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Revisiting the Power Explanation
"Walter Maxwell" wrote in message ... On Mon, 26 Mar 2007 20:41:42 -0400, "Jimmie D" wrote: I wish somone would convince my boss reverse power isnt real. Then he wouldnt be so angry about the power meter head I blew up because I forgot to put an attnuator on it. Even with the -20db of the coupler ther is still 20 watts peak on the reverse side. Jimmie, are you stating your belief that reverse, or reflected power isn't REAL power? Ya gotta be kidding! Listen to your boss, he's right! Walt, W2DU If reflected power wasnt real I wouldnt be in trouble or at least on the shame list. I picked up 30 db pad to put on the pwer meter some body started talking to me and I forgot to put the thing on before I took the reading. In other words my origional reply was a kind of tongue in cheek raz to anyone who doesnt believe reflected power is real. Jimmie |
Revisiting the Power Explanation
Richard Harrison wrote:
Power is acceptable and accepted. Why avoid the term? Jim Kelley, AC6XG, has convinced me to make the distinction between the nature of EM energy and the nature of EM power. Power is what exists at a point or plane. Energy is what is moving past the point or through the plane. Reflected power is measured at a point. Reflected energy is what is doing the moving past that point. In addition, there's the difference in definitions between the fields of RF engineering and the field of physics. In physics, zero work implies zero power. I am not avoiding power. I am avoiding "power waves" and "power flow". The dimensions of power flowing past a point would be watts/second. I don't know what physical quantity that would represent. -- 73, Cecil http://www.w5dxp.com |
Revisiting the Power Explanation
Walter Maxwell wrote:
"Jimmie D" wrote: I wish somone would convince my boss reverse power isnt real. Ya gotta be kidding! Yep, he is kidding. -- 73, Cecil http://www.w5dxp.com |
Revisiting the Power Explanation
Richard Clark, KB7QHC wrote:
"There is a continuum of phase relationships expressed in degrees between 0 and 360." We are discussing transmission lines and assuming near perfection. Indeed the phase of the wave depends on that of the generator when it was launched and the phase of the generator continues to advance with time, but a good line enforces its Zo, a resistance. Terman says on page 85 of his 1955 opus: "The incident wave on the transmission line can therefore be described as a voltage accompanied by a current that is everywhere in phase with and proportional to, the voltage and dropping back uniformly in phase as the load is approached." The transmission line treats the wave reflected from a discontinuity exactly the same as it does the incident wave. The reflected wave is identical with the incident wave except that it is traveling toward the generator. At an open circuit or high-impedance load, there is tto much line current for the high-impedance to accept gven the limited voltage. The surplus current must reverse phase and the wave must travel back toward the generator. There is no change in the phase of the voltage. At a short circuit or low-impedance load, there is too much line voltage for the low-impedance to accept, given the limited current. The surlus voltage must reverse phase and the wave must travel back toward the generator. There is no change in the phase of the current. There are two phasing conditions between the voltage and current on an ideal transmission line, 0 degrees and 180 degrees. Best regards, Richard Harrison, KB5WZI |
Revisiting the Power Explanation
Cecil Moore wrote:
Richard Harrison wrote: Power is acceptable and accepted. Why avoid the term? Jim Kelley, AC6XG, has convinced me to make the distinction between the nature of EM energy and the nature of EM power. Power is what exists at a point or plane. Energy is what is moving past the point or through the plane. Reflected power is measured at a point. Reflected energy is what is doing the moving past that point. In addition, there's the difference in definitions between the fields of RF engineering and the field of physics. In physics, zero work implies zero power. I am not avoiding power. I am avoiding "power waves" and "power flow". The dimensions of power flowing past a point would be watts/second. I don't know what physical quantity that would represent. Cecil, Utter nonsense. Jim was pulling your chain, and I guess you fell for it. I have been a professional physicist for nearly 40 years. Real physicists fully understand the difference between power as work and power as energy transport. Both definitions are used as needed. I would hazard a guess that most engineers understand and use both definitions as well. All of your ramblings about the difference between energy and power, as well as joules and watts, add nothing but noise to the discussion. It is highly likely that everyone reading this group understands the concept of time. 73, Gene W4SZ |
Revisiting the Power Explanation
Gene Fuller, W4SZ wrote:
"Utter nonsense." Cecil knows as well as anybody the difference between energy and power, Power is the rate of doing work. The electrical unit is the watt (joule/second). It is also the rate of transmission from source to load. A wattmeter keeps track of the rate of energy flow, and the watt-hour meter measures the integral of active power during the integration interval. The power company bills you for the energy consumed during an interval determined by successive watt-hour meter readings. Best regards, Richard Harrison, KB5WZI |
Revisiting the Power Explanation
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Revisiting the Power Explanation
On Mar 25, 8:23 pm, Cecil Moore wrote:
You are not allowed to deny the existence of the 450 ohm real world load while asserting that it still exists. If it exists, it dissipates the reflected energy. If it doesn't exist, it reflects the reflected energy. Please choose one or the other - obviously, you cannot have both at the same time. [snip] If reflected energy is not dissipated, it undergoes destructive interference and is redirected back toward the load as constructive interference instead of being incident upon the source. I like these two paragraphs. They describe exactly what I would expect the relationships to be were "reflected energy" to represent real energy. More importantly, they describe an expectation that is sufficiently precise to be falsifiable. But just to be sure I understand the meaning, let me restate them algebraically: Energy.reflected = Energy.dissipated + Energy.re-reflected If this is not what you meant, then please read no further until you correct my misinterpretataion. ---- Oh good. The interpretation is accepted. So let us call this "Cecil's Hypothesis": "If reflected energy is not dissipated, it is redirected back toward the load." Expressed as an equality: Energy.reflected = Energy.dissipated + Energy.re-reflected And for convenience, its power form: Preflected = Pdissipated + Pre-reflected Now all we need is a single example for which the equality does not hold and "Cecil's Hypothesis" will be disproved. Let us start with the example previously offerred: - Generator with 450 Ohm output impedance - connected directly to a 1000 foot line with 450 Ohm characteristic impedance - connected to a load with 75 Ohm input impedance. And to simplify computation and produce a numerical result, we add a few more details: - The generator is 2 Amp current source in parallel with a 450 Ohm resistor - The line is 31 wavelengths long Let the experiment begin: - The generator is off. - All currents, voltages, energies and powers are zero. - Turn on the current source. - The current source pushes 2 Amps. - This is divided equally between the 450 Ohm generator resistor and the 450 Ohm line. - The generator output voltage is 450 Volts. - The generator output current is 1 Amp. - 450 Watts is being put out by the generator - 450 Watts is being dissipated in the generator resistor - A real wattmeter (one that measures voltage and current) shows 450 Joules/s flowing down the line. (Is it okay to say that "450 Watts are flowing"?) - A directional wattmeter indicates that the "forward power" is 450 Watts and the "reflected power" is 0 Watts. - Vfwd is 450 Volts - Ifwd is 1 Amp - The load is happily oblivious to the oncoming onslaught. - 31 cycles later, the voltage, current and energy reach the load. - But there is an impedance discontinuity which creates a voltage reflection coefficient of -0.714 so the load refuses to accept all the incoming energy. - The voltage across the load is 128.6 Volts - The current into the load is 1.714 Amps - The power dissipated in the load is 220.4 Watts - Vref is -321.5 Volts - Iref is 0.714 Amps - Vfwd is still 450 Volts (the line is lossless for this example) - Ifwd is still 1 Amp - "Forward power" is 450 Watts - "Reflected power" is 229.6 Watts - 220.4 = 450 - 229.6 which is as expected and demonstrates the usefullness of a directional wattmeter for computing actual power transferred (or is it energy?). - Encountering the reflection has caused a change in the voltage and current and this change is now propagating back towards the generator. - 31 cycles later, this change in line conditions reaches the generator. - The conditions at the generator terminals are now the same as the load since it is an integral number of wavelengths from the load. - The voltage at the generator terminals is 128.6 Volts - The current out of the generator is 1.714 Amps - The "forward power" is still 450 Watts. - The "reflected power" has changed from 0 to 229.6 Watts. - There is 128.6 volts across the 450 Ohm generator resistor. - There is 0.286 Amps flowing in the 450 Ohm generator resistor - Because the 0.286 Amps in the 450 Ohm generator resistor plus the 1.714 Amps flowing into the line is 2 Amps, exactly the output of the current source, the system has now settled after one round trip. There is no re-reflection at the generator terminals (and no ghosts). - The power into the line is 220.4 Watts - The power into the 450 Ohm generator resistor is 38.6 Watts - 229.6 Watts of "reflected power" has reduced the dissipation in the generator resistor by 413.4 Watts. - Substituting into the hypothetical equality Preflected = Pdissipated + Pre-reflected 229.6 = -413.4 + 0 - The equality does not hold, thus disproving "Cecil's Hypothesis". It is instructive to also consider the case with a generator constructed as a 900 Volt voltage source connected to a 450 Ohm output resistor. In this case the 450 Watts dissipated for the first 62 cycles increases by 872 Watts to 1322 Watts when the reflection arrives. Similar to above there is no re-reflection or ghosts. Once again the hypothetical equality Preflected = Pdissipated + Pre-reflected 229.6 = 872 + 0 does not hold. But the principle of "conservation of energy" should hold whenever real energies are involved. Since it does not hold in this case, we can only conclude that the numbers computed for "forward and reverse power" do not represent any real energy flows but are merely conveniences to facilitate other computations. ....Keith |
Revisiting the Power Explanation
On 20 Mar, 08:37, Walter Maxwell wrote:
One of the issues discussed in this thread that Owen originated concerned whether or not reflected power enters the power amp and dissipates as heat in the plates of the amp. Some of the posters apparently are unable to appreciate that the reflected power does not cause heating of the amp, unless the reflected power detunes the amp and the amp is left detuned from resonance, which of course is not the correct manner of operating the amp. In the last post of the original thread I presented the details of an experiment I performed (one of many using the same procedure) on a Kenwood TS-830S transceiver that proves how and why reflected power in no way causes heating of the amp when the amp is properly adjusted in the presence of the reflected power. Usually, such a presentation as in the last post in that thread evokes a great deal of response, as for example, Art Unwin's. So I'm somewhat surprised, and a little disappointed that my post has resulted in total silence. Have my efforts in helping to solve the problem gone for naught? Walt, W2DU Walt you got your wish, over 100 posts and still coming. How many postings is it going to take to not only read what you said and also the hard part understand what you are saying? Only you can answer whether your help to solve the problem has gone for naught. Maybe at 200 postings you will feel a bit better about things but I doubt it. But I must thank you as you have greatly decreased the number ugly postings in my direction so please keep it up. Regards Art Art |
Revisiting the Power Explanation
Richard Clatk, WB7QHC wrote:
"This is, of course, contingent only when the line is terminated in its characteristic Z." Lines terminated in Zo don`t produce a reflection at the load. I did not make up the fact that the line forces the current to be exactlly in-phase or exactly out-of-phase with the voltage across the transmission line. I got it from Terman. On page 86 of his 1955 opus: "---This differs from Eqn (4-8) (Zo seen by the incident wave) only by the negative sign, which arises from the fact that the current in the reflected wave travels toward the generator whereas the current in the incident wave travels toward the load. Zo is a RESISTANCE. In a resistance, current is in-phase with its associated voltage. Waves traveling in either direction on a line see the same Zo. Best regards, Richard Harrison, KB5WZI |
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