Chipman does the Source Z appear relevant? You referenced equation 7.34. What do you see appearing in the illustration on the page opposite? Sorry, I don't see anything. I don't have the book. I drove over to Texas A&M to see what equation 7.34 really looks like when it is not forced into ASCII characters. Then there is no reason to expect you have access to any other page you asked me for is there? Such is the plight of speed reading. Everyone selects their favorite passage in ignorance to the whole of the work. I have recited enough extractions (including, additionally, my response to you above) to no refutation. I note that those who asked for those same examples continue to embrace Chipman despite refusing to observe his cautions and, frankly, you are less prepared than they to engage in that discussion much less debate. The only point of reciting the source is to establish a basis of common ground. Without that, it devolves to the common sense that the load and the source are interchangeable and both observe the same mechanics of reflection that exist as a terminus to a line. Cecil, I know that you have already stated as much. The quality (sic) of other discussion that usually attends this issue from more than a few correspondents, the source somehow deserves some special status where it magically exhibits no loss, no gain, no reflection, total reflection, and each-or-all uttered by those who go numb when asked just what quantitative value enforces such mysterious actions they purport to occur. Some suggest it is the imponderability of nature and the cosmos; others say confusion exists (but not in themselves - even when they stumble to answer the simplest question); one suggests that methods and accuracy are in doubt (and cannot say how much error, nor which method is vague); many say it doesn't matter (and they rage on demanding just that); and ALL of them cannot answer simple bench examples that confound their myopic theories. Such is the kulture of Institutionalized Ignorance that exists. 73's Richard Clark, KB7QHC |
Richard Clark wrote:
The quality (sic) of other discussion that usually attends this issue from more than a few correspondents, the source somehow deserves some special status where it magically exhibits no loss, no gain, no reflection, total reflection, and each-or-all uttered by those who go numb when asked just what quantitative value enforces such mysterious actions they purport to occur. Easy for you to say. Would you mind diagraming that sentence? Such is the kulture of Institutionalized Ignorance that exists. Indubitably. 73 de ac6xg |
Richard Clark wrote:
wrote: Sorry, I don't see anything. I don't have the book. I drove over to Texas A&M to see what equation 7.34 really looks like when it is not forced into ASCII characters. Then there is no reason to expect you have access to any other page you asked me for is there? Of course there is, Richard. I just bought a new Harley Road King Classic and relish any reason to ride it over to the Texas A&M library. ... the source somehow deserves some special status where it magically exhibits no loss, no gain, no reflection, total reflection, ... I agree with you on that point. But there seems to be little choice except the above due to measurement problems. Such is the kulture of Institutionalized Ignorance that exists. I tend to agree but the ignorance is caused by the difficulty in making meaningful measurements. It's like, "Prove that God doesn't exist." -- 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! =----- |
On Wed, 17 Sep 2003 13:28:51 -0700, Jim Kelley
wrote: Easy for you to say. Would you mind diagraming that sentence? Hi Jim, Odd that you should ask. In fact I could, and it would be quite easy except for the restraint of display imposed here. Unlike many of my cohorts working on their BA English, I took a bonehead class in sentence analysis and I am still proficient. Consult: http://www.cybernalysis.com/diagrams/diagrams.htm 73's Richard Clark, KB7QHC |
On Wed, 17 Sep 2003 16:46:23 -0500, Cecil Moore
wrote: Richard Clark wrote: wrote: Sorry, I don't see anything. I don't have the book. I drove over to Texas A&M to see what equation 7.34 really looks like when it is not forced into ASCII characters. Then there is no reason to expect you have access to any other page you asked me for is there? Of course there is, Richard. I just bought a new Harley Road King Classic and relish any reason to ride it over to the Texas A&M library. Fine, you can start with the illustration offered on the page facing your referenced equation. You may notice no one here is willing to give you any help. They act like this is minefield where any mis-step will make them an amputee candidate in the theory ward. ... the source somehow deserves some special status where it magically exhibits no loss, no gain, no reflection, total reflection, ... I agree with you on that point. But there seems to be little choice except the above due to measurement problems. Measurement problems are in direct proportion for those that try to first calibrate the bone density between their ears. The difficult part is using simple calipers and a common ruler. For newcomers it is easily within their skill. Two resistors, a hank of transmission line, less than an hour's effort and they are done with unambiguous results. Clearly many, older and slower "students" here prefer the comfort of ambiguity that so mimics their conditions of mental constipation. Such is the kulture of Institutionalized Ignorance that exists. I tend to agree but the ignorance is caused by the difficulty in making meaningful measurements. It's like, "Prove that God doesn't exist." Cecil, You stand head and shoulders above that crowd of midgets. Hit the road and prove you can find that page's illustration and I might suggest several more. Post your query (with proof of reading) from the student union computers and I will fulfill that offer to save you time; otherwise find yourself some hog babe along the way, follow the road out of Texas and enjoy life instead. Choices, choices, choices.... 73's Richard Clark, KB7QHC |
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"David Robbins" wrote in message ...
"David Robbins" wrote in message ... i think the more important thing now is to point out to the arrl the error of using that form of the reflection coefficient in place of the 'conventional' one in the latest antenna book so it doesn't become gospel in the future. Conventional RC formula is fine, just assume Zo is purely real, which is what you almost always do anyways. Slick |
"Dr. Slick" 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. 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. And the reflected power can never be greater than the incident. Classic rho does seem to produce all the proper voltages. And using the equations implemented in a directional voltmeter, we get back the expected Vfwd and Vrev. Using revised rho does not produce the voltages computed with circuit theory, nor those measured on the bench. So revised rho does not seem to be the answer to the power dilemma. 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. 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. Otherwise, there is no reasonable choice except to accept that classic rho correctly predicts the reflected voltage. ....Keith |
"Dr. Slick" wrote:
"David Robbins" wrote in message ... "David Robbins" wrote in message ... i think the more important thing now is to point out to the arrl the error of using that form of the reflection coefficient in place of the 'conventional' one in the latest antenna book so it doesn't become gospel in the future. Conventional RC formula is fine, just assume Zo is purely real, which is what you almost always do anyways. 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. 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. ....Keith |
Richard Harrison wrote:
Keith wrote: "Can anyone provide a clear definition of forward and reflected powers?" Power in the load = forward power - reflected power. Finally. You have seen the light. Once you know the value of Pload, you can pick any pair of numbers for Pfwd and Pref satisfying the above relation and you now KNOW Pfwd and Pref. This is about as useful as the concept gets. Toss it away. Stick with Vfwd and Vref; they always work. There is no need for caveats like 'sort of works when Z0 is mostly real'. And you won't be misled into questions like 'where does the reflected power go?' and 'how does reflected power fry the final?' ....Keith |
Keith wrote:
"Toss it away." Terman and Bird Electronic Corporation say, Power in the load = forward power - reflected power. Having long successful experience with the Bird wattmeter, I`ll stick with Terman and Bird. Best regards, Richard Harrison, KB5WZI |
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. -- 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! =----- |
Keith wrote:
"Stick to the incident and reflected voltage (or current) waves for analysis. They work." 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." With a little review of wave behavior on transmission lines (I like Terman) in almost any good text, it is seen that voltage and current continuously vary along a line containing a reflection. This results from interference between the forward and reflected waves. However, The power, forward or reflected, is free of the oscillations produced in the volts and amps by interference between forward and reflected waves. This steady power flow makes power the electrical property to measure and this was the impetus for the Bird wattmeter. It`s been around for about 50 years, and I`d wager it will be around for another 50 years. Best regards, Richard Harrison, KB5WZI |
Whichever nitwit of a Ph.D invented imaginary 'power waves' should be made
to provide a complete mathematical treatise, an indisputable proof of their existence, before going to bed on his wedding night. |
Roy Lewallen wrote:
So, I'll let you play with whatever mysterious equations you use to predict those two quantitites, whatever they are and whatever they mean to you, and I'll settle for just knowing all the voltages, currents, impedances, and powers. Point was that your fP didn't represent the total forward power and resulted in the false conclusion that reflected power was greater than forward power. If the total average Poynting vector points toward the load, it is impossible for the total reflected power to be greater than the total forward power. -- 73, Cecil, W5DXP |
Yep, I restricted my definition of "forward power" fP to the same one
you've always used -- the average power calculated from the forward V and I. And the "reverse power" rP to the same one you've always used -- the average power calculated from the reverse V and I. (It's what you've been calling the "power in the forward wave" and "power in the reverse wave" respectively.) And then, using those definitions of yours, I showed in the analysis that the "forward power" can be less than the "reverse power", while still delivering net power to the load. Cool, huh? Fact is, I'm not having any trouble at all resolving this. But then I don't have any investment in the notion of waves of average power bouncing around on a line. Roy Lewallen, W7EL Cecil Moore wrote: Roy Lewallen wrote: So, I'll let you play with whatever mysterious equations you use to predict those two quantitites, whatever they are and whatever they mean to you, and I'll settle for just knowing all the voltages, currents, impedances, and powers. Point was that your fP didn't represent the total forward power and resulted in the false conclusion that reflected power was greater than forward power. If the total average Poynting vector points toward the load, it is impossible for the total reflected power to be greater than the total forward power. -- 73, Cecil, W5DXP |
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Roy Lewallen wrote:
I showed in the analysis that the "forward power" can be less than the "reverse power", while still delivering net power to the load. Cool, huh? Actually pretty uncool. That's what happens when you don't deal with total average powers. You can come up with apparent violations of the conservation of energy principle that don't actually exist in reality. Your "forward power" is simply not all of the forward power. Just as explained in Dr. Best's QEX article, the constructive/destructive interference terms must also be taken into account. -- 73, Cecil, W5DXP |
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 |
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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