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#51
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Mismatched Zo Connectors
Cecil Moore wrote:
Gene Fuller wrote: This says that steady state depends on something else, namely the beginning and the end of the steady state condition. That is simply incorrect. In steady state conditions there is no concept of beginning or end. A 12VDC battery is sitting there with a 200 amp*hour charge. Are you asserting that there is no concept of where the 200 amp*hours came from? Please tell me you are not that stupid. Consider the one second long transmission line with 200W of forward power and 100W of reflected power. That requires 300 joules of energy during steady-state. If the 300 joules was not supplied during the transient state, then it must have magically appeared out of thin air in violation of the conservation of energy principle? Is that what you are trying to tell us? Cecil, You can wave your hands all you want, but it won't have much impact on the correct math and physics. Try writing the appropriate equations for your puzzler, in steady state conditions, and then figure out where to insert the transient behavior. Good luck. This is basic stuff taught in numerous math and technical courses. If don't accept the basic math, then I guess we will not agree. 73, Gene W4SZ |
#52
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Mismatched Zo Connectors
Gene Fuller wrote:
Try writing the appropriate equations for your puzzler, in steady state conditions, and then figure out where to insert the transient behavior. I have already provided the equations, Gene. In a one second long lossless transmission line, 200 watts of forward power equals 200 joules of energy in the forward wave. 100 watts of reflected power equals 100 joules in the reflected wave. Total joules in the transmission line equals 200 + 100 = 300 joules. The equations are trivial. -- 73, Cecil, W5DXP |
#53
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Mismatched Zo Connectors
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#54
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Mismatched Zo Connectors
On Sat, 26 Aug 2006 02:33:03 GMT, Gene Fuller
wrote: Cecil Moore wrote: Gene Fuller wrote: In steady state conditions there is no concept of beginning or end. If the 300 joules was not supplied during the transient state, then it must have magically appeared out of thin air in violation of the conservation of energy principle? Hi Gene, The bare contradiction is enough to condemn this thread. However, it does have its amusing character of "Who's on first?" Continuing that metaphor, Cecil would believe having been born on third base, that he had hit a triple to be there. ;-) 73's Richard Clark, KB7QHC |
#55
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Mismatched Zo Connectors
On Fri, 25 Aug 2006 20:10:14 -0700, Richard Clark
wrote: Continuing that metaphor, Cecil would believe having been born on third base, that he had hit a triple to be there. ;-) For the concept challenged, Being on third is the steady state. The transient state is one of: Being born; hitting a triple; hitting a double and then a batter advancing the runner(s); hitting a single (then like wise with the batter's assist); stealing a base; or two. The steady state also has to satisify other conditions that were taken up by transients like outs and innings. e.g. being on third with three outs does not mean you can stay on third. Thus the next transient is The side is retired (state change) or, as in this case of the bottom of the ninth and an untied score The game is over (solution). 73's Richard Clark, KB7QHC |
#56
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Error correction
A correction - insert dt instead of dz.
The fundamental partial differential equations of transmission lines are - - dv/dz = R + L*di/dt - di/dz = G + C*dv/dt where volts v and current i are incremental functions of distance and time, and z is incremental distance along line. Everything else follows. Similar equations can be written in terms of frequency. It is often easier to think in terms of Time and Distance rather than Frequency and Impedance. ---- Reg. |
#57
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Mismatched Zo Connectors
Cecil Moore wrote:
If I understand correctly, Roy's argument is that since the source is not supplying any steady-state energy to the lossless stub, there is no energy in the reflected wave within the stub. That sounds right... if the reflection coefficient is 1 then there's no net power flux into/through the line in steady state, and this can be described if you like by counterpropagating waves each carrying the same amount of energy. The problem is, in your other example where you say 200 joules in the forward wave + 100 joules in the reflected wave = 300 joules in the line total, you're neglecting the vector character of the power flux. Yes, the waves carry energy, but they carry it in different directions. The net power flux in the line with 200W forward power and 100W reflected power is 100W net power flowing to the load from the source. The real part of the Poynting vector of the reflected wave opposes that of the forward wave, as long as I got all the signs right. I don't think we can neglect the imaginary part of the Poynting vector, though. It's not zero and I think it represents the flow of the power in the stored fields in the line, and if we want to get the total energy in the line, we have to include the stored fields. Dan |
#58
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Mismatched Zo Connectors
Richard Clark wrote:
Continuing that metaphor, Cecil would believe having been born on third base, that he had hit a triple to be there. ;-) That is actually the other side of the argument. When an observer arrived after the game started, Cecil was on third base. Using steady-state logic, the newcomer assumes that Cecil is there without ever having to swing a bat. Someone looks at a transmission line during steady-state. The source is supplying 100 watts. The load is dissipating 100 watts. The forward power is 200 watts. The reflected power is 100 watts. The incorrect assumption is that the source is incapable of delivering the 200 watts of forward power and the 100 watts of reflected power. But the exact amount of energy required to support those values was provided to the transmission line before steady- state was reached. It was rejected by the load and is still there in the transmission line during steady-state. -- 73, Cecil http://www.qsl.net/w5dxp |
#60
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Mismatched Zo Connectors
Cecil,
You've set up a false dichotomy here. When I, and others, write "The electric field is the superposition of a forward and reverse traveling wave" maybe it would be better to say "The electric field has two terms, one that appears to be a forward traveling wave and one that appears to be a reverse traveling wave." or something like that. There's one electric field vector and one Poynting vector. Or there are two. The structure of the electric field and the structure of the real part of the Poynting vector both admit BOTH explanations of what's happening. You're not gonna get 300J in your one second line.... the stored energy flux in the line depends on the wavelength of the incident RF, and in retrospect, you might expect this from the fact that a misterminated line goes through cyclical impedance variations as you change its length (something that I know you're quite familiar with :-) ) I think the energy density per unit length in the line is proportional to the Poynting vector (or it's integral over the cable cross section, and the proportionality constant is the group velocity of the waves, I think) I left Jackson at work, so I'm not certain right now. What I am certain of is that you can't take the energy in the forward wave and add it to the energy of the reflected wave and get that there are 300J in a 1 second line carrying a 200W forward wave and a 100W reverse wave. Rather, there's a 100W net forward power flux and THAT will give you the energy contained in the part of the field that's actually moving from source to load. The energy contained in the reactive part has an integral that's going to cyclically vary with the length of the line, and sometimes goes through zero (kL or kL - phi equal to an integer multiple of Pi... or any integer multiple of a half wavelength, which happens to be the length of an impedance repeating line, eh?) Dan |
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