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Author Topic: Smudge proposed NMR experiment replication.  (Read 44157 times)

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with 1mm thick wire on the toroidal coil around it and some varnish it measures:
I.D. = 85mm
O.D. = 114mm
So with these diameters you will have 35% more wires on the inner circumference after bending the hose.  With 1 layer on the outer circumference, the depth of these bunched up wires on the inner one would be 0.2mm (when using a 0.1mm diameter wire).

Practically, I would make the bent hose's O.D. 118mm and I.D. 81mm, in order to give the shield a 2mm air gap from the toroidal coil and allow space for the overlap of the unconnected edges of the shield and for wire crossings of the toroidal coil, if you rewind it using the special winding technique, which cancels the circumferential current (...and the coil's associated sensitivity in the undesired direction).
   

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Thanks, it all seems so simple when you write it down that way  O0

The problem with the special winding technique for the water tube is that the tube with its toroidal coil is solidly covered with varnish / glue, so probably hard to unwind without damage.

Perhaps i should put up my smily face and ask Conrad for another one  C.C


Itsu
   

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Perhaps i should put up my smily face and ask Conrad for another one  C.C
Get two but make the water inlets much smaller.  I'll take one off you hands.
I can replicate your setup because I have these 140mm magnets and 112/90mm pancake coils.
   

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OK,  PM send,  not sure the smaller inlets would be possible easily, but Conrad will tell.

   
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Perhaps i should put up my smily face and ask Conrad for another one  C.C

Itsu

In principle it is no problem to send a new tube, but I am in the middle of a move from one apartment to another and my 3D printer will be one of the last items to be transported. It can well be up to a month till I am ready to 3D-print again.

There are so many other things to do till everything is set up in the new apartment. And the plague does not help at all when trying to get stuff I need to make the new apartment ready. The home improvement stores and garden centers are full of people which makes me hesitant to go shopping for the many little things I need to set up the apartment. At the moment I try to get plants and big pots to make my balconies green.

The move from the old apartment to the new was delayed for four months because of the Coronavirus. The craftsmen had many excuses for not keeping promises.

Greetings, Conrad
   

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Conrad,

no problem, your move is far more of importance now.

There must be more 3D printers around which can be used to print something similar.

We see/hear you again when settled in your new apartment.

Regards Itsu
   

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I put everything together (pancakes, toroidal coil, magnets, double distilled water in the tube, 5W power amplifier etc.) for seeing if there are some unforeseen problems like leakage etc.

No leakage has been seen, so thats OK, and the used power amplifier (100Khz to 60Mhz @ 5W) also behaves OK when driven into a 50 Ohm dummy load on the 4Mhz test frequency with a 1:1 SWR.

But when hooking it up to the input circuit via the 1:1 balun i get not enough power to effectively use the SWR meter, so it seems to me i have a very poor SWR no matter how to tune the 100pF input trimmer cap and the PA regulates back.

I will have to do more tests there to be able to put more power into the system or at least have a good SWR so the PA won't regulate back or get damaged.

Video here:  https://www.youtube.com/watch?v=57KryMiFsG8

Itsu

   

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It is not surprising, that the large cap in parallel with the signal source reflects so much energy back and causes a high SWR. Simulate it in PSpice and see...

The goal is to drive a high current through the pancake coils.

In cases when the frequency is low or the coil has a low inductance (or both), it is straightforward to drive high AC current through this coil using a low-impedance high-current amplifier, such as the EL2009.
In such cases, the coil’s impedance is low enough whereby it can be driven by an amplifier directly.

However, the impedance of a coil increases with frequency as |Z|=√(R2 + (2πfL)2), so at high-frequency, the coil's impedance is assumed to be very high.  See the impedance of my coil here as it reaches over 7kΩ.
Thus, a high-voltage driver is needed to drive high current into this coil.  These capacitors try to do that by swinging the front-end of your RF amplifier to high voltages. That's why you are measuring almost 200V at these coils' terminals (the current would be more interesting though, because it is the current which generates the magnetic field - the goal).

Anyway, what SWR do you get when the secondary of that 1:1 balun is loaded only with a 50Ω dummy load ?
   

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Below is more info on pri/sec electric shielding in transformers from Linear Circuit Design Handbook. This is about LF transformers, but it is also applicable to HF, except the shield's anatomy changes.

   

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Thanks verpies,

Your coil impedance around 4Mhz looks to be about 4800 Ohm, so indeed high compared to the low 50 Ohm of the PA.

According to the author Smudge, this is to be expected as he writes in his PDF (first one in post #1 of this thread) on page 11:
 
Quote
Figure 9 shows the basic circuit diagram where the input is assumed to be a low impedance signal generator that needs to be matched to the high impedance of the input resonant circuit.

This is simply achieved with the capacitor network shown.

The drive coils will have RF voltage across them that is several hundred volts.


But the PA needs to be matched correctly and the present capacitor network i have (10nF / 100pF) is not doing that.
Might be a good idea to use a simulator to find out what works better.

I will check out today what SWR i have behind the 1:1 balun when loaded with a 50 Ohm dummy.

Thanks for the info on the screening, i will try to download that handbook.

Itsu
   
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Have you all considered driving the pancake coils in series self-resonance.  This will give you the maximum available current if the PA can supply it.  The input matching would then be done with an additional series inductor of say ~2uH at 4MHz.

Just a thot.

regards,
Pm
   

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Have you all considered driving the pancake coils in series self-resonance. 
Yes, of course but I am waiting for Smudge to alter his design if this does not work.
   

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verpies,

Quote
Anyway, what SWR do you get when the secondary of that 1:1 balun is loaded only with a 50Ω dummy load ?

I measured the SWR when the secondary of that 1:1 balun is loaded only with a 50Ω dummy load to be 1:1.2, so a good match.



Partzman,

Quote
Have you all considered driving the pancake coils in series self-resonance.  This will give you the maximum available current if the PA can supply it.  The input matching would then be done with an additional series inductor of say ~2uH at 4MHz.


thanks for the input, not sure where your extra inductance has to go, could you specify?


Itsu
   

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I measured the SWR when the secondary of that 1:1 balun is loaded only with a 50Ω dummy load to be 1:1.2, so a good match.
So we cannot blame the balun for this.

On another subject - In a project like this it would be nice to have a high frequency magnetic field sensor.
Can you try if a miniature loop of wire* inserted into your current probe works as such magnetic sensor ? 

* For best results use that 2mm Litz wire to make that loop (the solder junction should be as short as possible).
   
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verpies,

I measured the SWR when the secondary of that 1:1 balun is loaded only with a 50Ω dummy load to be 1:1.2, so a good match.



Partzman,


thanks for the input, not sure where your extra inductance has to go, could you specify?


Itsu

Itsu,

Ignoring the present requirement for an isolating balun, the inductor would simply connect between the PA and the series pancake coils and a coupling cap would be used to remove any DC offset.  The basic difference is now the tap between the pancake coils is moving symmetrically above an below ground instead of remaining at ~0 volts with the balanced input. 

If the balun is still required to eliminate this single ended ac voltage, then the inductor would connect in one of the two balanced lines or could be split into two inductors, one in each input line.

The small inductor would produce a small phase shift and perhaps change the measured SRF of the pancake coils but this could be trimmed out.

Regards,
Pm
   

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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.


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

The linear plots of impedance magnitude |Z| vs. frequency, are attached below:

* because for cancelling the radial current it must be an even number

« Last Edit: 2020-07-28, 02:34:43 by verpies »
   

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Notice that this 45MHz peak, which appears when my coil is opened, is very close to a reflection from an unterminated transmission line, which has the length approximately equal to the length of the wire in the coil.
I wonder what the characteristic impedance of that coil is, ...when it is treated as a transmission line.

P.S.
From the lumped LC formula, the resonance frequency calculated with the capacitance and inductance measured @100kHz by the LCR meter, comes out to 2.7MHz.
   

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Itsu,

Ignoring the present requirement for an isolating balun, the inductor would simply connect between the PA and the series pancake coils and a coupling cap would be used to remove any DC offset.  The basic difference is now the tap between the pancake coils is moving symmetrically above an below ground instead of remaining at ~0 volts with the balanced input. 

If the balun is still required to eliminate this single ended ac voltage, then the inductor would connect in one of the two balanced lines or could be split into two inductors, one in each input line.

The small inductor would produce a small phase shift and perhaps change the measured SRF of the pancake coils but this could be trimmed out.

Regards,
Pm



PM,

thanks for the specification, the balun used is not an isolating one, its just there to balans (symmetrize) the input to the both pancake coils.

Itsu
   

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verpies,

Quote
On another subject - In a project like this it would be nice to have a high frequency magnetic field sensor.
Can you try if a miniature loop of wire* inserted into your current probe works as such magnetic sensor ?

* For best results use that 2mm Litz wire to make that loop (the solder junction should be as short as possible).


I can try that, i have 0.94mm litz wire coming next week, so i guess it would be better to use that.




Quote
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.


Wow, that is a beauty, nicely done  O0

I was wondering how to start such a coil, like from in the middle, but just using a split and a bridge lateron is an easy solution.

So the plots show that the impedance around 5Mhz is still high (3198 Ohm) so the need of impedance matching is still there.

Itsu
   

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I can try that, i have 0.94mm litz wire coming next week, so i guess it would be better to use that.
Why wait?
This is only a test whether it will be sensitive enough.


I was wondering how to start such a coil, like from in the middle, but just using a split and a bridge lateron is an easy solution.
I did the bridge in the middle because I wanted to access the middle, so I could investigate this coil as a transmission line, measure its self-capacitance, etc.
You can start from the middle if you plan to always have a short there.

So the plots show that the impedance around 5Mhz is still high (3198 Ohm) so the need of impedance matching is still there.
Yes, the coil's impedance maginitude |Z| crosses 50Ω at 405kHz and 16.75MHz, but the former one is due to the pure inductive reactance and the latter one is due to pure capacitive reactance... and the SWR never goes down below 1000.
   

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Quote
Why wait?
This is only a test whether it will be sensitive enough.

OK,  will try something today.

Quote
I did the bridge in the middle because I wanted to access the middle, so I could investigate this coil as a transmission line, measure its self-capacitance, etc.
You can start from the middle if you plan to always have a short there.

OK, guess i will do that then as i now have a ballpark length (3.2m) to go allthough i think i will aim for some more coverage of the tube by adding 2 or 4 more turns (12 or 14 turns).

Quote
Yes, the coil's impedance maginitude |Z| crosses 50Ω at 405kHz and 16.75MHz, but the former one is due to the pure inductive reactance and the latter one is due to pure capacitive reactance... and the SWR never goes down below 1000.

I don't see that (crosses 50Ω at 405kHz and 16.75MHz) in the plot, but i have no experience with this kind of equipment at all.

Seems very nice / handy to have though a VNA, would a HP 8753D or so be any good?

Itsu
   

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I don't see that (crosses 50Ω at 405kHz and 16.75MHz) in the plot, but i have no experience with this kind of equipment at all.
That's because the vertical scale of this plot is not zoomed-in, but you can extrapolate where the 50Ω horizontal line would be on these plots ...it's in the dips.

If I plot the inverse of the impedance's magnitude Y=1/|Z| (a.k.a. magnitude of Admittance) then it becomes more visible because Admittance is a more natural plot for shorted inductors.
The blue markers, in the plot attached below, show where the Admittance crosses the 20 milliSiemens (mS) line, which is equivalent to 50Ω.

Seems very nice / handy to have though a VNA, would a HP 8753D or so be any good?
That's expensive. At the frequencies we are experimenting with, this is a good & inexpensive choice.
BTW: He makes more experiments with this VNA in his subsequent videos.

Also, remember that your SA can make these |Z| vs. f  plots (...and |Y| vs. f plots) with the "impedance bridge" already. 
It cannot make the Smith chart, though, because it does not capture the phase information and thus the imaginary component of the impedance (reactance) is not distinguished.
   

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But the PA needs to be matched correctly and the present capacitor network i have (10nF / 100pF) is not doing that.
Low SWR is a general indicator of efficient energy transfer from the source to the load* (meaning: "low energy reflection coefficient"), but it is not an indicator of the maximum current flow in the coil.
Purely reactive components are perfect energy reflectors (by definition they have the reflection coefficient equal = 1 , the highest possible). They absorb the energy from the source and in the next quarter-cycle, they give it all back to the source, ...so the net energy transfer is zero over the entire cycle, but these circulating currents can be high, nonetheless.

Might be a good idea to use a simulator to find out what works better.
Watch a guy using a sim to match a loop antenna using Smudge's capacitor network.
https://youtu.be/weNwZ0D3txg

* The pancake coils are not the load. If you put a solid copper sheet next to them, then the induced eddy currents will heat this sheet and present a load to the source. ...the precessing protons will, too.
   

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Quote
On another subject - In a project like this it would be nice to have a high frequency magnetic field sensor.
Can you try if a miniature loop of wire* inserted into your current probe works as such magnetic sensor ?

* For best results use that 2mm Litz wire to make that loop (the solder junction should be as short as possible).

I made the below shown mini loop of litz wire and measured the magnetic field of the shown coil when fed by the FG with a 3.9Mhz AC sine wave.

Putting the probe close with the loop in the same direction as the coils windings, it picks up some signal, see screenshot.
Without the loop there is almost no signal.

Is this what you mean?

Itsu
   

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That's because the vertical scale of this plot is not zoomed-in, but you can extrapolate where the 50Ω horizontal line would be on these plots ...it's in the dips.

If I plot the inverse of the impedance's magnitude Y=1/|Z| (a.k.a. magnitude of Admittance) then it becomes more visible because Admittance is a more natural plot for shorted inductors.
The blue markers, in the plot attached below, show where the Admittance crosses the 20 milliSiemens (mS) line, which is equivalent to 50Ω.
That's expensive. At the frequencies we are experimenting with, this is a good & inexpensive choice.
BTW: He makes more experiments with this VNA in his subsequent videos.

Also, remember that your SA can make these |Z| vs. f  plots (...and |Y| vs. f plots) with the "impedance bridge" already. 
It cannot make the Smith chart, though, because it does not capture the phase information and thus the imaginary component of the impedance (reactance) is not distinguished.


Thanks for the additional info, seems it needs some experience to interpret the results.

That nanoVNA seems interesting, one can start to get the hang of it against very limited costs.
Seems there is already a V2 up to 3Ghz.
Those smith charts look very nice to have onboard.

I will seriously consider to buy one.

Itsu
   
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