but in all cases, they are doing it wrong and should be using 2 ports to measure cavity resonance. We have yet to see anyone do it this way.Todd
Quote from: sghill on 08/07/2015 07:46 pmThe testiness of this current discussion aside, I am genuinely interested in the reason why there is a difference between these two Q value viewpoints.As far as I can gather, @TT is saying, "How they did it". @rfmwguy is saying "How it should've been done." and @tleach is saying "This is how it seems to fit McCulloch's formula." Which BTW, McCulloch doesn't define how to measure Q. He simply redefines it as the number of bounces (reflections), in the time it takes the photon to decay to zero. So that's not even the same definition of the Q that is being kicked around here. @tleach was trying to bridge that gap.What I conclude is, the experimenters may or may not be measuring it consistently using the same methodology, but in all cases, they are doing it wrong and should be using 2 ports to measure cavity resonance. We have yet to see anyone do it this way.Todd
The testiness of this current discussion aside, I am genuinely interested in the reason why there is a difference between these two Q value viewpoints.
Quote from: sghill on 08/07/2015 07:46 pmThe testiness of this current discussion aside, I am genuinely interested in the reason why there is a difference between these two Q value viewpoints.In the EMDrive world measuring unloaded Q via S11 return loss at the 3bd down bandwidth is how it is measured.So say Shawyer, Prof Yang, Eagleworks and Prof Tajmar.
Quote from: WarpTech on 08/07/2015 07:55 pmQuote from: sghill on 08/07/2015 07:46 pmThe testiness of this current discussion aside, I am genuinely interested in the reason why there is a difference between these two Q value viewpoints.As far as I can gather, @TT is saying, "How they did it". @rfmwguy is saying "How it should've been done." and @tleach is saying "This is how it seems to fit McCulloch's formula." Which BTW, McCulloch doesn't define how to measure Q. He simply redefines it as the number of bounces (reflections), in the time it takes the photon to decay to zero. So that's not even the same definition of the Q that is being kicked around here. @tleach was trying to bridge that gap.What I conclude is, the experimenters may or may not be measuring it consistently using the same methodology, but in all cases, they are doing it wrong and should be using 2 ports to measure cavity resonance. We have yet to see anyone do it this way.ToddIt would be oh-so-simple to resolve this. Send a closed, resonant cavity to NIST, the British Standards Institute (BSI) or other reputable body with ONLY 1 PORT and have them measure Q. After they ask where the other port is, they would ask, why would you want to measure a closed cavity system with only one port? IOW, an open system, like an antenna, only needs a single port. A resonant cavity needs 2 to properly measure Q.I'll stand by this (un)controversial position regardless of the previous experimenters. Quite frankly, I'm surprised at their apparent lack of RF familiarity. This is not a slam, it is a known fact that RF engineering is taught less, practiced less and is receeding into the background of companies and institutions. Reason? Computer science boom and the "plug and play & throw away" mentality of electronics in general.Open system = 1 port Closed system (frustum) = 2 portCase closed. Its the last I will post on this matter at NSF.
I think the reason why all the EmDrive experimenters to date (Shawyer, Yang, Brady/White, Tajmar) measured the unloaded Q (or Qu) as the S11 1 port return loss (resonant frequency at maximum return loss dB -3dB off this peak return loss dB value) is not because they would not know or don't want to use the more official S22 2-port method, but because the Qu they get from the S11 1-port method is just a suitable value that can be plugged readily into their equations to calculate and predict theoretical thrust. Obviously the S22 2-port value does not provide a value usable to calculate thrust, at least according to their equations.
Quote from: flux_capacitor on 08/07/2015 08:39 pmI think the reason why all the EmDrive experimenters to date (Shawyer, Yang, Brady/White, Tajmar) measured the unloaded Q (or Qu) as the S11 1 port return loss (resonant frequency at maximum return loss dB -3dB off this peak return loss dB value) is not because they would not know or don't want to use the more official S22 2-port method, but because the Qu they get from the S11 1-port method is just a suitable value that can be plugged readily into their equations to calculate and predict theoretical thrust. Obviously the S22 2-port value does not provide a value usable to calculate thrust, at least according to their equations.transmission measurement is S12 or S21
Quote from: WarpTech on 08/07/2015 07:55 pmbut in all cases, they are doing it wrong and should be using 2 ports to measure cavity resonance. We have yet to see anyone do it this way.ToddSo Dr. Ray Kwok is also wrong?
Just a question regarding the Q measurement :Even if we know that the Q's are/were measured in the wrong way, wouldn't it be wise to continue with the faulty system, in order to make all those test comparative?If measurement standards are changed now to the correct method, all the data we have up till now becomes inaccurate or no longer useable, no?One can question the validity of the previous information bits that have been gathered, but this sure will not help... throw it all overboard then?
Q = f / deltaFwhere deltaF is the frequency spread between lower and upper -3 dB points.I think the differences discussed stem from how that -3 dB is measured, no?
Not coming from the RF world, I wonder why this whole discussion about Q can't settle on Q=2*π*energy_stored/energy_dissipated_per_cycle as a standard. Out of curiosity, isn't it possible to switch off a RF source fast enough (a few cycles) and get this Q value by observing only the time constant of the decay of amplitude with a minimally invasive probe ?BTW, not wanting to sound insistent but there is no answer to my questions about the relation of Q and "losses per round trip" (in the context of a linear resonant set up). If I am making a mistake by thinking "number of bounces" when hearing Q, then I'm not the only one : Think of it like the number of times a photon bounces inside a mirrored cavity. A clarification might be useful.
Quote from: frobnicat on 08/07/2015 10:41 pmNot coming from the RF world, I wonder why this whole discussion about Q can't settle on Q=2*π*energy_stored/energy_dissipated_per_cycle as a standard. Out of curiosity, isn't it possible to switch off a RF source fast enough (a few cycles) and get this Q value by observing only the time constant of the decay of amplitude with a minimally invasive probe ?BTW, not wanting to sound insistent but there is no answer to my questions about the relation of Q and "losses per round trip" (in the context of a linear resonant set up). If I am making a mistake by thinking "number of bounces" when hearing Q, then I'm not the only one : Think of it like the number of times a photon bounces inside a mirrored cavity. A clarification might be useful.My conjecture on this would be that, a small cone angle will cause more bounces and a higher Q. However, my feeling is that as the wave propagates from the small end to the big end, it is reflecting off the side walls. Each reflection off the side walls imparts a tiny bit of momentum to the frustum. So the more bounces off the side walls (not the end plates) will produce more thrust. This means that a slower group velocity, bouncing over a longer period of time, would give higher thrust. This leads to the idea that the frustum should be shaped more like a trombone with a long throat. But... none of the theories so far support this idea, but none have tried either.Todd