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Author Topic: Smudge proposed NMR experiment replication.  (Read 12354 times)
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The article does not mention whether the inter-turn capacitance CIT is the parallel capacitance CP depicted on the schematic diagram below:



This is important because any current flowing through the CIT distorts the direction of the magnetic field generated by the turns of the coil.  A current flowing in an external parallel capacitance, does not do that.

I assumed Cp was an external and have both bench built and simulated the concept with success but at lower frequencies than this NMR requires.

Any large Cit could be improved as you suggested by gaps in between the turns on the flat coil.

What amazed me was the difference in my tests between the series connected bucking coils with the facing windings going the same direction (no differential H field detected) and the parallel connected bucking coils with the facing windings counterwound (excellent differential H field detection)!

Regards,
Pm
   

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What amazed me was the difference in my tests between the series connected bucking coils with the facing windings going the same direction (no differential H field detected) and the parallel connected bucking coils with the facing windings counterwound (excellent differential H field detection)!
I just realized, that I might understand the phrase "differential H field " differently than you had meant it.  There are two plausible interpretations of that phrase in this context:
1) An amplitude of the difference between two H fields' magnitudes occuring at two different places or two different spatial directions (or two different times).
2) An amplitude of a single H field measured by an unloaded sensor coil connected to an oscilloscope with two probes that have floating grounds at the probes' ends*, which sense voltage amplitudes that are later subtracted.

* but these two grounds are connected together at the scope.
   
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I just realized, that I might understand the phrase "differential H field " differently than you had meant it.  There are two plausible interpretations of that phrase in this context:
1) An amplitude of the difference between two H fields' magnitudes occuring at two different places or two different spatial directions (or two different times).
2) An amplitude of a single H field measured by an unloaded sensor coil connected to an oscilloscope with two probes that have floating grounds at the probes' ends*, which sense voltage amplitudes that are later subtracted.

* but these two grounds are connected together at the scope.

Sorry, I should have been more distinct.  Your option 2) above is what I was meaning.  However, the sensor coils were loaded with 100k down to 1K resistors and exhibited power output within ~90% of the input power to the bucking coils.

Pm
   

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Ordered Litz wire parcel officially went missing  :(
New one is on its way

Itsu
   

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 :)


One day after the local PostOffice confirmed to me the parcel was missing (no status update since 12 Aug.) and it was confirmed missing by the Senders Post Organisation, it arrived this morning 11 Sept.

Unfortunately the sender already send a replacement, so unnecessary costs are being made.

Itsu
   

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First prototype coil made as weave-1 with 1mm nylon spacing.

Id=81mm, Od=120mm, 1mm diameter litz wire 200 strands x 0.04m.

Further measurements will follow.

Itsu
   

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First prototype coil made as weave-1 with 1mm nylon spacing.
You are getting better at this - I like the quality.
Please make the input to this coil using RF techniques. No screwy terminals and long leads...
   

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Further measurements will follow.
Because this coil is expected to have a high impedance at 3-4MHz please use the Series Measurement topology with your VNA for the first measurement.
This will give you the impedance of the coil or S21 vs. frequency, but it will not tell you the H-field amplitude vs. frequency, because the RF current does not have to flow through the coil's windings  (even when they are spaced apart).

To obtain the ratio of H-field amplitude / Input voltage amplitude vs. frequency, you'll need to use your H-field probe on the receiving port in S21 measurement, to sense the generated H-field. 
With only one coil, orient the H-Field probe for maximum amplitude.  However, with two coils, orient it to sense the magnetic flux in the radial direction.
   
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[clip]

  However, with two coils, orient it to sense the magnetic flux in the radial direction.


Is this correct?  Wouldn't you want to measure the H field in the axial direction?  Or are we saying there is a difference between the flux field of the coils and the H field?

Regards,
Pm
   

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I made some initial measurements on the new coil.

Number of turns is 10
Inductance measured at 100Khz = 16uH
Resistance = 0.4 Ohm

nanoVNA shows the below output using the shown connection to the coil (series measurement).

Itsu
   

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Is this correct?  Wouldn't you want to measure the H field in the axial direction?
No, the flux from the permanent magnets is in the axial direction.
The flux from the coils must be perpendicular to that axial flux, it must be e.g. Radial.  To be most sensitive in the radial direction, the sensing loop must be oriented as pictured in orange color on this diagram.
The video of my robotic positioner had the sensing loop oriented like that.

Or are we saying there is a difference between the flux field of the coils and the H field?
We are not.
   

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I made some initial measurements on the new coil.

Number of turns is 10
Inductance measured at 100Khz = 16uH
Resistance = 0.4 Ohm
Does the length of the wire used to wind this coil make sense as the one responsible for that 28MHz peak at Marker2, when you treat it as a shorted transmission line of half that length ?
Would you be willing to cut the coil at the middle (like I did with mine) and solder a 220Ω pot in the break to terminate it with a matching resistance that will inhibit reflections ?
BTW: you could measure the CIT directly this way, too.

P.S.
Does anyone remember the round trip time of that shorted coax in the Dally's contraption ?
« Last Edit: 2020-09-13, 23:01:31 by verpies »
   

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Quote
Does the length of the wire used to wind this coil make sense as the one responsible for that 28MHz peak at Marker2, when you treat it as a shorted transmission line of half that length ?



I started with a piece of 5m litz wire, and ended up cutting 88cm of both ends, so the coil itself is about 324cm.

Half of that is 162cm, so i do not see a reason there for the 28MHz (10m wavelength) peak.


Quote
Would you be willing to cut the coil at the middle (like I did with mine) and solder a 220Ω pot in the break to terminate it with a matching resistance that will inhibit reflections ?
BTW: you could measure the CIT directly this way, too.

I can try, but i am not sure it can be done now as i used some superglue at that start point.
I might destroy more then i like.


Quote
P.S.
Does anyone remember the round trip time of that shorted coax in the Dally's contraption ?

I don't, and i think it was never shown/mentioned.

Itsu
   
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No, the flux from the permanent magnets is in the axial direction.
The flux from the coils must be perpendicular to that axial flux, it must be e.g. Radial.  To be most sensitive in the radial direction, the sensing loop must be oriented as pictured in orange color on this diagram.
The video of my robotic positioner had the sensing loop oriented like that.
We are not.

OK, thanks.  I really meant co-axial but that doesn't change your answer any.  I don't want to take away from this thread but I will run tests to see why I detect a very strong co-axial H field in my setup as it could not be from the theoretical displacement current IMO as the voltage differential between adjacent turns is essentially zero.

Regards,
Pm
   

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Before splitting the coil in 2, i first wanted to try some H-field measurements.


Quote
To obtain the ratio of H-field amplitude / Input voltage amplitude vs. frequency, you'll need to use your H-field probe on the receiving port in S21 measurement, to sense the generated H-field.
With only one coil, orient the H-Field probe for maximum amplitude.  However, with two coils, orient it to sense the magnetic flux in the radial direction.


See below video on how i tried it, not sure its the correct way, especially the calibration of the "thru" part and to what value's i should look.

Video here: https://www.youtube.com/watch?v=xJtPG3lDiEM

Itsu

   

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I started with a piece of 5m litz wire, and ended up cutting 88cm of both ends, so the coil itself is about 324cm.
Half of that is 162cm, so i do not see a reason there for the 28MHz (10m wavelength) peak.
According to basic Ham math for quarter-wavelength stubs:
f[MHz] = 75[m/MHz] * Vf / L[m]

At 0.8 velocity factor (Vf):
A shorted 162cm stub will appear as an open at 37MHz, 111MHz, 185MHz, ...and all of the odd harmonics.
A shorted 162cm stub will appear as a short at 74MHz, 148MHz , 222MHz, ...and all of the even harmonics.

However at 0.6 velocity factor (Vf):
A shorted 162cm stub will appear as an open at 28MHz,  83MHz, 139MHz, ...and all of the odd harmonics.
A shorted 162cm stub  will appear as a short at 55MHz, 111MHz, 167MHz , ...and all of the even harmonics.

...all rounded up to the nearest MHz.
« Last Edit: 2020-09-15, 02:51:58 by verpies »
   

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See below video on how i tried it, not sure its the correct way, especially the calibration of the "thru" part and to what value's i should look.
Yes H-field "thru" calibration is difficult because you do not have a device that converts constant voltage amplitude (from the VNA) to constant amplitude H-field at various frequencies.
If your VNA had a constant current amplitude output then this would not be a problem.

Take a look at my attempt to do that some pages ago, where I used two identical 10mm shielded loops parallel to each other.  In that attempt the constant voltage amplitude signal from the VNA's transmitter obviously was inducing less and less H-field amplitude in the transmitting coil as the frequency was increasing (because the coil's impedance was increasing with frequency), so it was behaving like a low pass filter.  The receiver coil had the opposite characteristic.  Your H-field probe is flat, because it is compensated internally up to 70MHz.  You proved it by clamping the i-Probe on 15mm of Litz wire, which was shorting the VNA's transmitter....or was it the SA's TG ?

Anyway, fudging the "thru" by as a straight-through electric connection and assuming, that the H-field probe is flat, still lets us see the relative magnitudes of the H-Field and it is obvious that the field is the strongest on the middle of the winding (not to be confused with the center of the coil) and at 27.5MHz.

With one coil, the orientation of the H-field probe at the middle of the winding should be such as to maximize the amplitude sensed by it, ...but not so close that the capacitive coupling can take place.
With two coils and this probe between them, its orientation should be radial like in my video, period.

Let's investigate why the H-field's amplitude is so much higher at 27MHz than at 3MHz. Perhaps we can utilize the phenomenon responsible for it to our advantage.

When you cut the coil in the middle (take a look at the photo of my coil where I did it) please use the picopulser, tee, fast scope and 220Ω pot, to do a simple TDR experiment and find out what the characteristic impedance and velocity factor are, when this winding is treated as a transmission line.
   

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According to basic Ham math for quarter-wavelength stubs:
f[MHz] = 75[m/MHz] * Vf / L[m]

At 0.8 velocity factor (Vf):
A shorted 162cm stub will appear as an open at 37MHz, 111MHz, 185MHz, ...and all of the odd harmonics.
A shorted 162cm stub will appear as a short at 74MHz, 148MHz , 222MHz, ...and all of the even harmonics.

However at 0.6 velocity factor (Vf):
A shorted 162cm stub will appear as an open at 28MHz,  83MHz, 139MHz, ...and all of the odd harmonics.
A shorted 162cm stub  will appear as a short at 55MHz, 111MHz, 167MHz , ...and all of the even harmonics.

...all rounded up to the nearest MHz.

OK,  found the formula:
http://www.66pacific.com/calculators/full-wave-loop-antenna-calculator.aspx

L = 75 * Vf / F   or
F = 75 * Vf / L

But the key factor here is the velocity factor.

How do we know that for this SINGLE litz wire, i mean is the nylon inbetween the turns the dielectric causing the slowing effect? 

https://www.picwire.com/resources/technical-articles/velocity-factor/    mentions:

Material   c @ 1.0GHz     VOP         Delay (ns)
Nylon        4.5-3.6       47-53%      2.16-1.92

Itsu
   

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Yes H-field "thru" calibration is difficult because you do not have a device that converts constant voltage amplitude (from the VNA) to constant amplitude H-field at various frequencies.
If your VNA had a constant current amplitude output then this would not be a problem.

Take a look at my attempt to do that some pages ago, where I used two identical 10mm shielded loops parallel to each other.  In that attempt the constant voltage amplitude signal from the VNA's transmitter obviously was inducing less and less H-field amplitude in the transmitting coil as the frequency was increasing (because the coil's impedance was increasing with frequency), so it was behaving like a low pass filter.  The receiver coil had the opposite characteristic.  Your H-field probe is flat, because it is compensated internally up to 70MHz.  You proved it by clamping the i-Probe on 15mm of Litz wire, which was shorting the VNA's transmitter....or was it the SA's TG ?

Anyway, fudging the "thru" by as a straight-through electric connection and assuming, that the H-field probe is flat, still lets us see the relative magnitudes of the H-Field and it is obvious that the field is the strongest on the middle of the winding (not to be confused with the center of the coil) and at 27.5MHz.

With one coil, the orientation of the H-field probe at the middle of the winding should be such as to maximize the amplitude sensed by it, ...but not so close that the capacitive coupling can take place.
With two coils and this probe between them, its orientation should be radial like in my video, period.

Let's investigate why the H-field's amplitude is so much higher at 27MHz than at 3MHz. Perhaps we can utilize the phenomenon responsible for it to our advantage.

When you cut the coil in the middle (take a look at the photo of my coil where I did it) please use the picopulser, tee, fast scope and 220Ω pot, to do a simple TDR experiment and find out what the characteristic impedance and velocity factor are, when this winding is treated as a transmission line.


Yes, it was the SA's TG:
https://www.overunityresearch.com/index.php?topic=3924.msg83003#msg83003


Ok about cutting the coil in the middle:
https://www.overunityresearch.com/index.php?topic=3924.msg83295#msg83295

I will try to put a potmeter there, because as mentioned in my earlier post, knowing the characteristic impedance and velocity factor seems important.

So i treat the both single litz wires as a "cable" terminated by the 220 Ohm pot and adjusting for minimum reflections (impedance),  right?


Itsu
   

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How do we know that for this SINGLE litz wire, i mean is the nylon inbetween the turns the dielectric causing the slowing effect? 
When you treat this coil as a pair of Litz wires with the Nylon dielectric between them, then you get a coiled Twin-Lead transmission line shorted at the end.  The dielectric of the transmission line influences its velocity factor greatly. 
If you still have the Nylon fishing line spacer between the wire turns, remove it to leave air gaps behind.  Air is a better dielectric than Nylon.  Cyanoacrylate glue bonds very well to acrylic/Plexi, but not to Nylon.

So i treat the both single litz wires as a "cable" terminated by the 220 Ohm pot and adjusting for minimum reflections (impedance),  right?
Yes, also try to measure precisely the round trip delay of these reflections with a fast scope and the Picopulser using RF techniques, because this will give us the velocity factor. To maintain constant impedance, try to keep the leads to the coil spaced with the same gap as between the winding turns.

Yes, it was the SA's TG:
https://www.overunityresearch.com/index.php?topic=3924.msg83003#msg83003
I guess that was when you did not have the VNA yet.
You might want to repeat this experiment by clamping the i-Probe on a piece* of Litz wire, which shorts the VNA's transmitter.

* as short as possible !

« Last Edit: 2020-09-15, 17:03:08 by verpies »
   

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Quote
When you treat this coil as a pair of Litz wires with the Nylon dielectric between them, then you get a coiled Twin-Lead transmission line shorted at the end.  The dielectric of the transmission line influences its velocity factor greatly.
If you still have the Nylon fishing line spacer between the wire turns, remove it to leave air gaps behind.  Air is a better dielectric than nylon.  Cyanoacrylate glue bonds very well to acrylic/Plexi, but not to Nylon.

OK,  yes i still have the nylon between the litz wire.
I did remove the nylon which occupied the space where now the litz wire is and it did not came of easily.

Quote
Yes, also try to measure precisely the round trip delay of these reflections with a fast scope and the Picopulser using RF techniques, because this will give us the velocity factor. To maintain constant impedance, try to keep the leads to the coil spaced with the same gap as between the winding turns.

Ok, will try

Quote
I guess that was when you did not have the VNA yet.
You might want to repeat this experiment by clamping the i-Probe on a piece* of Litz wire, which shorts the VNA's transmitter.

Yes before i had the nanoVNA, so i can try it with the nanoVNA.

   

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Managed to remove the nylon spacers, but it weakens the whole pancake coil somewhat.
Had to restore some loosened litz with superglue.

Toke a nanoVNA scan, but somehow the saver program does not accept some values as i get this on connecting to the device:

2020-09-17 10:26:07,192 - NanoVNASaver.SweepWorker - WARNING - Got a non-float data value: -9.811650276 -0.571220398 (-9.811650276)
2020-09-17 10:26:07,393 - NanoVNASaver.SweepWorker - CRITICAL - Tried and failed to read data 1 20 times. Giving up.

So i only could take a screenshot of the nanoVNA device itself, see screenshot below.
Seems the resonance has shifted upwards to 9Mhz.

Anyway, will cut the middle and install a 220 Ohm pot to do some TDR measurements.

Regards Itsu
   

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2020-09-17 10:26:07,192 - NanoVNASaver.SweepWorker - WARNING - Got a non-float data value: -9.811650276 -0.571220398 (-9.811650276)
2020-09-17 10:26:07,393 - NanoVNASaver.SweepWorker - CRITICAL - Tried and failed to read data 1 20 times. Giving up.
Most likely that is an inferior USB cable.
A good USB cable should have 90Ω impedance and thick conductors.  It is hard to identify these without instruments and a good test jig.

Also, what is this "DISK: 99.9%" message ?

So i only could take a screenshot of the nanoVNA device itself, see screenshot below.
Seems the resonance has shifted upwards to 9Mhz.
Not only that but the linear S21 measurement shows the amplitude to be decreasing as the frequency increases almost monotonically.  The high frequency peak at ~32MHz is still there but it is much lower than the peak at low frequencies, which is as it should be because the reactance of an ideal inductor increases with frequency.
How was the H-probe oriented and and how was it positioned with respect to the winding ?
   

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Quote
That most likely is a inferior USB cable.
A good USB cable should have 90Ω impedance and thick conductors.  It is hard to identify these without instruments and a good test jig.

Also, what is this 99.9% disk message ?

So you mean the usb cable that connects between the nanoVNA and my PC?    Its the one supplied by the nanoVNA,  but i will try another one.

That 99.9% disk message seems to be the free space on an internal storage.

Quote
Not only that but the linear S21 measurement shows the amplitude to be decreasing as the frequency increases almost monotonically.  The high frequency peak is much lower than the peak at low frequencies.
How was the H-probe oriented and and how was it positioned with respect to the winding ?

I was using the series measurement, so not the H-field probe.

Itsu
   

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So you mean the usb cable that connects between the nanoVNA and my PC?
Yes

I was using the series measurement, so not the H-field probe.
Right, I should have seen it.
   
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