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Author Topic: Smudge proposed NMR experiment replication.  (Read 47114 times)

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I forgot to mention that while measuring a coil by VNA, the near H-field which surrounds the coil and stores its reactive energy* manifests itself as the imaginary component of its impedance (reactance) while the far-field EM radiation resistance manifests itself as the real part of its impedance (resistance) ...just like the ohmic resistance of the wire.
As usual, the energy stored in the near reactive field (magnetic or electric) does not dissipate and is recoverable*, while the EM energy dissipates permanently as it is radiated away - thus it is not recoverable.

* Later in the AC cycle, this energy will be returned to its source (i.e. to the parallel capacitor ...or to the serial capacitor through the PA).
   

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Quote from: Itsu on 2020-07-12, 21:28:59


    Still a question on my mind,   these 2 pancake coils are in series (bucking), but we have technically not a series capacitor, so does that make them a parallel LC circuit due to their interwinding (parallel) capacitance?


I think so, but I am not completely sure.  It sometimes looks like a mixed series/parallel circuit.
I would like others to analyze this question too: Partzman, Smudge, Centralflow ?

Anyway, the H-field's direction between the pancake coils will be different than outside them.  Amplitude too, especially if they are not identical.


I am asking because the high impedance (3KOhm) you measured during resonance (4.9Mhz) on your new pancake coil (also seen on my coils) using your VNA, points to parallel LC resonance (high impedance, low current and high voltage) i think.

If we do include the 100pF trimmer cap in this series pancake coil circuit it is a series LC circuit which in resonance would mean Low impedance, high current and low voltage.


So does it depends on where you measure (pancakes only or trimmer plus pancakes etc.) on what outcome impedance wise it has?


Itsu   
   

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So does it depends on where you measure (pancakes only or trimmer plus pancakes etc.) on what outcome impedance wise it has?
Of course it does.
All of the plots of my coil, that I've posted, were of the naked coil without any external capacitors connected.

The attached plot also shows the naked coil.
Note that its maximum inductive reactance (blue trace above the 0 line) does not coincide with the peak of the orange trace.

The orange trace is the real component* of the impedance which contains the radiation resistance, so it can be expected that at the peak of this trace, the coil radiates the most radio waves.
The blue trace is the imaginary component of the impedance (inductive reactance above the zero line, and capacitive below it), so it can be expected that at the peak of this trace, the coil builds up the highest near H-field.**
The green trace are the real and imaginary components combined ...or the magnitude of the complex impedance.  Note that this magnitude is sometimes smaller than the real component  :o

* it is a sum of the DC resistance, radiation resistance and additional resistances due to the skin-effect and proximity-effect.
** This is what you new Litzed loop magnetic probe would show.
« Last Edit: 2020-07-20, 14:36:30 by verpies »
   
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Of course it does.

The blue trace is the imaginary component of the impedance (inductive reactance above the zero line, and capacitive below it), so it can be expected that at the peak of this trace, the coil builds up the highest near H-field.


Are you sure about this?  Wouldn't the highest current and therefore the highest H field occur at the zero crossing point of the impedance component?

Also typically, inter-winding capacitance would model as a parallel capacitance and inter-coil capacitance along with any parasitics would model as series capacitance.
   

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Are you sure about this?  Wouldn't the highest current and therefore the highest H field occur at the zero crossing point of the impedance component?
Thanks for catching this ambiguity.  I meant it when all other things are being equal (including the current).
The way I wrote it suggests, that a higher current flows with higher magnitude of impedance when driven with a constant voltage - this is not true.

In the attached plots I changed the green plot to show the Admittance and the orange plot to show the Conductance. The reciprocals of what was shown before.
Now the peaks of these traces show frequencies where more current would flow when driven by a constant voltage.
And the green markers 1, 2, 4, 5, 7 illustrate where it approaches the standard impedance of 50Ω and maximal power transfer would occur.

Also typically, inter-winding capacitance would model as a parallel capacitance
Yes, so any added external capacitance will absorb this internal capacitance.

...and inter-coil capacitance along with any parasitics would model as series capacitance.
Which would explain that 2nd peak Itsu was observing with two coils.
   

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Of course it does.
All of the plots of my coil, that I've posted, were of the naked coil without any external capacitors connected.

The attached plot also shows the naked coil.
Note that its maximum inductive reactance (blue trace above the 0 line) does not coincide with the peak of the orange trace.

The orange trace is the real component* of the impedance which is contains the radiation resistance, so it can be expected that at the peak of this trace, the coil radiates the most radio waves.
The blue trace is the imaginary component of the impedance (inductive reactance above the zero line, and capacitive below it), so it can be expected that at the peak of this trace, the coil builds up the highest near H-field.**
The green trace are the real and imaginary components combined ...or the magnitude of the complex impedance.  Note that this magnitude is sometimes smaller than the real component  :o

* it is a sum of the DC resistance, radiation resistance and additional resistances due to the skin-effect and proximity-effect.
** This is what you new Litzed loop magnetic probe would show.



Great, stuff  O0

Quote
The attached plot also shows the naked coil.
Note that its maximum inductive reactance (blue trace above the 0 line) does not coincide with the peak of the orange trace.

Thats the difference i saw with the peak of my I-probe (blue) and the peak of the Power/SWR meter (orange).


Why do we have a second array of peaks around 29Mhz?  Does that mean 2 resonance points?

Itsu
   

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Why do we have a second array of peaks around 29Mhz?  Does that mean 2 resonance points?
2 ?!
We have many more!
See the plot below with the white background.  It was done with the closed coil up to 500MHz (warning: the VNA was not calibrated for this plot).

A badly terminated coax also has many diminishing│Z│peaks (see the plot with the black background and only 1 green trace). The white plot resembles that, except the green peaks are not equally spaced.

« Last Edit: 2020-07-13, 18:35:11 by verpies »
   

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So to demonstrate that the other peaks are caused by reflections. I have treated the coil as a coiled transmission line and have terminated it in the middle break with a 200Ω resistor.

Below is a TDR plot from a PulseGen+Scope with the white trace showing the reflected pulse from the terminated pancake coil and the yellow trace showing reflected pulses from the unterminated coil (open break).
Also, below is a 500MHz span VNA plot after the termination. Compare it to the 500MHz plot from the previous message (warning: the vertical scales are not the same).
   

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OK, the 50Mhz plot only showed 2 "resonance points", so therefor my remark about 2.

So those extra peaks are reflections not resonance points, in accordance with the theory which describes that a coil could have only 1 resonance point, at the frequency where Xc and Xl are equal  O0
   

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Below 2 SWR scans (0 to 500Mhz) of my 2 pancake coils with an open toroidal coil inbetween.

First is without input circuit, so naked coils,
second is with input circuit (balun, caps).

Both show a flat first 50Mhz with a high SWR
   

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Now, the naked plot is very different considering that your coil has almost the same amount of turns as mine (11 vs. 10) wound with the same wire diameter (but mine is Litzed) and almost the same OD and ID (mine is 112mm and 91mm,  respectively).

There are three differences, though:
  • Number of coils
  • Interconnects
  • Winding scheme

I think the first point might be the elephant in the room.

* I am assuming that you are making these measurements with your DSA815 + the VB1020 1-port "VSWR Bridge".
   

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Yes,  i agree, must be mainly the 2 coils in bucking series mode.

Correct on the setup

   

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Below 2 FM sweeps (1Khz to 10Mhz) made with the i-probe close up to the front pancake, middle of turns.

First is without input circuit, so naked coils,
second is with input circuit (balun, caps).


Finally, i made a sweep (1Mhz to 20Mhz in 10s) using the PA and looking at the power meter response.
Seems we have a varying input power into the circuit from 0.5W to 8W over this 20Mhz range.

Be aware there are many variables here like the linearity of the PA across this range, the possiblity of the PA to regulate power back in case of mismatch, the accuracy of the used power meter etc.

 
Video here: https://www.youtube.com/watch?v=ZKsKVZ5d38Q

Itsu
   

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Below 2 FM sweeps (1Khz to 10Mhz) made with the i-probe close up to the front pancake, middle of turns.
Lets call it an "H-probe" when used to sense external magnetic fields generated by other coils.
I like the idea. Actually I was just thinking of suggesting it to you but with the SA doing the sweeping and sensing.

I didn't watch the video yet - I'm mobile now on slow connection.
   

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Questions about the new build coils i will be making with 0.93mm litz wire.

Of these 2 pancake coils, one will be CW, the other CCW so they can be placed facing each other and be in bucking mode.

This means both pancake coils will be very close to the toroidal coil, only separated by the varnish and litz wire cloth and to each other (~14mm).

Also i will use thin (3mm) plexiglass as a former, so the magnets will be that close to the pancake coils and thus close to each other (~21mm)

How does this effect the setup, is it good or bad?
If bad is there an optimum distance between the severall layers?
What will be the approximate resonance frequency of the NMR?
Anything else?

Itsu
   

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Lets call it an "H-probe" when used to sense external magnetic fields generated by other coils.
I like the idea. Actually I was just thinking of suggesting it to you but with the SA doing the sweeping and sensing.

I didn't watch the video yet - I'm mobile now o slow connection.

Ok, H-probe it is  O0

I can try the SA for doing the sweeping and sensing.

Nothing heard from Conrad om my repeated request for the water tube 3D file, so he must have forgotten to send it and now is "on the move".


Is there someone else out there who had requested and received the 3D file when Conrad offered it and is willing to send it to me?
You can PM me here or via Overunity.com,  thanks.

Itsu
« Last Edit: 2020-07-16, 16:26:00 by Itsu »
   

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Questions about the new build coils i will be making with 0.93mm litz wire.
Of these 2 pancake coils, one will be CW, the other CCW so they can be placed facing each other and be in bucking mode.
Don't wind them yet, because I am making an experiment with a different winding method to investigate if the reflections, which I illustrated yesterday, will be different and better. The difference in this second symmetrical winding method is which wires cross on top, ( see the attached diagrams for the difference ).

Also, when pancake coils are wound in this symmetric way, the monikers CW and CCW lose sense, because the direction of the circumferential current can be reversed anytime be reversing the connection to the coil.

Finally, even if the coil was not wound in the symmetrical manner (just as an Archimedes spiral), then it seems to me that winding BOTH of them the same way and then flipping one over to face the other, would produce a bucking field.

This means both pancake coils will be very close to the toroidal coil, only separated by the varnish and litz wire cloth and to each other (~14mm).
Also i will use thin (3mm) plexiglass as a former, so the magnets will be that close to the pancake coils and thus close to each other (~21mm)
Anyway, I am attaching a simulation of the aiding and bucking fields of coaxial parallel pancake coils.  The magnitudes of both magnetic and electric fields are included (opposing ones are on the right (b) ).
This should illustrate to you how far the electric field extends.  The purpose of the Litz cloth is to ground these electric fields so the capacitive coupling does not allow them to reach the toroidal coil.

P.S.
If you want the e-book where these sims come from, PM me an FTP info where I can upload it.


« Last Edit: 2020-07-14, 11:19:40 by verpies »
   

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Don't wind them yet, because I am making an experiment with a different winding method to investigate if the reflections, which I illustrated yesterday, will be different and better.
The difference in this second symmetrical winding method is which wires cross on top, ( see the attached diagrams for the difference ).

Ok, i will wait, i still do not have the new litz, so have to wait anyway.


Quote
Also, when pancake coils are wound in this symmetric way, the monikers CW and CCW lose sense, because the direction of the circumferential current can be reversed anytime be reversing the connection to the coil.

Ok makes sense  :)

Quote
Finally, even if the coil was not wound in the symmetrical manner (just as an Archimedes spiral), then it seems to me that winding BOTH of them the same way and then flipping one over to face the other, would produce a bucking field.

well, i agree, but Smudge adviced here:
https://www.overunityresearch.com/index.php?topic=3924.msg82765#msg82765
to use symmetry to overcome the capacitive coupling between pancakes and toroidal coil by connecting the pancakes either from their outside connections (series connect them by their inner wires) or vv.

So then i had to flip one over.


Quote
Anyway, I am attaching a simulation of the aiding and bucking fields of coaxial parallel pancake coils.  The magnitudes of both magnetic and electric fields are included (opposing ones are on the right (b) ).
This should illustrate to you how far the electric field extends.  The purpose of the Litz cloth is to ground these electric fields so the capacitive coupling does not allow them to reach the toroidal coil.

P.S.
If you want the e-book where these sims come from, PM me an FTP info where I can upload it.



I have no FTP at the moment (no more fixed IP), but will set one up soon.

Itsu

   

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Video here: https://www.youtube.com/watch?v=ZKsKVZ5d38Q
I watched the video.
Does your power meter measure the real power or the apparent power ?
i.e. does your power meter read zero when you connect only a high Q capacitor at its output ?
   

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Apparently apparent power (reactive power and true power) as with a 624nF capacitor at its output
and sweeping from 1 to 20Mhz, i see a similar output pattern as yesterday.

Itsu
   

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Apparently apparent power (reactive power and true power) as with a 624nF capacitor at its output
and sweeping from 1 to 20Mhz, i see a similar output pattern as yesterday.
So that explains it.
The readings shown in the video do not necessarily mean that the energy provided by the PA goes into the pancakes and does useful/useless* work there.  These readings can mean, that most of the energy provided by the PA is returned to it.
Do you have a recirculator so you can separate the forward power from the reflected power ?

* Joule heating, ultrasonics, radio wave emmission into the far-field.
   

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Lets call it an "H-probe" when used to sense external magnetic fields generated by other coils.
I like the idea. Actually I was just thinking of suggesting it to you but with the SA doing the sweeping and sensing.


H-probe measurement (0 to 50Mhz) on the front pancake middle of windings using the TG and SA

Itsu
   

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Measurments of my precision pancake coil:

OD: 112mm
ID: 91mm
Winding: 1 layer, 10 turns* of 1mm Litz wire (68 strand), 3.2m long, antiradial weave 1.


When the break is opened:
Intrawinding capacitance: 174.75pF (@100kHz)
ESR: 218Ω (@100kHz)

When the break is closed:
Inductance: 19.65µH (@100kHz)
DC resistance: 0.22Ω

Measurments of my 2nd precision pancake coil:

OD: 112mm
ID: 91mm
Winding: 1 layer, 10 turns* of 1mm Litz wire (68 strand), 3.2m long, antiradial weave 2.


When the break is opened:
Intrawinding capacitance: 173.85pF (@100kHz)
ESR: 195Ω (@100kHz)

When the break is closed:
Inductance: 19.72µH (@100kHz)
DC resistance: 0.22Ω
   

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So that explains it.
The readings shown in the video do not necessarily mean that the energy provided by the PA goes into the pancakes and does useful/useless* work there.  These readings can mean, that most of the energy provided by the PA is returned to it.
Do you have a recirculator so you can separate the forward power from the reflected power ?

* Joule heating, ultrasonics, radio wave emmission into the far-field.

Ok,   no, i do not have a recirculator, and it seems most are designed for high to very high frequencies.


By the way, i found this informative website:  https://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/Homebrew_RF_Circuit_Design_Ideas.htm


Measurments of my 2nd precision pancake coil:

OD: 112mm
ID: 91mm
Winding: 1 layer, 10 turns* of 1mm Litz wire (68 strand), 3.2m long, antiradial weave 2.


When the break is opened:
Intrawinding capacitance: 173.85pF (@100kHz)
ESR: 195Ω (@100kHz)

When the break is closed:
Inductance: 19.72µH (@100kHz)
DC resistance: 0.22Ω


So the Weave 2 design does not show any significant difference from the weave 1 design, at least looking at these parameters.

Itsu
   

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So the Weave 2 design does not show any significant difference from the weave 1 design, at least looking at these parameters.
Yes, but there is a small difference in reflection.  I'll post scopeshots later.

Also, take a look at this video for a guy making and using a directional coupler for quite a low frequency.
https://youtu.be/byF1FLdbUiA
https://youtu.be/iBK9ZIx9YaY

Your VSWR Bridge is a similar device, but it has 3 terminals and might burn out under high power.

   
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