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Tom Donaly wrote:
I should have known I could depend on you, Cecil. Negative power is a fallout of the math model being used. For instance, V*I*cos(theta) is negative when theta=180 degrees. theta=180 degrees for reflected power. If you have a windmill generator connected to your house power wiring, you can cause your watthour meter to run backwards. By convention, that would be negative energy consumption. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
On Thu, 23 Jun 2005 15:08:09 GMT, "Tom Donaly"
wrote: The whole exercise is nonsensical Hi Tom, You are FOURTH in a growing list of correspondents in the past couple of weeks who've intentionally posted bum science to prove a point. Unfortunatle none of those correspondents have made any pointed impression on the bums they wanted to prove science to. 73's Richard Clark, KB7QHC |
Tom Donaly wrote:
Owen wrote: Tom Donaly wrote: Given a 396 meter length of Radio Shack RG58. At 250 kiloherz TLD says (after some manipulation) that it has a propagation constant of 689.6 X 10^-9 + j7.933 X 10^-3. Zo is 50 -j4.344. Feed it with a Doesn't that imply that the the matched line loss at 0.25MHz is 689.6E-9*20*e^1*100 dB/100m? That is 0.0006dB/100m, it seems too good to be true! Owen Hi Owen, It is too good to be true. (Just consider it came from an unusually good batch.) The whole exercise is nonsensical, though, because it results in negative power and a negative SWR. Increase the loss to a more realistic value and the negative power goes away as does the negative SWR while the absolute value of the reflection coefficient is still greater than 1. I was hoping I could get some kind of nut philosophical justification for negative average power out of Cecil, but you sprang the trap. 73, Tom Donaly, KA6RUH Well, you are right I was fooled by your statement "Given a 396 meter length of Radio Shack RG58" which seemed to say a real cable. I suspect the source of "negative power" values stems from the assumption that Power=Real(Vf*If*-Vf*Ir*), whereas it is my understanding that the power flow at a point on the line is Real(V*I*) or Real((Vf+Vr)*(If-Ir)* which expands to Real(Vf*If* - Vf*Ir* + Vr*If* - VrIr*) so that when Power=Real(Vf*If*-Vf*Ir*) is assumed, two of the terms (- Vf*Ir* + Vr*If*) are being ignored. The real part of (-Vf*Ir* + Vr*If*) is zero when Zo is real, so they can be ignored for calculating real power when Zo is real. In the case of your example, but using real RG58C/U (and Zo is not real), it looks to me like Real(Vf*If*-Vf*Ir*) is negative out to about 60m from the load, but Real(V*I*) is always positive and always grows toward the generator. A graphic showing the behaviour of the terms is at http://www.vk1od.net/RG58sol.gif . Owen PS: My notation: the * postfix unary operator means complex conjugate, ie (If-Ir)* means compex congugate of (If-Ir). If you are having trouble viewing the gif file because it has been zoomed to fit in the browser window, most modern browsers allow you to zoom it up to 100% size. In Windows Exploder, hold your cursor over the image until a little "Expand" control appears, click the "Expand" control and there you go. In Firefox, just click on the image. |
Owen wrote:
I suspect the source of "negative power" values stems from the assumption that Power=Real(Vf*If*-Vf*Ir*), whereas it is my understanding that the power flow at a point on the line is Real(V*I*) or Real((Vf+Vr)*(If-Ir)* which expands to Real(Vf*If* - Vf*Ir* + Vr*If* - VrIr*) so that when Power=Real(Vf*If*-Vf*Ir*) is assumed, two of the terms (- Vf*Ir* + Vr*If*) are being ignored. The real part of (-Vf*Ir* + Vr*If*) is zero when Zo is real, so they can be ignored for calculating real power when Zo is real. In the case of your example, but using real RG58C/U (and Zo is not real), it looks to me like Real(Vf*If*-Vf*Ir*) is negative out to about 60m from the load, but Real(V*I*) is always positive and always grows toward the generator. A graphic showing the behaviour of the terms is at http://www.vk1od.net/RG58sol.gif . I meant to also mention the response of a typical reflectometer. The type of reflectometer that samples the voltage at a point, and the current at the same point to create a voltage proportional to that current, (such that the magnitude and phase of the voltage due to the current sample is equal to and opposite to the voltage sample when the line is terminated in its characteristic impedance,) then adds the two voltages algebraicly and measures the magnitude of the resultant responds to Vr, or if it is turned around, to Vf. Such an instrument does not respond to the two cross product terms terms discussed in the quote. So even if you adjusted such a reflectometer so that it nulled on the true Zo of the cable by adjusting the phase and amplitude of the voltage same wrt current sample, it would respond to Vf and Vr and could not be scaled to read forward and reverse power (since it doesn't respond to the cross product terms). So, in Tom's case (though with real RG58), a reflectometer of that type, properly nulled and scaled in Watts would indicate more reflected power than forward close to the load. The instrument is correctly responding to the magnitude of the reflection coefficient, but scaling in Watts can only be valid for Zo real. Owen PS: My notation: the * postfix unary operator means complex conjugate, ie (If-Ir)* means compex congugate of (If-Ir). If you are having trouble viewing the gif file because it has been zoomed to fit in the browser window, most modern browsers allow you to zoom it up to 100% size. In Windows Exploder, hold your cursor over the image until a little "Expand" control appears, click the "Expand" control and there you go. In Firefox, just click on the image. |
Richard Clark wrote:
On Thu, 23 Jun 2005 15:08:09 GMT, "Tom Donaly" wrote: The whole exercise is nonsensical Hi Tom, You are FOURTH in a growing list of correspondents in the past couple of weeks who've intentionally posted bum science to prove a point. Unfortunatle none of those correspondents have made any pointed impression on the bums they wanted to prove science to. 73's Richard Clark, KB7QHC Hi Richard, mea culpa, only what I posted was more parody than science. At least it was meant to be. Nothing, short of a visitation by The Creator will make some of the correspondents to this newsgroup change their thinking about how transmission lines behave. However, for those who aren't sure of whether they're right or not, the contemplation of absurdity may sharpen the understanding. (Or it may not. I'll try not to post things like this in the future, but can't guarantee forebearance.) 73, Tom Donaly, KA6RUH |
Tom Donaly wrote:
Nothing, short of a visitation by The Creator will make some of the correspondents to this newsgroup change their thinking about how transmission lines behave. Hey Tom, what do you think about Ramo & Whinnery's assertion in _Fields_and_Waves_... 2nd edition, page 291 " .. we are often most interested in the ratio of power in the reflected wave to that in the incident wave, and this ratio is given by the square of the magnitude of [rho], as can be shown by considering the Poynting vectors: Pz-/Pz+ = |rho|^2 (3)" Note the plural "Poynting vectors", Pz- for reflected power and Pz+ for forward power. -- 73, Cecil, http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
Nothing, short of
a visitation by The Creator will make some of the correspondents to this newsgroup change their thinking about how transmission lines behave. ================================= Not even The Creator will get ME to change MY thinking. Not even if the so-called SWR meter is located in the right place. Not even if it's a Bird with wings. |
Owen wrote:
Tom Donaly wrote: Owen wrote: Tom Donaly wrote: Given a 396 meter length of Radio Shack RG58. At 250 kiloherz TLD says (after some manipulation) that it has a propagation constant of 689.6 X 10^-9 + j7.933 X 10^-3. Zo is 50 -j4.344. Feed it with a Doesn't that imply that the the matched line loss at 0.25MHz is 689.6E-9*20*e^1*100 dB/100m? That is 0.0006dB/100m, it seems too good to be true! Owen Hi Owen, It is too good to be true. (Just consider it came from an unusually good batch.) The whole exercise is nonsensical, though, because it results in negative power and a negative SWR. Increase the loss to a more realistic value and the negative power goes away as does the negative SWR while the absolute value of the reflection coefficient is still greater than 1. I was hoping I could get some kind of nut philosophical justification for negative average power out of Cecil, but you sprang the trap. 73, Tom Donaly, KA6RUH Well, you are right I was fooled by your statement "Given a 396 meter length of Radio Shack RG58" which seemed to say a real cable. I suspect the source of "negative power" values stems from the assumption that Power=Real(Vf*If*-Vf*Ir*), whereas it is my understanding that the power flow at a point on the line is Real(V*I*) or Real((Vf+Vr)*(If-Ir)* which expands to Real(Vf*If* - Vf*Ir* + Vr*If* - VrIr*) so that when Power=Real(Vf*If*-Vf*Ir*) is assumed, two of the terms (- Vf*Ir* + Vr*If*) are being ignored. The real part of (-Vf*Ir* + Vr*If*) is zero when Zo is real, so they can be ignored for calculating real power when Zo is real. In the case of your example, but using real RG58C/U (and Zo is not real), it looks to me like Real(Vf*If*-Vf*Ir*) is negative out to about 60m from the load, but Real(V*I*) is always positive and always grows toward the generator. A graphic showing the behaviour of the terms is at http://www.vk1od.net/RG58sol.gif . Owen PS: My notation: the * postfix unary operator means complex conjugate, ie (If-Ir)* means compex congugate of (If-Ir). If you are having trouble viewing the gif file because it has been zoomed to fit in the browser window, most modern browsers allow you to zoom it up to 100% size. In Windows Exploder, hold your cursor over the image until a little "Expand" control appears, click the "Expand" control and there you go. In Firefox, just click on the image. I used the complicated expressions for V and I found in _Field and Wave Electromagnetics_ by David K. Cheng on page 468, and then found the power by taking the real part of VI*. If you're interested in doing it the hard way, you can first find V and I, then take (V + ZoI)/2Sqrt(ReZo) and call that a. Then take (V - Zo*I)/2Sqrt(ReZo) and call that b. The power is then just |a|^2 - |b|^2. This last comes from _Microwave Engineering Using Microstrips_ by E.F. Fooks and R.A Zakarevicius. Before doing any of this, though, be sure the propagation constant is the right one for the Zo of the line. The books give the propagation constant for a transmission line as sqrt((R + jwL)(G + jwC)), and Zo as sqrt((R + jwL)/(G + jwL)) so it's reasonable to assume the two are related. In other words, it doesn't seem as if you can just pick numbers out of a hat for the two quantities and expect them to mean anything. 73, Tom Donaly, KA6RUH |
"Cecil Moore" wrote in message ... Tom Donaly wrote: Nothing, short of a visitation by The Creator will make some of the correspondents to this newsgroup change their thinking about how transmission lines behave. Hey Tom, what do you think about Ramo & Whinnery's assertion in _Fields_and_Waves_... 2nd edition, page 291 " .. we are often most interested in the ratio of power in the reflected wave to that in the incident wave, and this ratio is given by the square of the magnitude of [rho], as can be shown by considering the Poynting vectors: Pz-/Pz+ = |rho|^2 (3)" Note the plural "Poynting vectors", Pz- for reflected power and Pz+ for forward power. -- 73, Cecil, http://www.qsl.net/w5dxp Cecil, concerning Poynting vectors, may I refer you to the last paragraph of Page 8-3 in Reflections 1, or the last paragraph beginning on Page 8-2 of Reflections 2. Walt, W2DU |
Walter Maxwell wrote:
"Cecil Moore" wrote: ... Ramo & Whinnery's assertion in _Fields_and_Waves_... 2nd edition, page 291 " .. we are often most interested in the ratio of power in the reflected wave to that in the incident wave, and this ratio is given by the square of the magnitude of [rho], as can be shown by considering the Poynting vectors: Pz-/Pz+ = |rho|^2 (3)" Note the plural "Poynting vectors", Pz- for reflected power and Pz+ for forward power. Cecil, concerning Poynting vectors, may I refer you to the last paragraph of Page 8-3 in Reflections 1, or the last paragraph beginning on Page 8-2 of Reflections 2. Thanks Walt, it's been about 15 years since I first read that short and sweet chapter in "Reflections 1". -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
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