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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. SP3L

    SP3L Ham Member QRZ Page

    Version 2.0 released!
    We would like to invite you to download the enhanced version of our calculator.

    After analyzing a number of new measurement results (a huge thanks to Nikos, SV1IYF), we improved the accuracy of the calculator. We introduced two new parameters you are supposed to set when designing the inductor:
    • Ratio d/D
    • Winding

    Ratio d/D is the ratio of the conductor diameter to the jacket diameter of the insulated wire used for winding the inductor. In case of a coaxial cable, this is the outer conductor (shield) diameter to the jacket diameter.

    There are two settings possible for the Winding parameter: “normal” and “cross”. The cross winding is shown below.

    Both new parameters impact the inductor self-capacitance - Cp. The winding set to “normal” will result in a higher Cp than the “cross” setting. The higher d/D setting will create higher Cp than the lower d/D setting. Higher Cp shifts the inductor self-resonance frequency to a lower value and reduces inductor impedance at higher frequencies. More often than not, you will want to keep the Cp as low as possible. But sometimes you may want to increase the Cp to align the inductor self-resonance frequency with the antenna operating frequency. At self-resonance, the inductor has the highest |Z| and R.

    Some guidance on the d/D ratios:
    • 0.35 very thin insulated wires
    • 0.5 ~ 0.65 typical insulated wires
    • 0.7 ~ 0.85 coax cables
    • 0.9 enameled wires

    And here is a list of some popular coaxial cables used in the RF chokes/baluns and their d/D ratios (rounded to 0.05):
    RG58: 0.7 (
    RG174: 0.7 (
    RG174: 0.8 (
    RG316: 0.8 (
    RG400: 0.8 (
    RG179: 0.8 (
    RG302: 0.85 (

    You can reproduce the results of the earlier calculator versions (1.0 through 1.3) if you set the Ratio d/D to 0.8 and the Winding to “normal” in version 2.0.

    If you looked closely at the first picture in this post, you certainly noticed the so called “spin buttons” next to the Number of turns setting. Except for using the drop down menu when selecting the number of turns (as in the previous versions of the tool), you can also use these new buttons. Every click changes the turn number up or down and you can also click and hold the up or down arrow causing the number to change continually. You can do that while watching the chart and conveniently see how the number of turns affects the impedance/resistance of the inductor.

    Another novelty is the addition of three “combo” cores in the Material & O.D./Height parameter.
    A “combo” core consists of two cores made of different ferrite materials like 31 and 61 or 43 and 61. We do not consider such construction as very useful design but we included the above three combo cores so that you could calculate inductors built on them and draw your own conclusions.

    In our opinion, instead of making combo cores, it is better to use two (or more) inductors connected in series and wound on single cores made of different materials. And in this way, we have arrived to another new feature in version 2.0 – the InSeries sheet. This new sheet enables you to calculate the total |Z| and R of up to six inductors connected in series. Moreover, you can store such total impedance/resistance plots as a reference or a “snapshot”. You can then compare the |Z| and R plots of other combinations of inductors with your previously stored snapshot.

    See below some exemplary calculation made in the InSeries sheet.

    The reference plot is:

    Let’s now compare two inductors in series:

    with our reference (snapshot):

    And now, 4 inductors in series:


    As you can see, a set of four inductors can create a miraculous “superchoke” for 80-10 m bands.

    There are a few more enhancements in version 2.0, like the chart in the Permeabilities sheet in which you can, for example, compare the permeabilities of the Fair-Rite and Amidon materials – see below.

    Or you can display the resultant permeability a combo core would have in comparison with the permeabilities of its component core materials:

    Don’t wait but go to this page:

    and download your copy of the TFCI Calculator version 2.0 along with its help file.

    Have fun!

    Dan & Jacek
    K1LH likes this.
  2. KN5L

    KN5L Ham Member QRZ Page

    Hi All,

    I've lost confidence that measuring common mode choke Balun complex impedance is a good figure of merit indicator. Mostly because it's rather difficult to accurately measure Ferrite complex impedance.

    For comparing Baluns, my preference is evaluating Balun measured Common Mode Rejection (CMR). Evaluating several 43 material Baluns:

    I've evaluated four CMR measurement methods. The four methods and SimSmith model show here:

    Within above link, also compared coax versus parallel conductor. Little to no difference in CMR. Magnet wire easier to work with.

    Demonstrated measuring CMR using a signal generator and oscilloscope as a RF volt meter:

    I am unable to correlate EZNEC model Balun inductance to measured common mode current. Antenna field testing:

    Simple third wire EZNEC model does demonstrate TL length versus common mode current behavior. Shown in "TL Length versus EZNEC Current" plot. Though measured common mode current does not match EZNEC model.

    There is some measured TL length versus common mode current variance as shown in "50W TL Length vs CMC" table.

    John KN5L
  3. AC6LA

    AC6LA Ham Member QRZ Page

    I personally do not have the metrology chops to question any of John's measurements but I do remember that Owen Duffy wrote a few things on the subject of Common Mode Rejection. Doing a search for "CMRR" on Owen's blog produced these hits:

    Along with these references:
    Anaren: Measurement techniques for Baluns
    Skelton: Measuring HF Balun Performance

    John, do you have any comments on what Owen had to say?

    Dan, AC6LA
  4. SP3L

    SP3L Ham Member QRZ Page

    Hi John,

    could we compare the results you achieved using with your method versus the prediction of the calculator?
    It would be great if you could share with me the |Z| & R vs. F plots of the baluns/chokes you have measured. If you do not have the |Z| values calculated, it can also be R & X versus F.

  5. KN5L

    KN5L Ham Member QRZ Page

    Hi Jacek,

    Measured one common Balun configuration for Dan. FT240-43 with ten turns. Complex impedance measurement and SimSmith model found in "Estimating Stray Capacitance" section in:

    John KN5L
  6. SV1IYF

    SV1IYF Ham Member QRZ Page

    My guess is that it will track OK at the HF spectrum, but above this will have worse performance due to the shunt capacitance of the ground plane (printed circuit board) with the in and out wire leads. It will also demonstrate a lower resonant frequency.
  7. KN5L

    KN5L Ham Member QRZ Page

    SV1IFY is correct. Any, and I mean any, physical change in test fixture will change stray capacitance. Which is why it is near impossible to accurately measure high inductance inductors at high frequency. Which is why previously I commented "I've lost confidence that measuring common mode choke Balun complex impedance is a good figure of merit indicator. Mostly because it's rather difficult to accurately measure Ferrite complex impedance."

    John KN5L
  8. SV1IYF

    SV1IYF Ham Member QRZ Page

    The nice thing about this calculator is that it will give the amateur constructor an "ideally" peak impedance frequency bellow which it gives robust values. Among less experienced amateurs the notion of squizing more turns to improve performance may led the exact opposite result. For the conservative constructor a back-off from the calculated ideal peak point is advisable as the self capacitance is of the order of a fraction of a pF and stray capacitance of the same order of magnitude may be lurking around. For example the calculator addresses a "naked" inductor with very short leads. Start adding a PL connector, a pair of mounting bolts, some longer leads because the box in hand was a bit oversized, some fixing resin with a high dielectric constant and you pass over to the area of diminishing returns.
  9. KN5L

    KN5L Ham Member QRZ Page

  10. AC6LA

    AC6LA Ham Member QRZ Page

    I've also wondered about the proximity of the ground plane in John's test jig.

    The appendix of the K9YC New Choke Cookbook shows this test jig:


    A different although similar test jig is shown on pg 117 of the K9YC Coaxial Transmitting Chokes slide presentation:


    Earlier (S11) measurements made by K9YC's "anonymous collaborator" as shown on the various charts in the Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing were done using the test jig shown on pg 72 of the above Coaxial Transmitting Chokes presentation:


    And finally, G3TXQ (SK) used this test jig as shown at the bottom of his Common-mode chokes web page:


    In all of those setups neither the windings of the choke nor the input/output leads were near a ground plane. Contrast that with John's test jig that was used to measure the CM choking impedance of 10 turns on an FT240 core (section "Estimating Stray Capacitance" here):


    John, by any chance have you measured a "known" load, such as a high value resistor with the easily quantified inductance of the axial leads, in order to provide a sanity check on any choke CM impedance measurements?

    Again I must stress that I personally don't have the skill needed to make any of these measurements.

    Dan, AC6LA

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