I used a single loop and the special prepared coax loop (see above video) for some measurements, but it seems that there is hardly any difference in signal pickup between those 2 loops (both 10Vpp at resonance).
The lack of difference means that the capacitive coupling does not play a great role IN THAT FIELD ORIENTATION.
I must confess that i had to make 3 of those coax (RG58) loops as the first 2 seem to work very erratically (signal / no signal, depending how to hold the loop), so it can be that there is something wrong, but as far as i can see (and measure) this last one should be OK.
Yes, the erratic performance could be due to the construction of the loop (the aluminum shield does not solder well and is prone to bad contacts and shorts), however there is another explanation which should be investigated further.
Namely, does the axial* movement of the sensing loop cause the received signal to go through zero amplitude?
On the video, I saw you making only the radial move with the sensing loop. I love that glass cube, btw !
It is important that the sensing loop is very planar (flat) - its shadow should be a straight line. The planes of the 3 coils must be parallel all the time. A 2x thinner wire would allow for more selective axial positioning, too (e.g. the same wire that the toroidal coil is made with).
*
"axial" as in moving along the axis of the pancake coils and the sensing loop, ...in other words: initially close to one pancake, next exactly between pancakes and finally close to the other pancake. 2 problems noted:
# self resonance frequency very (to) low 3.3Mhz for NMR
Yes, and that means that achieving a high amplitude H1 field at a higher frequency will require a lot of driving power. Paradoxically, the high circulating current I
C at resonance is accompanied by low current draw from the generator I
G. This is caused by high "Current Magnification" or the ratio I
C / I
G also known as Q. Remember, that it is the high I
C which is responsible for generating the high amplitude of the desired H1 field - not the I
G. This helpful effect does not occur as much outside of resonance.
Also, above the self-resonance frequency, more current flows in the intrawinding capacitance (inter-turn capacitance or "Cit" in Partzman's parlance) than in the turns of the coil.
Furthermore, the interwinding current increases through the interwinding capacitance ("Ciw" in Partzman's parlance).
Currents flowing through the Cit and Ciw generate magnetic fields H
2 and H
3, respectively, which are oriented in a different direction than the desired field H
1. Smudge please comment on this.
# if coax loop is OK, then very much crosstalk is present
It was the entire idea of this experiment to demonstrate this.
This means the the toroidal coil should be wound with a special scheme (analogous to weave1 or weave2), that will cancel its circumferential induction.
P.S.
The Cit for one of my coils is 175pF (measured at 100kHz with an LCR meter).
The Ciw for a pair of my coils is 19.7pF at 2mm, 8.4pF at 9mm, 5.8pF @ 16mm, 4.3pF at 23mm (all measured at 100kHz when coaxial and parallel)
My detailed coil dimensions and other data are here.
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Topic: Smudge proposed NMR experiment replication. (Read 127081 times)
