secondary resonance vs working frequency of the secondary coil
IamSmooth, Sat Nov 11 2006, 01:03AMI was reading through a Tesla Coil design paper and it distinguishes between self-resonant frequency and working frequency. The Fres is the natural frequency for the L and C values of the secondary. It describes the working frequency as being based on considering that the secondary coil's wire length is 1/4 of the wavelength (wavelength = c/f; wirelength*4 = wavelength).
My question is to which frequency does one tune the primary LC circuit; or, should the working frequency and self-resonant frequency be the same?
Re: secondary resonance vs working frequency of the secondary coil
Marko, Sat Nov 11 2006, 01:15AM
We simply forget 1/4 wave thing since TC is not an antenna.
Atleast I do so, although I see some people hunting for their secondary wire length to be closely 1/4 of operating wavelength for reasons I can't understand.
Since wire is coiled up and it's business is not to radiate EM from all I figured out it doesn't have any effect on TC operation.
Marko, Sat Nov 11 2006, 01:15AM
We simply forget 1/4 wave thing since TC is not an antenna.
Atleast I do so, although I see some people hunting for their secondary wire length to be closely 1/4 of operating wavelength for reasons I can't understand.
Since wire is coiled up and it's business is not to radiate EM from all I figured out it doesn't have any effect on TC operation.
Re: secondary resonance vs working frequency of the secondary coil
Hazmatt_(The Underdog), Sat Nov 11 2006, 03:33AM
It's really hard to get the 1/4 wave to match the particular secondary because of self capacitence. Then you're changing the system again with the topload.
What you're looking for is basically:
LpCp = LsCs
I knew where the secondary performed best from a few previous runs, so I had a ballpark figure. I had to change some of the wiring on the primary side. I measured the primary side wiring and coil, then adjusted so the total was Lp. Afterward I varied the inductance slightly and the performance dropped off, so I knew I was in the right area.
So basically, if you know your sum Cs, Ls, Cp, you can measure your connections with a meter, adjust, and you get really close ;)
Matt
***edit**
You know, there is one thing I forgot to mention, and its exactly what we're talking about in Phyisics right now. Your wavelength calculation is wrong actually, because there is a transmission impedance of the wave. What this means to you is that c, the speed of light, is not actually the speed of light. The wave actually bounces back and forth inside the conductor as it propagates.
conductor
--------------------------- --
/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/ --------> wave propagation
------------------------------
Now what you would have expected if your wave speed were actually c would be:
conductor
----------------------------
------ --------------------------> wave propagation
----------------------------
The permitivity of the conductor 'slows' the wave speed down, and if you look this up in a Belden catalog its something like .66 * speed of light, and that also varies between cables too.
so f = c/v; yea, for light and ideal cases, but not so much here.
Hazmatt_(The Underdog), Sat Nov 11 2006, 03:33AM
It's really hard to get the 1/4 wave to match the particular secondary because of self capacitence. Then you're changing the system again with the topload.
What you're looking for is basically:
LpCp = LsCs
I knew where the secondary performed best from a few previous runs, so I had a ballpark figure. I had to change some of the wiring on the primary side. I measured the primary side wiring and coil, then adjusted so the total was Lp. Afterward I varied the inductance slightly and the performance dropped off, so I knew I was in the right area.
So basically, if you know your sum Cs, Ls, Cp, you can measure your connections with a meter, adjust, and you get really close ;)
Matt
***edit**
You know, there is one thing I forgot to mention, and its exactly what we're talking about in Phyisics right now. Your wavelength calculation is wrong actually, because there is a transmission impedance of the wave. What this means to you is that c, the speed of light, is not actually the speed of light. The wave actually bounces back and forth inside the conductor as it propagates.
conductor
--------------------------- --
/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/ --------> wave propagation
------------------------------
Now what you would have expected if your wave speed were actually c would be:
conductor
----------------------------
------ --------------------------> wave propagation
----------------------------
The permitivity of the conductor 'slows' the wave speed down, and if you look this up in a Belden catalog its something like .66 * speed of light, and that also varies between cables too.
so f = c/v; yea, for light and ideal cases, but not so much here.
Re: secondary resonance vs working frequency of the secondary coil
WaveRider, Sat Nov 11 2006, 10:11AM
The TC resonator has a lot in common with the 1/4 wave helical resonator. However, being a close wound helix with dimensions much smaller than a wavelength at the operating frequency, it does not radiate very much. Another consequence of the close windings is that the physical length of the wire in the coil is longer than 1/4 wavelength, even though the magnification effect at the open end is alot like the magnification one gets at the open end of a 1/4 wave transmission line. This is because the turns are closely inductively/capacitively coupled. Likewise, if the turns are not so close (loosely wound helix), the electrical length of the helix winding wire approaches 1/4 wave at the fundamental resonance...and if the physical dimensions are a significant fraction of a wavelength, you will get radiation in an open structure!
Tuning the primary gives you a coupled tuned circuit. The coupling gives you a so called frequency-splitting effect. There are two resonances that move around as you tune the primary and vary the coupling. The maths is straightforward, but can be a little tedious.. Do you have a good circuits book? I also believe the ARRL handbook talks a bit about doubly-tuned circuits...
WaveRider, Sat Nov 11 2006, 10:11AM
The TC resonator has a lot in common with the 1/4 wave helical resonator. However, being a close wound helix with dimensions much smaller than a wavelength at the operating frequency, it does not radiate very much. Another consequence of the close windings is that the physical length of the wire in the coil is longer than 1/4 wavelength, even though the magnification effect at the open end is alot like the magnification one gets at the open end of a 1/4 wave transmission line. This is because the turns are closely inductively/capacitively coupled. Likewise, if the turns are not so close (loosely wound helix), the electrical length of the helix winding wire approaches 1/4 wave at the fundamental resonance...and if the physical dimensions are a significant fraction of a wavelength, you will get radiation in an open structure!
Tuning the primary gives you a coupled tuned circuit. The coupling gives you a so called frequency-splitting effect. There are two resonances that move around as you tune the primary and vary the coupling. The maths is straightforward, but can be a little tedious.. Do you have a good circuits book? I also believe the ARRL handbook talks a bit about doubly-tuned circuits...
Re: secondary resonance vs working frequency of the secondary coil
Sulaiman, Sat Nov 11 2006, 10:21AM
TC programs often give the frequency at which the inductor alone is self resonant
'Wheeler' Inductance in parallel with 'Medhurst' Capacitance
This frequency will be well above the 1/4-wavelength frequency for normal coils (H/D = 2 to 5)
Adding a topload capacitance reduces this resonant frequency
A toroid of similar dimensions to the inductor bent into a circle
brings the resonant frequency close to 1/4 wave
An even larger topload can be used for operation below 1/4 wavelength
with a lot of charge available for streamer growth.
Operating without a topload produces the highest voltages part way down the coil from the top
(causing corona/sparks to come from the winding)
operating at 1/4 wave or lower frequency ensures the highest voltage is at the topload
Sulaiman, Sat Nov 11 2006, 10:21AM
TC programs often give the frequency at which the inductor alone is self resonant
'Wheeler' Inductance in parallel with 'Medhurst' Capacitance
This frequency will be well above the 1/4-wavelength frequency for normal coils (H/D = 2 to 5)
Adding a topload capacitance reduces this resonant frequency
A toroid of similar dimensions to the inductor bent into a circle
brings the resonant frequency close to 1/4 wave
An even larger topload can be used for operation below 1/4 wavelength
with a lot of charge available for streamer growth.
Operating without a topload produces the highest voltages part way down the coil from the top
(causing corona/sparks to come from the winding)
operating at 1/4 wave or lower frequency ensures the highest voltage is at the topload
Re: secondary resonance vs working frequency of the secondary coil
Marko, Sat Nov 11 2006, 12:57PM
No, it simply can't be true.
In any LC, peak voltage is always going to appear between 'ends' of the coil itself, and I think it's pretty obivous thing.
Some people obivously aren't even sure if it's good or bad to have 'omg 1/4 wavelength' secondary and nobody explained yet how it really affects coils in any way.
Does it really *mysteriously* teleport peak of voltage to somewhere else?
Or causes secndary to emit more magnetic component like antenna, wastng power?
Or soemthing else?
All that about 'omg 1/4 wl' seems like having no physical basis and seem to be just placebo statisfaction for some coilers, while others seem to spark very well without even mentioning it.
I feared to open such a topic before but now it seems that this needs to be solved right now for sake of 4hv.
Due to my ignorance I need something to be step-by step explained, and it doesn't seem possible when opinions are divided like such...
So, again, why to tune a secondary coil like it's some kind of 75 ohm antenna (how is it important, and why?) while lots of other coilers (steve ward, conner, TDU, EVR ...) don't mention a thing about it and are still fine?
Marko, Sat Nov 11 2006, 12:57PM
Operating without a topload produces the highest voltages part way down the coil from the top
(causing corona/sparks to come from the winding)
No, it simply can't be true.
In any LC, peak voltage is always going to appear between 'ends' of the coil itself, and I think it's pretty obivous thing.
Some people obivously aren't even sure if it's good or bad to have 'omg 1/4 wavelength' secondary and nobody explained yet how it really affects coils in any way.
Does it really *mysteriously* teleport peak of voltage to somewhere else?
Or causes secndary to emit more magnetic component like antenna, wastng power?
Or soemthing else?
All that about 'omg 1/4 wl' seems like having no physical basis and seem to be just placebo statisfaction for some coilers, while others seem to spark very well without even mentioning it.
I feared to open such a topic before but now it seems that this needs to be solved right now for sake of 4hv.
Due to my ignorance I need something to be step-by step explained, and it doesn't seem possible when opinions are divided like such...
So, again, why to tune a secondary coil like it's some kind of 75 ohm antenna (how is it important, and why?) while lots of other coilers (steve ward, conner, TDU, EVR ...) don't mention a thing about it and are still fine?
Re: secondary resonance vs working frequency of the secondary coil
WaveRider, Sat Nov 11 2006, 01:28PM
Speaking from the viewpoint of a CW coil: The arc at the top of the coil behaves like a resistive load(with a bit of capacitive loading too). If you want to get the best transfer of power into that arc, you need to "match" the driving amplifier to the coil.
Before the arc is struck (before breakout), the coil behaves as a resonator loaded only by its internal losses and the driving circuit. Resonator Q is quite high (several hundred or so). Power from the driving amplifier (or switch) goes into building up the resonant oscillations of the resonator and some is dissipated in the circuit resistances. When the oscillations build up enough, the air around the breakout point breaks down. This appears like extra loss in the resonant circuit. Since the plasma is conductive almost like a metal, extra capacitance appears...lowering the resonant frequency a little (this is easily verified with a CW coil). Since we have this plasma sucking power out of our resonator, we need the amplifier to supply power in the most efficient way to the resonator....this means an attemt at matching. This is difficult because the plasma is not a constant impedance...it varies with power level and time (it "flickers" like a flame).. The best hope is to pick a power level and design a matching network around the expected feedpoint impedance for that power level (arc size). This is what I did with my 250 W CW coil and I can say with confidence that it works!
With pulsed coils, the design criteria are somewhat different. The idea seems to me (and the Steves, EVR, et al can elaborate on this) to dump as much energy into the coil resonator as quickly as possible to get the big sparks. Here the idea of matching in the CW sense does not work, because you are talking about dumping energy in a short pulse with as little pulse spreading as possible so you get very high voltages with sudden breakout at extreme peak powers. In short, the design goals are different...
As Firkragg adamantly and correctly states, this cannot be true. The fundamental resonance of the TC without the topload is the 1/4-wave like (even tho' we all know the winding is longer than 1/4 wave) mode with (nearly) max current at the base (ground-plane) and max voltage at the top. Adding a topload reduces the frequency of resonance. The max voltage will still be at the top of the coil, altho' the "magnification" effect will be somewhat less. (consider the voltage magnification in a resonator formed using a less-than-1/4-wave length of transmission line with a capacitor at the end compared to the pure 1/4 wave transmission line.) Theoretically, a large number of modes can be excited on a helical resonator. We all shoot for the fundamental mode which has a voltage peak at the top. . Only if you excite a "higher-order" mode on the helix will you get voltage peaks "in the middle" somewhere. This happens if you choose an operating frequency higher than the fundamental (first) resonant frequency. However, sparks can appear in the middle of you resonator for a variety of reasons (e.g. insulation breakdown, etc.)
WaveRider, Sat Nov 11 2006, 01:28PM
So, again, why to tune a secondary coil like it's some kind of 75 ohm antenna (how is it important, and why?) while lots of other coilers (steve ward, conner, TDU, EVR ...) don't mention a thing about it and are still fine?
Speaking from the viewpoint of a CW coil: The arc at the top of the coil behaves like a resistive load(with a bit of capacitive loading too). If you want to get the best transfer of power into that arc, you need to "match" the driving amplifier to the coil.
Before the arc is struck (before breakout), the coil behaves as a resonator loaded only by its internal losses and the driving circuit. Resonator Q is quite high (several hundred or so). Power from the driving amplifier (or switch) goes into building up the resonant oscillations of the resonator and some is dissipated in the circuit resistances. When the oscillations build up enough, the air around the breakout point breaks down. This appears like extra loss in the resonant circuit. Since the plasma is conductive almost like a metal, extra capacitance appears...lowering the resonant frequency a little (this is easily verified with a CW coil). Since we have this plasma sucking power out of our resonator, we need the amplifier to supply power in the most efficient way to the resonator....this means an attemt at matching. This is difficult because the plasma is not a constant impedance...it varies with power level and time (it "flickers" like a flame).. The best hope is to pick a power level and design a matching network around the expected feedpoint impedance for that power level (arc size). This is what I did with my 250 W CW coil and I can say with confidence that it works!
With pulsed coils, the design criteria are somewhat different. The idea seems to me (and the Steves, EVR, et al can elaborate on this) to dump as much energy into the coil resonator as quickly as possible to get the big sparks. Here the idea of matching in the CW sense does not work, because you are talking about dumping energy in a short pulse with as little pulse spreading as possible so you get very high voltages with sudden breakout at extreme peak powers. In short, the design goals are different...
Operating without a topload produces the highest voltages part way down the coil from the top
(causing corona/sparks to come from the winding)
operating at 1/4 wave or lower frequency ensures the highest voltage is at the topload
As Firkragg adamantly and correctly states, this cannot be true. The fundamental resonance of the TC without the topload is the 1/4-wave like (even tho' we all know the winding is longer than 1/4 wave) mode with (nearly) max current at the base (ground-plane) and max voltage at the top. Adding a topload reduces the frequency of resonance. The max voltage will still be at the top of the coil, altho' the "magnification" effect will be somewhat less. (consider the voltage magnification in a resonator formed using a less-than-1/4-wave length of transmission line with a capacitor at the end compared to the pure 1/4 wave transmission line.) Theoretically, a large number of modes can be excited on a helical resonator. We all shoot for the fundamental mode which has a voltage peak at the top. . Only if you excite a "higher-order" mode on the helix will you get voltage peaks "in the middle" somewhere. This happens if you choose an operating frequency higher than the fundamental (first) resonant frequency. However, sparks can appear in the middle of you resonator for a variety of reasons (e.g. insulation breakdown, etc.)
Re: secondary resonance vs working frequency of the secondary coil
Marko, Sat Nov 11 2006, 02:17PM
The first part of your text (about loaded and unloaded impedances) is familiar enough to me - that's etc. why class E coils are so difficult anyway (very narrow frequency band and sensitivity to load impedance changes).
I wasn't asking about secondary and spark impedances anyway, but what tuning to 1/4 wl frequency helps system in any mean!
It has been explained on richie burnett's site niely long ago (site is now down, sadly
)
On his site, the thing aboul 1/4 wl also wasn't mentioned anywhere IIRC.
Yeah, I agree that.
And from all I know, tuning the coil's 1/4 wl frequency to it's LC resonant frequency practically has no effect on TC operation.
Marko, Sat Nov 11 2006, 02:17PM
The first part of your text (about loaded and unloaded impedances) is familiar enough to me - that's etc. why class E coils are so difficult anyway (very narrow frequency band and sensitivity to load impedance changes).
I wasn't asking about secondary and spark impedances anyway, but what tuning to 1/4 wl frequency helps system in any mean!
It has been explained on richie burnett's site niely long ago (site is now down, sadly
) On his site, the thing aboul 1/4 wl also wasn't mentioned anywhere IIRC.
The fundamental resonance of the TC without the topload is the 1/4-wave like (even tho' we all know the winding is longer than 1/4 wave) mode with (nearly) max current at the base (ground-plane) and max voltage at the top. Adding a topload reduces the frequency of resonance. The max voltage will still be at the top of the coil, altho' the "magnification" effect will be somewhat less. (consider the voltage magnification in a resonator formed using a less-than-1/4-wave length of transmission line with a capacitor at the end compared to the pure 1/4 wave transmission line.) Theoretically, a large number of modes can be excited on a helical resonator. We all shoot for the fundamental mode which has a voltage peak at the top. . Only if you excite a "higher-order" mode on the helix will you get voltage peaks "in the middle" somewhere. This happens if you choose an operating frequency higher than the fundamental (first) resonant frequency. However, sparks can appear in the middle of you resonator for a variety of reasons (e.g. insulation breakdown, etc.)
Yeah, I agree that.
And from all I know, tuning the coil's 1/4 wl frequency to it's LC resonant frequency practically has no effect on TC operation.
Re: secondary resonance vs working frequency of the secondary coil
WaveRider, Sat Nov 11 2006, 02:41PM
Firkragg, I think the confusion comes about because the first resonance the TC resonator behaves a bit like a 1/4-wave transmission-line resonator, even tho' it is not really the frequency corresponding to the "1/4 wavelength" frequency, as you put it.. .
In fact, I've done the simulations which clearly show the current distribution along the coil windings. Qualitatively, they look conspicuously like the current distribution on a 1/4-wave transmission-line resonator... Unfortunately, no one seemed to be too interested in the results to discuss them much....
WaveRider, Sat Nov 11 2006, 02:41PM
Firkragg, I think the confusion comes about because the first resonance the TC resonator behaves a bit like a 1/4-wave transmission-line resonator, even tho' it is not really the frequency corresponding to the "1/4 wavelength" frequency, as you put it.. .
In fact, I've done the simulations which clearly show the current distribution along the coil windings. Qualitatively, they look conspicuously like the current distribution on a 1/4-wave transmission-line resonator... Unfortunately, no one seemed to be too interested in the results to discuss them much....
Re: secondary resonance vs working frequency of the secondary coil
Terry Fritz, Sat Nov 11 2006, 08:11PM
Hi,
Paul Nicholson has done many studies of secondary resonators too:
I have done it the hard way
I trust all these simulations by now independent studies are giving the same answers?
Programs like JAVATC can calculate everything for typical coil use to very high accuracy now:
Nobody bases anything on "wire length" these days. It does not matter. The secondaries coil's inductance and self capacitance and the added capacitance of the top load rule the resonant frequency and that can be calculated by computer.
Cheers,
Terry
Terry Fritz, Sat Nov 11 2006, 08:11PM
Hi,
In fact, I've done the simulations which clearly show the current distribution along the coil windings. Qualitatively, they look conspicuously like the current distribution on a 1/4-wave transmission-line resonator... Unfortunately, no one seemed to be too interested in the results to discuss them much....
Paul Nicholson has done many studies of secondary resonators too:
I have done it the hard way
I trust all these simulations by now independent studies are giving the same answers?
Programs like JAVATC can calculate everything for typical coil use to very high accuracy now:
Nobody bases anything on "wire length" these days. It does not matter. The secondaries coil's inductance and self capacitance and the added capacitance of the top load rule the resonant frequency and that can be calculated by computer.
Cheers,
Terry
Re: secondary resonance vs working frequency of the secondary coil
Sulaiman, Sat Nov 11 2006, 09:05PM
AFAIK most people buld a TC to get impressive/spectacular arcs
the result will be (mostly) determined by power throughput
so close-coupling, large topload etc. works best.
Tesla didn't want corona/sparks/arcs, high-voltage high-capacitance was more his thing
(wireless transmission of power etc.)
Terry, your graphs clearly show the wavelength characteristics of your resonator;
the coil with topload (toroid similar to coil ... operating below 1/4 wavelength)
shows a nearly linear volts/inch - lumped model reasonably accurate.
the unloaded coil clearly shows the tapering off of volts/inch near the top
if you extrapolate the centre region of the slope
you should get about 90 kV at the topload instead of 70 kV
signs of non-lumped operation.
The resonance effect of Inductance and capacitance
is more significant than the 1/4-wavelength effects
so for practical high power long arc TC forget wire-length effects
and operate well below 1/4 wavelength resonance. (large topload)
but the effect is real and present.
Sulaiman, Sat Nov 11 2006, 09:05PM
AFAIK most people buld a TC to get impressive/spectacular arcs
the result will be (mostly) determined by power throughput
so close-coupling, large topload etc. works best.
Tesla didn't want corona/sparks/arcs, high-voltage high-capacitance was more his thing
(wireless transmission of power etc.)
Terry, your graphs clearly show the wavelength characteristics of your resonator;
the coil with topload (toroid similar to coil ... operating below 1/4 wavelength)
shows a nearly linear volts/inch - lumped model reasonably accurate.
the unloaded coil clearly shows the tapering off of volts/inch near the top
if you extrapolate the centre region of the slope
you should get about 90 kV at the topload instead of 70 kV
signs of non-lumped operation.
The resonance effect of Inductance and capacitance
is more significant than the 1/4-wavelength effects
so for practical high power long arc TC forget wire-length effects
and operate well below 1/4 wavelength resonance. (large topload)
but the effect is real and present.
Re: secondary resonance vs working frequency of the secondary coil
IamSmooth, Thu Nov 16 2006, 06:45PM
So, if I wanted to resonant coil of my secondary with the top load to match the 1/4 wavelength frequency should I use 66-80% the speed of light for the calculation? Does anyone think this will truly affect coil performance?
IamSmooth, Thu Nov 16 2006, 06:45PM
Hazmatt_(The Underdog) wrote ...
The permitivity of the conductor 'slows' the wave speed down, and if you look this up in a Belden catalog its something like .66 * speed of light, and that also varies between cables too.
so f = c/v; yea, for light and ideal cases, but not so much here.
The permitivity of the conductor 'slows' the wave speed down, and if you look this up in a Belden catalog its something like .66 * speed of light, and that also varies between cables too.
so f = c/v; yea, for light and ideal cases, but not so much here.
So, if I wanted to resonant coil of my secondary with the top load to match the 1/4 wavelength frequency should I use 66-80% the speed of light for the calculation? Does anyone think this will truly affect coil performance?
Re: secondary resonance vs working frequency of the secondary coil
Marko, Thu Nov 16 2006, 06:56PM
As said, it won't. Just calculate your inductance, self-capacitance and topload capacitance to get your resonant frequency out.
Marko, Thu Nov 16 2006, 06:56PM
Does anyone think this will truly affect coil performance?
As said, it won't. Just calculate your inductance, self-capacitance and topload capacitance to get your resonant frequency out.
Re: secondary resonance vs working frequency of the secondary coil
Sulaiman, Thu Nov 16 2006, 08:28PM
If you design your topload to resonate the coil at the 'quarter vavelength' frequency
(assuming c=speed of light in vacuum)
you will certainly be in the right area (below 1/4 wavelength) due to v<c.
So using the speed of light in vacuum with the length of wire to (wrongly) estimate 1/4 wavelength operation the coil will be almost a lumped equivalent/at a good frequency.
Adding a second tuned circuit (the primary) causes frequency-splitting anyway
so the 'operating' frequency will be neither 1/4 wavelength nor 1/2.pi.sqrt(LsCs)
Sulaiman, Thu Nov 16 2006, 08:28PM
If you design your topload to resonate the coil at the 'quarter vavelength' frequency
(assuming c=speed of light in vacuum)
you will certainly be in the right area (below 1/4 wavelength) due to v<c.
So using the speed of light in vacuum with the length of wire to (wrongly) estimate 1/4 wavelength operation the coil will be almost a lumped equivalent/at a good frequency.
Adding a second tuned circuit (the primary) causes frequency-splitting anyway
so the 'operating' frequency will be neither 1/4 wavelength nor 1/2.pi.sqrt(LsCs)
Re: secondary resonance vs working frequency of the secondary coil
teravolt, Fri Nov 17 2006, 05:27AM
unless your trying to make a tesla to a specfic frequency 1/4 wave is accademic. When I make a coil I just mesure it then use top load to tune it. Of more inerest is frequency splitting and tuning low.
teravolt, Fri Nov 17 2006, 05:27AM
unless your trying to make a tesla to a specfic frequency 1/4 wave is accademic. When I make a coil I just mesure it then use top load to tune it. Of more inerest is frequency splitting and tuning low.
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