I am looking for a design for an absorption cell in the range 10 MHz to
10 GHz. My intention is to measure the absorption of a crystal powder.
The cell should have coaxial connections, no waveguides. The two-port cell will be put into an oscillationf field, RF is injected into port 1 and a power meter on port 2 measures the transmitted power. The magnetic field will be modulated to make detection easier.
My first idea is to use a copper tube with 10 mm ID two semirigid coax cables soldered into one end and the other with a plug.
Not necessarily "better", but how about dimensioning your copper tube and placing the connectors so as to make a re-entrant resonant cavity? Fabricate it so you can put a RF-transparent container of the powder under test inside, then remove it. Compare the transmission loss with and without the powder (a "tare" measurement with the powder container present, but empty, would be helpful).
Yes, indeed. What do you intend to measure ? You can measure nuclear magnetic resonance and you can measure electron spin resonance. Both being standard approaches of physical chemistry. The two differ by 3 orders of magnitude in the same field. Forget to measure 10MHz to 10GHz in the same resonator. What quantity of powder do you want to measure at what field ? What is the linewidth and what will be the absorption? Meaning what is the concentration of measureable particles ?
Whatever it is you put between the connectors WILL BE a waveguide and WILL HAVE many orders of resonant MODES.
The behavior of the powder will be wildly different depending upon its spatial location.
Your first step is to understand what things like TM(1,3) mean.
The problem is enormously more complex than you think it is.
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Many thanks,
Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
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It is an ESR like measurement. My Frequency of interest is between 4 and 5 GHz, fields will be up to
0.5 T, but I dont know the exact frequency yet. I know about ESR setups, but I only found designs with waveguides. The cell has to be quite small (1cm). This is for someone who wants to measure his sample in my microwave setup, so it should be a simple design, just for a fast try.
I know the cell act as a cavity resonator, but well below its cutoff frequency. The cell has to be small since it has to fit into a 11 mm bore cryostate. Anyway a resonance would not be a problem thince the absorption line will be modulated by an external field (lock-in), and the absolute value is not important. I am looking for a proven simple design, nothing for high precision. I know a lot of these experiments are done with simple cells as the one I described, I just don't want to design my own. Another idea would be to use a rigid coax, drill a hole into the shield, extract the dielectric and put the sample inside.
Usually such measurements have incident power, reflected power and transmitted power. Your goal is to have zero reflected power, meaning a perfect energy transfer into the cavity. You can't have a 10MHz to 10GHz cavity even for a coaxial one. On the cavity you'll find multi resonant modes (ie say at 5Ghz,
5.5Ghz, etc) for every resonant mode the amount of transmitted energy will be different. Step one is to calibrate your cavity and see what you've got on the transmitted side without any powder. A Vector Network Analyzer (but also a good detection diode and an analogic microvoltmeter ) could help you alot.
The ESR measurements are all done in resonant cavities with a Q in the several 1000 for CW mode. This also with lock-in, modulated fields and such. What makes you believe to get sufficient signal without a resonant cavity ?
The sample is a molecule which contains some magnetic atoms. This should/could be a ferromagnet, and the resonance of the ferromagnet would be visible, that's what I'll be looking at.
Well, if you have some idea where to look for resonance, that'll narrow down the design options considerably. Frinst at first you specified the range from 10 MHz to 10 GHz which seems a bit unneccesarily broad.
Now, can you give us a few more details that might help us with your design, like exactly what sort of resonance is this other person looking for? What is the material? Does your existing equipment provide for a static magnetic field, or just high-power RF, and in what range(s)? FTM what's the range of RF available? I'm just saying that the desired resonance may not _be_ measurable with your equipment, or it may be mistaken for something else, or some other effect may be mistaken for _it_.
If all the other person wants to do is go/no-go verify ferromagnetism in the material there're much simpler ways to do it.
Interesting. Before invesing considerable amounts into specialty gear I'd suggest to visit the next ESR/NMR Lab and do some measurements there. I'm aware there are not too many, but still. If you tell them something about interesting materials, and possible publications, they might do a measurement for you.
What is the material? a strange molecule/cluster Does your existing equipment
the or just high-power RF
both, a strong static field up to 0.5 T and an RF Field from 10 MHz to
20 GHz. I will modulate the static field and look with a lock-in at the RF power. The sample space is cylindrical with 11 mm diameter and 50 mm length, the experiment will be done at 2 K.
My real area of work is the influence of microwave radiation on magnetic nanostructures, i.e. small rings and wires with
200 nm width. These measurements are very broadband, from 10 MHz to 20 GHz. Unfortunately it is impossible fabricate these structures with a controlled impedance over this range. (the geometric shape is fixed by the physical properties needed, there need to be other contacts to the sample which partially shorten the RF...) I have a very bad VSWR, which I need to compensate, because this signal is stronger than the effect measured... So you are right, resolving the desired signal will be difficult.
Yes, I know. This thing is a molecule whose owner wants to measure some energy levels and transitions between them.
Oh, Konstanz. They'd have an ESR lab there. So if you're having a resonator with your material in it, at least they'd have the sweepers and magnets. They'd also have the helium equipment. And whatever they don't have, we have it in Zuerich.
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