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#780
by
SleeperService
on 18 Feb, 2015 14:25
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#781
by
Notsosureofit
on 18 Feb, 2015 14:26
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@ Star-Drive
Trying to put together a reference table.
Did they calculate Q values for all of the COMSOL modes shown ?
As RODAL asked, I'm assuming these are w/o dielectric ?
Also, can they calculate a dispersion relation for the cavity including the dielectric ?
Thanks
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#782
by
lele
on 18 Feb, 2015 14:32
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If I understood correctly some things in this thread, EM radiations heat the frustum, which changes its shape, which changes the resonant frequency of the cavity, so it's difficult to keep the EM source tuned.
Would cooling (with flowing water for example) the copper plates 1) reduce the variation of the resonant frequency, by reducing the variation of temperature, thus reducing the geometrical changes of the frustum 2) not interfere with the thrust produced ?
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#783
by
Mulletron
on 18 Feb, 2015 14:38
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#784
by
Rodal
on 18 Feb, 2015 14:39
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If I understood correctly some things in this thread, EM radiations heat the frustum, which changes its shape, which changes the resonant frequency of the cavity, so it's difficult to keep the EM source tuned.
Would cooling (with flowing water for example) the copper plates 1) reduce the variation of the resonant frequency, by reducing the variation of temperature, thus reducing the geometrical changes of the frustum 2) not interfere with the thrust produced ?
Cooling from the outside may actually be counterproductive, as it may increase the thermal gradient through the thickness (it would be induction-heated from the inside and cooled from the outside hence a large thermal gradient) and deform the
very thin NASA EM Drive shell due to the non-uniform aspect of the induction heating due to the non-uniform EM field.
One solution (already offered by Shawyer) is to use Invar, an old alloy that has very low thermal expansion, another solution is to increase the thickness of the EM Drive. NASA was constrained to use very thin copper for the EM Drive because they wanted to minimize the mass on their torque pendulum.
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#785
by
SleeperService
on 18 Feb, 2015 14:45
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If I understood correctly some things in this thread, EM radiations heat the frustum, which changes its shape, which changes the resonant frequency of the cavity, so it's difficult to keep the EM source tuned.
Would cooling (with flowing water for example) the copper plates 1) reduce the variation of the resonant frequency, by reducing the variation of temperature, thus reducing the geometrical changes of the frustum 2) not interfere with the thrust produced ?
Water cooling would either have to be internal to the thrust measuring device or external.
If external, then you have the problem of the coupling of the pipework and the externally powered thrust of the coolant.
If internal then you get the added mass of coolant, cooling jackets, pumps and radiators. And you have an pump running in close proximity to the cavity.
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#786
by
Mulletron
on 18 Feb, 2015 14:45
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#787
by
Notsosureofit
on 18 Feb, 2015 14:50
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#788
by
SleeperService
on 18 Feb, 2015 14:53
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#789
by
SleeperService
on 18 Feb, 2015 15:00
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"The function of the PLL is to compare the distributed clock to the incoming reference clock, and vary the phase and frequency of its output until the reference and feedback clocks are phase and frequency matched."
At which spatial point shall we measure the freq. to compare with the incoming freq.?
Please forgive me if I am wrong and/or naive about this...
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#790
by
Rodal
on 18 Feb, 2015 15:00
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Or a phase locked loop circuit.
http://en.m.wikipedia.org/wiki/Phase-locked_loop#Frequency_synthesis
How would apply this to tracking the resonant freq of the cavity to the input freq of the loop antenna?
I also like Mulletron's original idea of distributing the spectral power density of the input into a spectral band instead of a spike at a single frequency. As Mulletron pointed out, magnetrons used by Juan Yang in China and Shawyer in the UK deserve attention.
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#791
by
Giovanni DS
on 18 Feb, 2015 15:06
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A feedback circuit would be required in order to keep the VCO on the resonant frequency. If such a feedback is possible then a simple MCU could be programmed to do that using an ADC as output, it would not be a PLL anyway but more similar to an FLL.
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#792
by
SleeperService
on 18 Feb, 2015 15:07
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Maybe that was behind Cannae's technique of putting notches in his cavity. It produces more thrust with a broad, 'dirty' spectrum from a magnetron than a 'pure' sinusoidal source..
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#793
by
SleeperService
on 18 Feb, 2015 15:09
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A feedback circuit would be required in order to keep the VCO on the resonant frequency. If such a feedback is possible then a simple MCU could be programmed to do that using an ADC as output, it would not be a PLL anyway but more similar to an FLL.
Where does the feedback signal come from?
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#794
by
matthewpapa
on 18 Feb, 2015 15:18
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I have 2 high performance boxes at my house you may want to consider using for simulation purposes
2p Sandy Bridge (16 cores, 32 threads total @2.8 GHz) about 50 GB of DDR3 memory
2p Haswell (20 cores, 40 threads total @3.1 GHz) and 32 GB of DDR4 memory
PM me if that can help at some point
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#795
by
Giovanni DS
on 18 Feb, 2015 15:20
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A feedback circuit would be required in order to keep the VCO on the resonant frequency. If such a feedback is possible then a simple MCU could be programmed to do that using an ADC as output, it would not be a PLL anyway but more similar to an FLL.
Where does the feedback signal come from?
This is the problem
I do understand controls and realtime systems but microwaves and resonant cavities are not my field. How do you know it is resonating?
In theory the thrust could be your feedback, you tune the VCO in order to maximize that.
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#796
by
SleeperService
on 18 Feb, 2015 15:34
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A feedback circuit would be required in order to keep the VCO on the resonant frequency. If such a feedback is possible then a simple MCU could be programmed to do that using an ADC as output, it would not be a PLL anyway but more similar to an FLL.
Where does the feedback signal come from?
This is the problem I do understand controls and realtime systems but microwaves and resonant cavities are not my field. How do you know it is resonating?
In theory the thrust could be your feedback, you tune the VCO in order to maximize that.
I agree, the only way you could do it is to lock the thrust to the freq. How you would do that though to avoid false maxima is beyond me.
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#797
by
Mulletron
on 18 Feb, 2015 15:40
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Resonant cavities are also filters. You monitor a sample port to ensure the cavity is resonant at the desired frequency.
Another control loop compensating for doppler drift is needed. Shawyer mentions this type of control loop over at emdrive.com.
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#798
by
SleeperService
on 18 Feb, 2015 15:44
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Resonant cavities are also filters. You monitor a sample port to ensure the cavity is resonant at the desired frequency.
Another control loop compensating for doppler drift is needed. Shawyer mentions this type of control loop over at emdrive.com.
It seems to me that it matters where you put the sample port with a truncated frustum.
Am I wrong? Would a sample port to feedback the PLF work in any location?
I'll leave Doppler drift for another time...
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#799
by
Mulletron
on 18 Feb, 2015 15:54
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I do understand controls and realtime systems but microwaves and resonant cavities are not my field. How do you know it is resonating?