Smudge proposed NMR experiment replication.

[Itsu]

some test runs of:  1)  naked pancake coils powerd by the PA.
                           2)  pancake coils powerd by the PA, through a symmetrizing 1:1 balun and the parallel cap (Smudge's style).
                           3)  pancake coils powerd by the PA, and the parallel cap (Smudge's style).

Position of the H-probe as shown in the above picture.

Be aware, PA can only handle from 100Khz, so sweep is from 100Khz to 50Mhz.

Is this what you expect?

[Verpies]

For maximum pickup i need to have the litz loop on the opposite side / curve of the pancakes, but it won't fit.

It is a silly problem and a serious problem at the same time.

BTW: Don't you get a better signal when the plane of the Litz loop is perpendicular to the planes of the pancake coils ?

[Itsu]

Yes, silly and serious, i agree, but with how the H-probe is now, not easily fixed.

The signal is the strongest with the loop as parallel to the pancake coil turns as possible, but due to the distance of the loop from the pancake coil a 45° angle is optimum (60ma) while at 90° (perpendicular and touching) it drops to 52mA.

Also the curves of the loop and the pancake coil turns need to be the same for best pickup.

Measuring the 4 outer turns of the both coils (front left,  front right, rear left, read right) shows great difference in signals with front coil left max.

Measuring inside the pancake coils is very limited and only possible the way i showed in the above picture.

I put the H-probe loop inbetween the both front and rear coils, but as they show different signals strengths (on the outsides) i do not know what the correct balanced position is (max i guess, but due to its
bucking positions it might be min. too).

Itsu

[Verpies]

some test runs of:  1)  naked pancake coils powerd by the PA.
                           2)  pancake coils powerd by the PA, through a symmetrizing 1:1 balun and the parallel cap (Smudge's style).
                           3)  pancake coils powerd by the PA, and the parallel cap (Smudge's style).

Are these the new coils or old ones ?

Be aware, PA can only handle from 100Khz, so sweep is from 100Khz to 50Mhz.

That's no problem.  Noting interesting happens below 400kHz anyway.

Is this what you expect?

For the naked coils without balun - yes.  In this case it can be seen that the amplitude of the H-fields decreases with frequency until 5.3MHz as it should with inductive reactance.  After that, the coil's SRF takes over and peaks at 8.7MHz.

In the sweep without the balun and with caps (Smudge style) the trend is reversed.  The H-filed's amplitude increases almost monotonically up to 8.7MHz as if it was determined by capacitive reactance, but it stays at low amplitude and before 8.7MHz it does not reach the -10dB level that the naked coils reached (I assume the H-probe has not moved and the amplitudes are comparable).
There is a little disturbance around 600kHz (I'll ignore for now).  The LC resonance is not visible at 4MHz but the coil's SRF is still prominent at 8.7MHz.  This is just wrong. The monotonic increase and low amplitude, too.

The sweep with the balun and with the caps clearly shows the LC resonance at 4MHz and coil's SRF at 8.7MHz at high amplitudes, which I expected, but the only difference from the previous case is the presence of the balun.  The balun should not make this difference !

I am missing the sweep with naked coils powered by the balun to understand why that balun made such a difference.

[Verpies]

The signal is the strongest with the loop as parallel to the pancake coil turns as possible

The H-field generated by the pancake coils (between them) should be perpendicular to the magnetic field from the magnets^*.  That's the whole idea of this opposing pancake coils arrangement !
This pancake H-field should also be perpendicular to the H-field that the toroidal coil would make, if it was powered.

I am very concerned if it is not so.

Also the curves of the loop and the pancake coil turns need to be the same for best pickup.

I do not understand that. Do you mean, their radii of curvature need to be the same ?

Measuring the 4 outer turns of the both coils (front left,  front right, rear left, read right) shows great difference in signals with front coil left max.

Oops. I do not understand again what front-rear means in this context, as well as left-right.  This is probably because my pancake coils are laying on a wooden table and yours are standing up.

Measuring inside the pancake coils is very limited and only possible the way i showed in the above picture.

So the only solution is to use a smaller sensor or spread them apart.

I put the H-probe loop inbetween the both front and rear coils

Did you mean to write "front and rear coil" ?  The plural suggests four coils in total.

but as they show different signals strengths (on the outsides) i do not know what the correct balanced position is (max i guess, but due to its bucking positions it might be min. too).

Take a look at Smudge's FEMM simulation.  The angles that the flux makes with respect to the planes of the pancake coils, are very important.
Watch out, as far as I remember, his sim was rotated counterintuitively and used current strips. Ask him for clarification if you cannot visualize the pancake wires on his sim.

* _{...and parallel to the planes of the pancake coils. Thus the H-probe's loop should encompass the most flux lines between pancake coils, when the plane of the probe's loop is perpendicular to their planes and to their radii.}

[Itsu]

Are these the new coils or old ones ?

No,  i am still waiting which coil (weave1 or weave2) is best,     or did you interpret this sentence of my:   Ok,  is one more symmetric (better) then the other? as "one" (as in weave1) is the better one with your   "Yes" answer?

For the naked coils without balun - yes.  In this case it can be seen that the amplitude of the H-fields decreases with frequency until 5.3MHz as it should with inductive reactance.  After that, the coil's SRF takes over and peaks at 8.7MHz.

In the sweep without the balun and with caps (Smudge style) the trend is reversed.  The H-filed's amplitude increases almost monotonically up to 8.7MHz as if it was determined by capacitive reactance, but it stays at low amplitude and before 8.7MHz it does not reach the -10dB level that the naked coils reached (I assume the H-probe has not moved and the amplitudes are comparable).
There is a little disturbance around 600kHz (I'll ignore for now).  The LC resonance is not visible at 4MHz but the coil's SRF is still prominent at 8.7MHz.  This is just wrong. The monotonic increase and low amplitude, too.

The sweep with the balun and with the caps clearly shows the LC resonance at 4MHz and coil's SRF at 8.7MHz at high amplitudes, which I expected, but the only difference from the previous case is the presence of the balun.  The balun should not make this difference !

I am missing the sweep with naked coils powered by the balun to understand why that balun made such a difference.

I just found out that for the "3)  pancake coils powerd by the PA, and the parallel cap (Smudge's style)." test i seem to have a loose soldering connection, so that might be the problem, will redo all tests....

The H-field generated by the pancake coils (between them) should be perpendicular to the magnetic field from the magnets.  That's the whole idea of this pancake coil arrangement.
This pancake H-field should also be perpendicular to the H-field that the toroidal coil would make, if it was powered.

I am very concerned if it is not so.

well, there is a detectable H field perpendicular to the pancake coils, but the loop picks up a stronger amplitude when parallel or semi parallel.

I do not understand that. Do you mean, their radii of curvature need to be the same ?

No,  see picture, the curve of the loop should match the curve of the pancake coils like thid (( and not like this  )(

Oops. I do not understand again what front-rear means in this context, as well as left-right.

see picture

So the only solution is to use a smaller sensor or spread them apart.

yes a smaller sensor would keep the pancakes at their present distance.

Did you mean to write "front and rear coil" ?  The plural suggests four coils in total.

yes

Take a look at Smudge's FEMM simulation.  The angles that the flux makes with respect to the planes of the pancake coils, are very important.
Watch out, as far as I remember, his sim was rotated counterintuitively and used current strips. Ask him for clarification if you cannot visualize the pancake wires on his sim.

This below sim picture?

[Verpies]

or did you interpret this sentence of my:   Ok,  is one more symmetric (better) then the other? as "one" (as in weave1) is the better one with your   "Yes" answer?

Yes

I just found out that for the "3)  pancake coils powerd by the PA, and the parallel cap (Smudge's style)." test i seem to have a loose soldering connection, so that might be the problem, will redo all tests....

So if you have the caps out, maybe you could sweep them individually with TG+SA to see what thair self-resonance frequencies are.

well, there is a detectable H field perpendicular to the pancake coils,

There is a perpendicular field between them ?!  I expected the flux between the pancake coils to be parallel to their planes. This also means, that for the best pickup, the plane of the H-probe's loop should be perpendicular^* to this flux.

No,  see picture, the curve of the loop should match the curve of the pancake coils like thid (( and not like this  )(

I understand now, but that is not supposed to pick up the flux between the pancake coils the best, because in such orientation, the plane of the H-probe's loop is parallel to the pancake's flux, so almost no flux enters the loop.

This below sim picture?

Yes

* _{...and perpendicular to the planes of the pancake coils.}

[Itsu]

There is a perpendicular field between them ?!  I expected the flux between the pancake coils to be parallel to their planes. This also means that the plane of the H-probe's loop, should be perpendicular to this flux for best pickup.

What i mean is that when the H-probe loop is held perpendicular to the pancake coil windings, there is a signal being picked up (H-field), but not as strong when the H-probe loop is parallel to these windings.

[Verpies]

What i mean is that when the H-probe loop is held perpendicular to the pancake coil windings,

Ooff, so that's OK.
The H-probe's loop, which is perpendicular to the pancake coil windings and their radii, is most sensitive to the magnetic flux which is parallel to the pancake coil windings and their radii.
In other words: The H-probe's loop encompasses the most flux lines between pancake coils, when the plane of the probe's loop is perpendicular to their planes AND to their radii. I have depicted this optimal position and orientation of the probing loop as a red line on Smudges's diagram attached below.

Any deviation from these optimal angles can be attributed to the fringing of the field when the probing loop is not placed precisely between the two pancake coils. The fringing also happens more when the pancakes are farther apart.

P.S.
I have also added little blue circles to Smudge's sim, to illustrate the crossections of the pancake's winding wires ...and crossections of cylindrical permanent magnets (without their flux)

[Itsu]

Yes

ok,  will be building weave1 coils then.

So if you have the caps out, maybe you could sweep them individually with TG+SA to see what thair self-resonance frequencies are.

Below 3 screenshots of sweeps (0 to 600Mhz) of the input caps:

1) 100pF trimmer cap sweep 0 to 600Mhz using TG and SA
2) 10nF cap sweep 0 to 600Mhz using TG and SA
3) 100pF plus 10nF Smudge style sweep 0 to 600Mhz using TG and SA

Itsu

[Verpies]

1) 100pF trimmer cap sweep 0 to 600Mhz using TG and SA

Is the dip approximately at 500kHz real or an artifact of the SA's display ?

[Itsu]

As far as i can see its real, as i saw it on severall frequency ranges.


Here a short video of the H-probe orientation:  https://www.youtube.com/watch?v=CJOMrmeu7cQ


Itsu

[Verpies]

As far as i can see its real, as i saw it on severall frequency ranges.

You know that a 100pF capacitor should exhibit a decreasing S21 attenuation as the frequency increases, which is an upward slope on your display.  See my S21 sweep of a 100pF cap. So what inductance is causing the S21 attenuation to increase (downward slope) from 0dB^* do -25dB between 0Hz and 500kHz ?
If this was happening at higher frequencies, I would attribute the increasing attenuation to the inductance of the interconnects and cap's leads, but at such low frequency it does not make sense. Especially the cap's low attenuation^* at 0Hz (9kHz).

I treat your TG as Port1 and SA's input as Port2 so the transmittance/attenuation from Port1 to Port2 is S21 in RF parlance (yes, it is backwards).

Here a short video of the H-probe orientation:  https://www.youtube.com/watch?v=CJOMrmeu7cQ

OK, that helps me a lot with your orientational terminology.

....but I saw no H-field probing between the coils , ...which is where the direction of the H-field matters most.
Perhaps this video will help:
https://youtu.be/2xy3Hm1_ZqI

* _{Is the cap's attenuation at 0Hz (9kHz) equal to 0dB or -10dB ?  I can't see it on such a wideband sweep. On my sweep, the attenuation of the 100pF cap is -63dB at 9kHz}

[Itsu]

Nice video, so there are commercial H-field probes.

Concerning the trimmer cap performance, i redid the sweep, but now from 0 to 1Mhz and the dip seems to be around 60Khz, so probably is indeed due to some SA artifact around the beginning of the frequency range.

See screenshot 1 and video   



For the H-probe measurement inbetween the coils, there is hardly any room to manoeuvre, see also the video.



Seen your 100pF cap measurement, so here one also from 0 to 50Mhz, see screenshot 2.
Guess my Rigol looses some accuracy when spanning large ranges.

video:  https://www.youtube.com/watch?v=2PQu_UjG92Q

Itsu

[Verpies]

Nice video, so there are commercial H-field probes.

Yes, but none that I've seen are made properly, i.e. with Litz center conductor and Litz cloth for the shield of the loop. Because if this, their amplitude vs. frequency relationship is not flat. At just several MHz, the skin effect adds to the resistance and the solid shield gets eddy currents induced in it, which opposes the external magnetic field.

Concerning the trimmer cap performance, i redid the sweep, but now from 0 to 1Mhz and the dip seems to be around 60Khz, so probably is indeed due to some SA artifact around the beginning of the frequency range.

I hope it is only an artifact.
Does another 100pF cap (a fixed one - not a trimmer) exhibit this artifact, too ?

[Itsu]

Does another 100pF cap (a fixed one - not a trimmer) exhibit this artifact, too ?

Not the 10nF cap also tested earlier, but will check a fixed 100pF cap.


I redid some PA tests:


1)  naked pancake coils powerd by the PA.
2)  pancake coils powerd by the PA, through a symmetrizing 1:1 balun and the parallel cap (Smudge's style).
3)  pancake coils powerd by the PA, and the parallel cap (Smudge's style).


See screenshots:

Itsu

[Verpies]

Seen your 100pF cap measurement, so here one also from 0 to 50Mhz, see screenshot 2.
Guess my Rigol looses some accuracy when spanning large ranges.

video:  https://www.youtube.com/watch?v=2PQu_UjG92Q

Ok, so now everything is clear.
The dip is an artifact of calibration with a test fixture which is inappropriate at these frequencies.

If you do a test fixture like the one I did (or a better one without adapters), then this kHz dip will go away.  See the attached photo and zoom into it, to see important details.

[Itsu]

Ok,  i will try tomorrow.

The fixed 100pF cap shows the same 58Khz dip, so will see tomorrow if it goes away.

Itsu

[Verpies]

I redid some PA tests:

I hope that the H-probe was between the pancake coils (as much as it can be) and did not move between the tests. If it moved - ignore most what I wrote below.

1)  naked pancake coils powerd by the PA.

The two peaks that you have marked are to be expected. One is due to SRF of the coils and the other one is due to their mutual inductance and is described in that e-book. I am attaching an excerpt from it below.

Note, that you are measuring the amplitude of the H-field generated by these pancake coils, and it is a measure that is unencumbered by any parasitic and imperfections of the coils and driving circuit.  In other words, this H-field is the field that the protons in the water will be subjected to.

Anyway, look at the amplitude of the H-field at the target frequency of 4MHz.  To me it looks like it is -13dB. Notice that this is the highest amplitude of all of these 3 tests !!!

2)  pancake coils powerd by the PA, through a symmetrizing 1:1 balun and the parallel cap (Smudge's style).

Here you added the balun and parallel caps and the appearance of peak 1 @4MHz reflects it (the other peaks hove shifted slightly by inconsequentially), however the amplitude at that peak is only -34.47dB, that is ~21dB lower than with the naked coils ! 21dB less power means less than 0.001% of the H-field amplitude you had with the naked coils !

3)  pancake coils powerd by the PA, and the parallel cap (Smudge's style).

Here you added the parallel caps only and the appearance of peak 1 @4MHz reflects it (the other peaks hove shifted slightly by inconsequentially),
Notice that this peak at 4MHz is much higher than you had in sweep #2 with the balun, but still lower than with the naked coils.
The amplitude at that peak is only -18.62dB, that is still ~5dB lower than with the naked coils ! 5dB less power means less than 50% of the H-field amplitude you had with the naked coils.

So as these test show now, you get the best H-field amplitude by driving the pancake coils directly without the caps and balun.  I wonder why... overvoltaged caps maybe?


BTW: It would be easier to do the calculations if you had the vertical units on the SA set to dBmV.

[Itsu]

Ok, so now everything is clear.
The dip is an artifact of calibration with a test fixture which is inappropriate at these frequencies.

If you do a test fixture like the one I did (or a better one without adapters), then this kHz dip will go away.  See the attached photo and zoom into it, to see important details.

I made a similar fixture like you used using SMA connectors, see picture, but the result is identical as before, see screenshot.

I have ordered some sma adapters, so will redo this test using other shorter coax to see if that makes some difference.

Itsu

[Itsu]

So as these test show now, you get the best H-field amplitude by driving the pancake coils directly without the caps and balun.  I wonder why... overvoltaged caps maybe?


BTW: It would be easier to do the calculations if you had the vertical units on the SA set to dBmV.

So if i do not use the PA, then we would eliminate the overvoltage on the caps as the cause.
Will try that tonight, both in dBm as dBmV.

[Verpies]

I made a similar fixture like you used using SMA connectors, see picture, but the result is identical as before, see screenshot.

Did you calibrate/normalize by shorting the new fixture ?

[Verpies]

So if i do not use the PA, then we would eliminate the overvoltage on the caps as the cause.

I do not know with certainty that the caps are overvoltaged.
Without the PA the H-field amplitude will be much lower, won't it ?

[Itsu]

Did you calibrate/normalize by shorting the new fixture ?

Yes, as usual

I do not know with certainty that the caps are overvoltaged.
Without the PA the H-field amplitude will be much lower, won't it ?

Yes, much lower, but hopefully still enough.

[Itsu]

Redone the H-probe test in 2 situations without PA in dBmV

1)  naked pancake coils powerd by the PA.
2) 
3)  pancake coils powerd by the PA, and the parallel cap (Smudge's style).

Itsu