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microwave absorption cell

Discussion in 'Electronic Design' started by [email protected], Jan 25, 2007.

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  1. Guest


    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.

    Any better ideas?

  2. Guest

    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).

    Mark L. Fergerson
  3. 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
    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 ?

  4. 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.

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  5. Guest

    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.

    Thank you

  6. Guest

    Hi Don,

    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.

    Thanks for your input
  7. Guest

    Hello Mark,

    that's a good idea,
    I fould a similar design for a reentrant cavity in Rev. Sci. Instrum.
    67 (12), December 1996, p 4294, this
    seems to be a popular method.

  8. vasile

    vasile Guest

    Hi Daniel,

    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
    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 magnetic field will be modulated to make
  9. 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 ?

  10. Guest

    Hello Rene,

    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.

  11. Guest

    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.

    Mark L. Fergerson
  12. 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.

    Where are you located ?

  13. Guest

    Hi Mark,

    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
    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
    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
    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

    Thank you

  14. Guest

    Hello Rene,

    I am located in Konstanz, quite near to Switzerland.
    of couse that is the best option, though not as technically
    I already got some interesting ideas by not going over to the NMR lab.

  15. 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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