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#101
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Revisiting the Power Explanation
Keith Dysart wrote:
It would be much more convincing if you were to compute the magnitude of the ghost rather than handwaving an answer. I made the rough computation of -25 dB before I responded. It is below the ability of the human eye to detect TV ghosting. -- 73, Cecil http://www.w5dxp.com |
#102
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Revisiting the Power Explanation
Dave wrote:
forget power, forget energy, they are products of other calculations. all that is needed is impedance and voltage OR current. once you know voltage OR current and the impedance you can calculate all the rest. What would you do if you were dealing with EM light waves instead of EM RF waves? Optical physicists don't have the luxury of dealing with "impedance and voltage OR current". They are forced to deal with the essence of EM waves which is photonic energy. They have done it for over a century and have learned a lot about the nature of EM waves. They did not "forget power, forget energy". -- 73, Cecil http://www.w5dxp.com |
#103
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Revisiting the Power Explanation
On Mar 24, 8:45 am, Cecil Moore wrote:
Keith Dysart wrote: It would be much more convincing if you were to compute the magnitude of the ghost rather than handwaving an answer. I made the rough computation of -25 dB before I responded. It is below the ability of the human eye to detect TV ghosting. Would you care to reveal the details of the calculations so that we can locate the error(s) that lead to a non-zero answer? ....Keith |
#104
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Revisiting the Power Explanation
Keith Dysart wrote:
On Mar 24, 8:45 am, Cecil Moore wrote: I made the rough computation of -25 dB before I responded. It is below the ability of the human eye to detect TV ghosting. Would you care to reveal the details of the calculations so that we can locate the error(s) that lead to a non-zero answer? As I said, it was a rough computation based on your 450 ohm series padding resistor and a lot of simplified assumptions about the configuration of the system. In order to avoid ambiguity, would you please give us better specifications for the source? Ramo and Whinnery warn us not to draw any conclusions about what happens inside a Thevenin equivalent source so your specification must be an achievable well-defined real-world source. How about a class-A linear amplifier with no filters, no impedance transformation, and no protection circuitry? Vrms/Irms = 75 ohms Vrms*Irms*cos(0) = 1 watt The amount of interference cannot be calculated without magnitude and phase. Shall we assume that the 450 ohm line is an integer number of wavelengths and lossless? i.e. Z0-matched to 75 ohms? If you require detailed calculations, more specified information is needed. -- 73, Cecil http://www.w5dxp.com |
#105
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Revisiting the Power Explanation
On Mar 24, 10:02 am, Cecil Moore wrote:
Keith Dysart wrote: On Mar 24, 8:45 am, Cecil Moore wrote: I made the rough computation of -25 dB before I responded. It is below the ability of the human eye to detect TV ghosting. Would you care to reveal the details of the calculations so that we can locate the error(s) that lead to a non-zero answer? As I said, it was a rough computation based on your 450 ohm series padding resistor and a lot of simplified assumptions about the configuration of the system. In order to avoid ambiguity, would you please give us better specifications for the source? Ramo and Whinnery warn us not to draw any conclusions about what happens inside a Thevenin equivalent source so your specification must be an achievable well-defined real-world source. How about a class-A linear amplifier with no filters, no impedance transformation, and no protection circuitry? Vrms/Irms = 75 ohms Vrms*Irms*cos(0) = 1 watt The amount of interference cannot be calculated without magnitude and phase. Shall we assume that the 450 ohm line is an integer number of wavelengths and lossless? i.e. Z0-matched to 75 ohms? If you require detailed calculations, more specified information is needed. Standard analysis can tell us quite a bit about the situation without the need for further information. The information we have: - 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. Remembering that reflection coefficient is RC = (Z2-Z1)/(Z2+Z1) let use compute the RCs at all the connection points: RCgen-line = (450-450)/(450+450) - 0 RCline-load = (75-450)/(75+450) - -0.714 RCline-gen = (450-450)/(450+450) - 0 Let the output of the generator into a 450 Ohm load be Vout. Forward voltage supplied by the generator into the line: Vfwd = Vout This is also the Vfwd that reaches the load (for the purposes of example we are dealing with lossless lines). The reflected voltage at the load: Vref = Vfwd * RCline-load = Vfwd * -0.714 = Vout * -0.714 The voltage applied to the load: Vload = Vfwd + Vref = Vout * 0.286 The voltage reflected from the load that arrives back at the generator: Vref = Vout * -0.714 The amount of Vref reaching the generator that is reflected back to the load: Vrefref = RCline-gen * Vref = 0 * Vref = 0 There for no ghosts since there is no re-reflection. A few other things of possible interest... Power delivered to the load: Pload = Vload * Vload / 75 = Vout * Vout * 0.0816 / 75 = Vout * Vout * .0011 This is the actual power that flows in the line. Forwarwd power as indicated by a directional wattmeter in the line: Pfwd = Vfwd * Vfwd / 450 Reverse power as indicated by a directional wattmeter in the line: Prev = Vref * Vref / 450 Net power flowing in the line: Pfwd - Prev = (Vfwd*Vfwd)/450 - (Vref*Vref)/450 = Vout*Vout/450 - Vout*RC*Vout*RC/450 = Vout*Vout*(1-(RC*RC))/450 = Vout * Vout * 0.0011 Which is the same as the load power; this being as expected since directional wattmeters are useful for obtaining the net power. It is also useful to know what we can not compute from the provided information.... The voltage at the generator output and its phase with respect to the current since this is dependant on the velocity factor of the line and the signal frequency. Though if the output is DC (e.g. a step function) than this can be easily computing for the situation after one round trip delay. Generator dissipation since we would need to know the internal construction of the generator and the actual voltage and current phase at the output of the generator. ----- Still, using conventional techniques we have derived quite a bit about the system without knowing the internals of the generator. Cecil, are you saying that your techniques can not produce any derivations without knowing the internal construction of the generator and the length of the line (in wavelengths)? ....Keith |
#106
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Revisiting the Power Explanation
"Cecil Moore" wrote in message ... Dave wrote: forget power, forget energy, they are products of other calculations. all that is needed is impedance and voltage OR current. once you know voltage OR current and the impedance you can calculate all the rest. What would you do if you were dealing with EM light waves instead of EM RF waves? Optical physicists don't have the luxury of dealing with "impedance and voltage OR current". They are forced to deal with the essence of EM waves which is photonic energy. They have done it for over a century and have learned a lot about the nature of EM waves. They did not "forget power, forget energy". -- yes they do. but photons are propagating em waves, they aren't currents and voltages, they are E and B fields. you can do the exact same calculations using either E or B and the refractive index or epsilon/mu. again, all you need is 2 of them, and you can compute the third, and then you know it all. |
#107
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Revisiting the Power Explanation
Keith Dysart wrote:
Standard analysis can tell us quite a bit about the situation without the need for further information. The information we have: - Generator with 450 Ohm output impedance There are a number of techniques to ensure that the reflected waves encounter an impedance of 450 ohms but none of those techniques are implemented in amateur radio transmitters. Your generator is obviously equipped with a circulator load of 450 ohms, 450 ohm attenuation pads, or some active feedback. Given that, your analysis is correct but moot. Your point is already known and accepted by every initiated person including me. I freely admit that if the proper circulator load exists or if 450 ohm attenuator pads exist, they will dissipate the reflected power by heating up the circulator load resistor or the pads. But it's tough to argue that there is no energy in the reflected waves while they are being used to boil water. -- 73, Cecil http://www.w5dxp.com |
#108
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Revisiting the Power Explanation
Dave wrote:
yes they do. but photons are propagating em waves, they aren't currents and voltages, they are E and B fields. you can do the exact same calculations using either E or B and the refractive index or epsilon/mu. again, all you need is 2 of them, and you can compute the third, and then you know it all. True, but how do they measure two of them? Ever tried to measure the E-field of visible photons? Hint: It is an irradiance (power density) time- averaged measurement proportion to E^2. -- 73, Cecil http://www.w5dxp.com |
#109
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Revisiting the Power Explanation
Richard Clark wrote:
Cecil Moore wrote: your analysis is correct but moot Is he stealing your style? "Moot" is an interesting word, Richard. From Webster's - "moot - 1. a: debatable, b: disputed" -- 73, Cecil http://www.w5dxp.com |
#110
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Revisiting the Power Explanation
On Sat, 24 Mar 2007 11:40:37 -0500, Cecil Moore
wrote: your analysis is correct but moot Is he stealing your style? |
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