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#11
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Revisiting the Power Explanation
On Tue, 20 Mar 2007 22:57:48 GMT, Walter Maxwell wrote:
On Tue, 20 Mar 2007 22:06:35 GMT, Owen Duffy wrote: I don't recall seeing experimental results to convincingly demonstrate that the PA is a linear source, though I have seen those that suggest otherwise. If the source cannot be proven to be sufficiently close to a linear source, then the basis for arguing the implicit conjugate match dissolves. Owen, despite our previous discussion, I have explained many times that even though the PA source upstream of the tank circuit is non-linear (and no one's saying it isn't), the energy storage in the tank makes the output of the tank a linear source, no matter what the shape of the current wave form may be at the input. The output of the tank is proved linear because the voltage/current ratio at the output is non-varying and the shape of the voltage and current wave forms are essentially sine waves. Consequently, the output circuit can be represented by a Thevenin source that supports both a conjugate match and the maximum power transfer theorem. Are you now denying that the output of a PA with the routine Q of 10 to 12 is not substantially a sine wave? If you agree that it is a sine wave, then why are you arguing that there is no basis for a conjugate match? However, none of the responses above respond to the issue of why the reflected power does not cause heating of the amp, which is what my treatise was all about. Walt, W2DU In the fourth line in the first paragraph above the word 'time' was inadvertantly omitted. It should have read ....the output is non-time varying and the shape... Sorry about that, Walt, W2DU |
#12
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Revisiting the Power Explanation
"Owen Duffy"
No one has demonstrated that using equivalent impedances etc is not a valid analysis of the steady state behaviour. _________ A reflection is a reflection. The reflection of a ~steady-state r-f source may produce a different perceived/effective result than if that source includes transients (modulation), but such does not negate the existence of reverse/reflected power in the steady-state case. Decades of experience with analog broadcast TV transmission systems demonstrate that the reflected power from a mismatch at the transmit antenna produces an amplitude variation (ripple) and other effects across the r-f and demodulated video channel bandwidths that are directly related to the magnitude of the antenna mismatch and the round-trip propagation time of the transmission line between the tx and the antenna (period = 1 cycle per ~491 feet of air-dielectric line). This is evident not only from accurate measurements made via a highly directional r-f coupler sampling forward power at the tx end of the transmission line, but also from results seen on the screen of TV sets viewing those transmissions. I suspect, Owen, that you would agree that this example originates from a "practical" system. The r-f power supplied even by a CW source is subject to the same amount of reflected power for a given antenna mismatch, which will have an appropriate effect on system performance. Whether or not that reflected power/performance effect is important (or even recognized as existent) is the issue at hand. RF |
#13
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Revisiting the Power Explanation
"Walter Maxwell" wrote
... I have explained many times that even though the PA source upstream of the tank circuit is non-linear (and no one's saying it isn't), the energy storage in the tank makes the output of the tank a linear source, no matter what the shape of the current wave form may be at the input. The output of the tank is proved linear because the voltage/current ratio at the output is non-varying and the shape of the voltage and current wave forms are essentially sine waves. Consequently, the output circuit can be represented by a Thevenin source that supports both a conjugate match and the maximum power transfer theorem. ______________ If this statement about the tank circuit being ~ a linear source is valid, does that mean that any load-reflected power that appears across the output terminals of the tx stops at the tank circuit, and never sees the non-linear, non-matching Z of the active PA? And if so, would that also mean that such a tx would not be prone to producing r-f intermodulation components when external signals are fed back into the tx from co-sited r-f systems? Yet experience shows that this is not the case for ~closely spaced interfering signals. The only mitigation for this for a PA with a tank circuit is the rejection of that tank circuit to those off-freq, external signals, and to the resulting IM products generated by mixing with the main tx signal in the active (and non-linear) PA stage of that tx. And the tank has VERY low rejection to load reflections of the signal bandwidth to which it is tuned. Also to be considered are the modern broadband (88-108MHz) FM broadcast transmitters, which have no tank circuits, but except for some designs incorporating balanced 3 dB hybrid combiners are affected by load reflections about the same as a tx with a tuned tank circuit. RF |
#14
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Revisiting the Power Explanation
"Richard Fry wrote
(period = 1 cycle per ~491 feet of air-dielectric line). _____________________ Sorry, make that 1 cycle == per MHz of bandwidth ==, per ~491 feet of transmission line. RF |
#15
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Revisiting the Power Explanation
On Tue, 20 Mar 2007 19:19:26 -0500, "Richard Fry" wrote:
"Walter Maxwell" wrote ... I have explained many times that even though the PA source upstream of the tank circuit is non-linear (and no one's saying it isn't), the energy storage in the tank makes the output of the tank a linear source, no matter what the shape of the current wave form may be at the input. The output of the tank is proved linear because the voltage/current ratio at the output is non-varying and the shape of the voltage and current wave forms are essentially sine waves. Consequently, the output circuit can be represented by a Thevenin source that supports both a conjugate match and the maximum power transfer theorem. ______________ If this statement about the tank circuit being ~ a linear source is valid, does that mean that any load-reflected power that appears across the output terminals of the tx stops at the tank circuit, and never sees the non-linear, non-matching Z of the active PA? Richard, my earlier treatise considers only tube-type PA's with pi-network output coupling circuits used in the Amateur Service, such as the Kenwood TS-830S on which my measurements were made. It was not intended to consider PA's used in the tv service. Sorry, I didn't make this distinction earlier. And if so, would that also mean that such a tx would not be prone to producing r-f intermodulation components when external signals are fed back into the tx from co-sited r-f systems? This issue is irrelevant, because the signals arriving from a co-sited system would not be coherent with the local source signals, while load-reflected signals are coherent. The destructive and constructive interference that occurs at the output of a correctly loaded and tuned PA requires coherence of the source and reflected waves to achieve the total re-reflection of the reflected waves back into the direction toward the load. Yet experience shows that this is not the case for ~closely spaced interfering signals. The only mitigation for this for a PA with a tank circuit is the rejection of that tank circuit to those off-freq, external signals, and to the resulting IM products generated by mixing with the main tx signal in the active (and non-linear) PA stage of that tx. Again, Richard, this condition is irrelevant to the re-reflection of the waves reflected by the load, because the relevant signals are not coherent. And the tank has VERY low rejection to load reflections of the signal bandwidth to which it is tuned. This may be true for PAs with bandwidths wider than those occurring in ham tx. However, the destructive and constructive interference between the reflected and source waves in a correctly loaded and tuned ham tx results in total re-reflection of the reflected waves. Also to be considered are the modern broadband (88-108MHz) FM broadcast transmitters, which have no tank circuits, but except for some designs incorporating balanced 3 dB hybrid combiners are affected by load reflections about the same as a tx with a tuned tank circuit. And still further, Richard, the FM transmitters you refer to above are not in the same category as those used in tube rigs used by hams. Incidentally, Richard, have you really reviewed the report of my TS-830S experiment? Walt |
#16
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Revisiting the Power Explanation
Cecil Moore wrote:
Richard Harrison wrote: Terman says on page 96 of his 1955 opus: "The reflected wave is identical with the incident wave except that it is traveling toward the generator." Gene needs to tell us how the TV modulation that causes ghosting makes its predictable round trips to the source and back without the aid of the reverse traveling wave. Cecil, It is interesting that you can be so precise at times and so sloppy at other times. I very carefully limited my discussion to steady state conditions, which is what everyone is already talking about in this case. You then conveniently inject modulation into the mix, completely ignoring what I said. Do the math and show us how my comment is in error. Add the two traveling waves and see if you get the summation to be precisely a standing wave plus a residual forward traveling wave. Go back and reread your quoted references and try to figure out if anything I said is different from your gurus. 73, Gene W4SZ |
#17
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Revisiting the Power Explanation
"Walter Maxwell" wrote
(RF): And if so, would that also mean that such a tx would not be prone to producing r-f intermodulation components when external signals are fed back into the tx from co-sited r-f systems? This issue is irrelevant, because the signals arriving from a co-sited system would not be coherent with the local source signals, while load- reflected signals are coherent. The destructive and constructive interference that occurs at the output of a correctly loaded and tuned PA requires coherence of the source and reflected waves to achieve the total re-reflection of the reflected waves back into the direction toward the load. But even for coherent reflections, if the PA tank circuit has very low loss for incident power (which it does), why does it not have ~ equally low loss for load reflections of that power? Such would mean that load reflections would pass through the tank to appear at the output element of the PA, where they can add to its normal power dissipation. Also, does not the result of combining the incident and reflected waves in the tx depend in large part on the r-f phase of the reflection there relative to the r-f phase of the incident wave? And the r-f phase of the reflection is governed mostly by the number of electrical wavelengths of transmission line between the load reflection and the plane of interest/concern -- which is independent of how the tx has been tuned/loaded. If the ham transmitter designs that your paper applies to produce a total re-reflection of reverse power seen at their output tank circuits, then there would be no particular need for "VSWR foldback" circuits to protect them. Yet I believe these circuits are fairly common in ham transmitters, aren't they? They certainly are universal in modern AM/FM/TV broadcast transmitters, and are the result of early field experience where PA tubes, tx output networks, and the transmission line between the tx and the antenna could arc over and/or melt when reflected power was sufficiently high. RF |
#18
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Revisiting the Power Explanation
On Mar 20, 3:43 pm, Cecil Moore wrote:
I understand what happens to the direction and momentum in the reflected wave when it encounters an impedance discontinuity at some distance from the source, e.g. a Z0-match. What happens to the direction and momentum in the reflected wave when it encounters a non-dissipative resistance at the source? For some years now, you have been arguing the reality of 'reverse power'. 'Reverse power' has served you well in that it appears to offer reasonable explanation for some phenomena: - 'forward power' minus 'reverse power' yields transferred power - circulators - TV ghosting - dissipation of pulses in generators But there are some challenges to the premise of 'reverse power': - where does the 'reverse power' go? - why does the change in dissipation of a generator when 'reverse power' changes depend more on the design of the generator than on the magnitude of the 'reverse power'? In an attempt to resolve these, you have apparently done extensive studies in optics looking for an explanation based on constructive and destructive interference but are still left with the question you posed above and others, like the one below from another of your posts: All one has to do to calculate the reflected power dissipated in the source is to understand the constructive and destructive interference occurring at the source output terminal. THIS IS EASIER SAID THAN DONE. [emphasis mine] Like myself, others have encountered difficulties with the premise of 'reverse power'. But we have taken a different path to enlightenment than yours; we have given up on the premise that 'reverse power' represents something that is real. To do this, we have had to find alternative explanations to all the phenomena listed above, but once this was done, life was good. I would suggest that you try trodding this path. Make a list of phenomena that you think are explained by 'reverse power'. For each phenomena, explore the possibility of alternative explanations that do not require 'reverse power'. When you have an explanation for each, test the explanations against each other to ensure they are self-consistent, then take the body of non-'reverse power' explanations and compare it the body of 'reverse power' explanations. Which is more complete? Which violates fewer fundamentals? You have believed in 'reverse power' for so long that you will probably find this path difficult. Make a conscious effort when thinking about circulators, for example, not to give up because it does not explain ghosting. Work out the solution to ghosting later. Similarly, when working on steady-state examples, do not confuse yourself with transients. Do those later. And when exploring a phenomena using a hypothetical generator, do not simply give up because it does not accurately model a real transmitter. Much can be learned from the simplifications of ideal voltage and current sources. Those who have already trodden this path are, I am quite sure, willing to assist you in finding the solutions, if you are willing to learn, rather than tossing distractions into the discussion. Save the other phenomena that trouble you for a later discussion. Keep the discussion on track. You can not lose if you take this path. In the best ending, you end up with a coherent explanation for all the phenomena and can give up on your search for solutions to the troubling issues posed by 'reverse power' and the vanishing of the energy. But even if you do not change your view you will have a better appreciation of the alternative explanations and should be better able to partake in debates on their correctness. You could start by providing a list of phenomena for which you think the reality of 'reverse power' is the only viable explanation and offer a willingness to learn about alternative explanations. ....Keith |
#19
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Revisiting the Power Explanation
Keith wrote:
"For some years now, you have been arguing the reality of "reverse power". For good reason. You feed a transmission line into an open circuit at its far end, and the power arriving at the open has no where to go but to return towards its generator. What happens at the generator upon arrival of the power reflected from the mismatched load depends on the vector values of incident and reflected waves as well as the impedance of the generator. Searching the net for "reflected r-f power" returned over 25,000 examples. Belief in reverse power is obviously common. Best regards, Richard Harrison, KB5WZI |
#20
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Revisiting the Power Explanation
On Mar 21, 12:08 pm, (Richard Harrison)
wrote: Keith wrote: "For some years now, you have been arguing the reality of "reverse power". For good reason. You feed a transmission line into an open circuit at its far end, and the power arriving at the open has no where to go but to return towards its generator. What happens at the generator upon arrival of the power reflected from the mismatched load depends on the vector values of incident and reflected waves as well as the impedance of the generator. Searching the net for "reflected r-f power" returned over 25,000 examples. Belief in reverse power is obviously common. Best regards, Richard Harrison, KB5WZI You are certainly correct; many people believe in reflected power, though I've always found that to be a poor basis for my own beliefs. You have also provided the classic example where the numerology works and 'reverse power' offers a tidy explanation. I am sure this neat example is the basis for many people's belief. What drove me to look at alternate explanations for these kinds of examples was that the 'reverse power' explanation fails miserably when the power gets back to the generator. Having another explanation for this classic example lets one let go of 'reverse power' which solves the challenges at the generator end. When 'reverse power' is not real, the question of where it goes becomes irrelevant. ....Keith |
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