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New tool - toroidal ferrite core inductor calculator

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by SP3L, Jun 14, 2021.

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  1. KN5L

    KN5L Ham Member QRZ Page

    Hi Dan,

    What makes S21 through accuracy a possibility is ability to calibrate out fixture reactance. Fixture calibration process referenced several times in previous referenced web pages:
    https://www.kn5l.net/S21adapter/FR43-material/calibration/

    Added 3.3k 1/4w resistor measurement to above link. Resistor element is capacitive. As stated above, radial leads are calibrated out during calibration process.

    John KN5L
     
  2. KN5L

    KN5L Ham Member QRZ Page

    Must also mention, during FT240-43 10T measurement. Balun was rotated away from board.
    Which is consistent with estimated capacitance of 63fF, or near zero capacitance. And SRF well above 30MHz. IF capacitance was an issue, SRF would be much lower.

    John KN5L
     
  3. KN5L

    KN5L Ham Member QRZ Page

  4. SV1IYF

    SV1IYF Ham Member QRZ Page

    This is a good sanity test. Measure the resonance frequency like in the photo and then again having the leads perpendicular to the ground plane. I have done something similar by adding some parallel capacitance of the order of 1 pF, measure the latter independently and finally solve for self C. I was working with 140 size cores so self C was higher and if memory serves the backing off of the resonant frequency was of the order of 20 MHz for an 8 turn mono wire winding.
     
  5. SP3L

    SP3L Ham Member QRZ Page

    Hi John,

    I found the screenshot from the SimSmith program on you webpage. I think I identified the R and X of the choke correctly. See below:
    upload_2021-7-12_10-28-20.png

    I compared your results with the calculator prediction (black solid line is R, black dashed line is X). The settings were: d/D=0.35 and winding=normal.
    upload_2021-7-12_10-29-13.png

    Our tool predicted self-resonance at about 15 MHz. That's where the dashed line crosses Y=0.
    Your measurements and calculations predicted it well above 30 MHz.

    I changed the capacitance Cp used in our tool to the lower value of Cpmod (modified Cp). When I set Cpmod=0.1 Cp the agreement was not bad but still not perfect:
    upload_2021-7-12_10-33-26.png

    When I set Cpmod=0.05 Cp the agreement was practically perfect:
    upload_2021-7-12_10-34-5.png

    Now, Cpmod=0.05 Cp = 0.056 pF

    I am afraid this is rather unrealistic value for a 10-turn choke on the 2.4" core of 43 material (Fair-Rite). Our calculator assumes Cp=1.11 pF (and even more for the higher d/D ratios) what seems to be more realistic.
    Is it possible that you overcompensated the test setup?

    73
    Jacek
     
  6. SV1IYF

    SV1IYF Ham Member QRZ Page

    From the picture Jacek it looks to me rather close to 300 deg winding, definitely not a 360 deg one i.e. 12 angular segments of which 10 are occupied by the winding.
     
    Last edited: Jul 12, 2021
  7. SP3L

    SP3L Ham Member QRZ Page

    IMHO, this should not reduce the Cp to such extraordinary low value. But let's wait for John's comment.
     
  8. SV1IYF

    SV1IYF Ham Member QRZ Page

    Additionally, I would look further into John's calibration process for the following two (conflicting) reasons.
    1) The resistor used looks like the carbon type the manufacturing technique of which etches a spiral on them to get the designated value. This works like a miniature coil in HF. Metal film resistors avoid this. 2) It has a larger diameter compared to the wire used for the short calibration, hence introducing some extra shunt capacitance. I would use a metal film resistor bended a bit away from the ground plane and it goes without saying that the short wire would had the same shape. In this manner we are in error of some additional L (over-compensated), which compared to the one to be measured is negligible like that of the straight wire used in the short calibration.
     
    Last edited: Jul 12, 2021
  9. KN5L

    KN5L Ham Member QRZ Page

    Hi Nikos, https://www.kn5l.net/S21adapter/FR43-material/calibration/ is updated with new measurement using #24, which is slightly smaller than resistor leads, calibration wire.

    Hi Jacek, https://www.kn5l.net/Balun-CM/FT240/ is updated with improved SimSmith Analysis Z model and measurement trace labels.

    FT240-43 10T Z measurement, was performed over several days with fresh calibrations to help mitigate measurement errors.

    My observation is only hobby price range VNA/Calibration devices, which has capability to perform these measurements at this level of accuracy is VNWA and calibration standards.

    I will gladly help anyone, with a VNWA and appropriate calibration standards, perform these measurements.

    John KN5L
     
  10. SV1IYF

    SV1IYF Ham Member QRZ Page

    Thank you John for considering my notes. When I wrote about larger diameter I was not referring specifically to the wire leads of the resistor. It is the tubular body of the latter that is, in bulk, responsible for the extra shunt capacitance. If you bare with me suggesting another alternative you could use another resistor modified for the short calibration. Get a resistor of as low a value as you can find and solder a very thin wire across and as close to the body as possible. This jumper should be positioned diametrically opposite the ground plane during the shorted calibration.

    73 Nikos
     
    Last edited: Jul 12, 2021

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