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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 |
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