On Sat, 9 Feb 2019 12:12:53 -0500, "Carl"
"Lasse Langwadt Christensen" wrote in message
lørdag den 9. februar 2019 kl. 02.55.29 UTC+1 skrev George Herold:
On Friday, February 8, 2019 at 7:13:52 PM UTC-5, John Larkin wrote:
On Fri, 8 Feb 2019 06:05:42 -0800 (PST), George Herold
On Thursday, February 7, 2019 at 8:12:05 PM UTC-5, Jeff Liebermann
On Thu, 7 Feb 2019 06:33:34 -0800 (PST), George Herold
Hi all, A (mostly clueless) physics type wants to do
some RF stuff. (measuring nmr signals.. protons,
~5-25 MHz range) He's got something working, but
construction is ugly.
The uglier the antenna, construction, or layout, the better it
I was thinking of recommending an ARRL handbook, is
any version better than others?
I don't know. I have some old ARRL Handbook issues at home that I
rarely read or use.
5-25 MHz is practically DC as far as breadboarding is concerned.
Right... but of concern is not just bread boarding, but in nmr you
typically have a coil down the end of a probe/ transmission line and
there is some thought needed to get good coupling into the coil.
There's usually a tuning network and a low-noise (sometimes cryo, a
"chilly probe") preamp in a box just outside the main field. Some sort
of t/r switching, too, since there's only one sample coil for transmit
So we've done things a few different ways. (Our latest nmr is all
done by Norman, and I'm mostly clueless on the details.)
Tuning near the coil is what we do now. But the first one
had a coil on a fixed length of coax, (less than 1/4 wavelength)
and a tuned series inductor for the transmitter tuning.
(I'm not sure how the input was tuned, parallel C I assume.
Norman did the hard parts here too. :^) You've got to have
some Q on the input.
So my boss asked today if we could make a cheap kit/ gizmo
that would let people 'see' proton nmr, pulse a coil,
look at the ring down. Insert water sample into coil,
see ring down change.
So (to have fun) the first question is how big is the
sample/magnetic field? At ~2.5kHz you can see nmr in
a beer can size of water in the Earth's field ~0.6 Gauss
In a permanent magnet ~0.5 Tesla (5 k Gauss) you
can see a signal from .. well a few good sized
drops of water. at ~20 MHz.
(proton gyromagnetic ratio is ~42 MHz/ T
4.2 kHz/ Gauss.)
I've got some 8" Helmholtz coils that do
~100 Gauss at 3A. But lotsa turns and spendy.
Some permanent magnets and a yoke? The field
homogeneity of little permanent magnetics is
going to suck. Field homogeneity is either
less volume or less Q, T2 broadening in the
something like this? https://www.thingiverse.com/thing:942257
It looks cute but the pictures show a lot of slop in the magnet position so
I doubt it will achieve anywhere near that field map. Not that I can
eyeball sub-part per thousand differences from a map like that but I've
spent some time playing with arrays just like that and testing small
position and tilt errors. Also, given the size of the "bore" opening
compared to the length of the magnets pretty much only a sub, sub millimeter
thick slice across the center will be homogeneous. The distance from the
closest side of the homogenous region needs to be at least two (three is
better) bore diameters from the end of the magnet segments to limit the
effect of field falloff as you approach the ends. Again, percent is easy,
sub part per thousand gets hard. You could probably shorten the magnet
length with some form of end cap but then you lose the clear bore access..
After doing a lot of comparisons I decided that just using two discs and
having side access perpendicular to the magnetic field gave the same or
greater field strength and homogeneity over an appreciable volume for the
same cost and much easier construction. I'm not a magnet physicist, this is
worth what you are paying, YMMV, etc., etc.
. Other things I played
with: making the discs thicker doesn't get that much more field, using a
yoke of flat plates across the outer faces of the discs with one or two side
panels gave more field but added a lot of weight and didn't really change
the simulated homogeneity so it was a wash compared to just buying thicker
discs, and trying a few simple pole cap shapes helped the homogeneity but
lowered the field since the magnets were now further apart for the same
clear gap. I was bored, curious, and had a pc and FEMM so I explored
Maybe one could solder a bunch of unshielded drum core inductors to a
PC board and glue a couple of them to the front or back of your disk
magnets. Make an active shim system. That would allow George's system
to be a shimming trainer too.
a magnetic around in space an find one of the two sweet spots for it.