• swadesh
    1
    I am trying to do some free space calibration to extract material data from unknown arbitrary material. I am struggling with the calibration part as the s parameters i am getting is not good enough to get proper value. Can anybody please suggest the calibration procedure?
    I have E8361 keysight VNA. I have Type N short, open, load, through (50ohm) but no standard precise calkit. What should I do?
  • Yassen Mikhailov
    1
    Dear Swadesh,

    I would recommend that you perform a free space TRL calibration. The through is simply the distance between the two ports (antennas) without any sample. Reflect is a reflective plate in the sample holder. Line is the additional distance between the two ports depending on the measurement frequency - take the quarter wavelength of the mid-band frequency and move the second antenna by the corresponding length - example X-band - 9 GHz - 8 mm = line is through length + 8 mm.

    Make sure you specify a sexless connector with a characteristic impedance of 377 Ohms.

    Hope this helps.


    Yassen
  • David
    1
    @swadesh

    The classical method used in free-space/microwave materials characterization testing is the "Gated Reflect Line" (GRL) method. There are other methods as-well, some new and some old.

    The Keysight Web page has a lot of documentation about this subject, see below. Take your time and dig around on the Keysight page links I provide. You will see documentation on interpreting the sweeps and how to extract the S-parms from the traces. Also I include a couple of non-Keysight links below. One link has a free to download .pdf. The other looks like an interesting paper, but it is locked-up behind the infamous IEEE paywall along with a lot of other (mostly) taxpayer and non-IEEE funded papers.

    You should be able to put together some sort of test jig with what you have depending on the material type and characteristics. But you provide NO information on what you are trying to characterize and NO information on what options your VNA/PNA has. You will need some obvious things like quality cables and connectors/adapters, torque wrenches, etc., and some not so obvious stuff like standard horns or launchers, waveguide or launcher to coax terminations, standard materials for differential comparison, etc... I recommend you connect your VNA to a computer, it will help to setup your test jig and interpret the results. The proper way to do this is to buy the correct Materials Measurement Software Suite for your instrument from Keysight! Along with it comes support.

    I think that's about all I can contribute here. Good luck, David in Florida...

    --------------------

    * Keysight Technologies N1500A Materials Measurement Suite

    N150xA and 8507xE Series: Customer Support Home Page:

    http://na.support.keysight.com/materials/

    https://www.keysight.com/en/pd-2491705-pn-N1500A/materials-measurement-suite

    https://literature.cdn.keysight.com/litweb/pdf/5992-0263EN.pdf

    http://na.support.keysight.com/materials/help/85071webhelp/85071Freespace_Measurement_Example.htm

    http://na.support.keysight.com/materials/help/85071webhelp/85071_Opt100_GRL_Calibration.htm

    http://na.support.keysight.com/materials/help/85071webhelp/85071_Opt100_GRL_Measurements_Considerations.htm

    - Powerful free space calibration and gating techniques [GRL or "Gated Reflect Line" Method]

    Gated isolation/ response calibration reduces errors from diffraction effects at the sample
    edges, and multiple residual reflections between the antennas. Gated reflect line (GRL)
    calibration makes accurately calibrating in free space fast and easy. A software wizard
    automatically sets up all the free space calibration definitions and network analyzer
    parameters, saving engineering time. It reduces costs associated with TRM and TRL
    calibration methods by eliminating the need for micro antennae positioners or direct receiver
    access.

    There are now two ways to perform GRL calibration. The original 2 tier GRL calibration
    technique converts a previously saved full 2-port coaxial or waveguide calibration into a full
    2-port free space calibration by measuring two additional standards, the empty free space
    fixture and a metal plate. With 1 tier GRL cal, a waveguide or coaxial cal is not required
    before the metal plate and empty fixture measurements. Because 1 tier GRL cal requires a
    longer alias free time domain span, it is best suited for set ups that don’t have long cables,
    when the frequency span is not extremely wide, or a large number of points is not desired.
    GRL calibration requires a network analyzer with full S-parameter (S11, S21, S12, S22)
    capability and time domain option. An appropriate free space fixture with metal calibration
    plate is also needed.

    * Free-Space Materials Characterization by Reflection and Transmission Measurements using Frequency-by-Frequency and Multi-Frequency Algorithms
    by Fábio Júlio F. Gonçalves 1,2 [OrcID] , Alfred G. M. Pinto 2, Renato C. Mesquita 1 [OrcID] , Elson J. Silva 1 and Adriana Brancaccio 3,* [OrcID]
    1 Department of Electrical Engineering, Federal University of Minas Gerais, Belo Horizonte 31270-901, Brazil
    2 Pitágoras Faculty, Belo Horizonte 31210-800, Brazil
    3 Department of Engineering, University of Campania “Luigi Vanvitelli”, 81031 Aversa, Italy
    * Author to whom correspondence should be addressed.
    Received: 12 September 2018 / Accepted: 12 October 2018 / Published: 18 October 2018

    https://www.mdpi.com/2079-9292/7/10/260/htm

    * A new free-space calibration technique for materials measurement

    Publisher: IEEE

    Author(s): Philip G. Bartley ; Shelley B. Begley

    https://ieeexplore.ieee.org/document/6229351

    Abstract:
    A new method for performing a full two-port s-parameter calibration in free-space is presented. The proposed calibration technique computes the error coefficients from measurements made on an empty fixture and a measurement made on a metal plate of known thickness. Time-domain gating was employed. This technique requires fewer and simpler standards than the existing thru-reflect-line (TRL) and thru-reflect-match (TRM) calibration techniques. It requires the same calibration standards as the gated-reflect-line (GRL) method but doesn't require performing a coaxial or waveguide calibration at the end of the cables connected to the antennas. Permittivity calculated from measurements, calibrated using this technique, made on a material sample appear to be superior to results published using the TRL and TRM calibration technique and the same as those for a GRL calibration.

    Published in: 2012 IEEE International Instrumentation and Measurement Technology Conference Proceedings

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