1:1 Choke Toroid Question

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N1MVP, May 30, 2021.

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  1. N1MVP

    N1MVP Ham Member QRZ Page

    Hello all...
    I have been reading about building a 1:1 choke, and ran into a question. I have read some of Jerry Sevick's papers and books, and he uses toroids with permeability of 250, and sometimes lower. Common practice now seems to be to use a 43 mix (850 permeability), or 31 mix (1500 permeability). I am wondering why there is such a large difference in permeability suggestions.
    I know there is a lot more to consider than just permeability when choosing toroid cores (efficiency, impedance, temp ratings, frequency range, and the list goes on). I'm looking for a high level list of pros and cons (not too technical) for high vs. low permeability cores in a 1:1 bifilar wound choke. Frequency range of interest is 80 through 10 meters, and 200 watts of power.

    Thank you.
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    Sevicks work is based on what was available then. Much better material is available now... That makes his work dated and not very useful...
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  3. N1MVP

    N1MVP Ham Member QRZ Page

    Yeah... I was wondering if that was the case. Between the 43 and 31 mixes for this choke, I'm leaning toward 31. Your thoughts...?
    Can you recommend any other authors I should be reading? There is an ocean of stuff out there, and some are probably better than others.
  4. NK7Z

    NK7Z XML Subscriber QRZ Page

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  5. 4X1SO

    4X1SO Ham Member QRZ Page

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  6. WA4SIX

    WA4SIX Ham Member QRZ Page

    In this writeup, on the bottom of page 7, you will see, "Chokes in Series: In general, any combination of chokes can be used in series to provide the desired choking impedance over the desired bandwidth. Their combined choking impedance, RS, will be the sum of their RS values on each band. For example, two 12-turn RG400 or Teflon chokes provide at least 8KΩ from 80 to 15M and 6KΩ on 10M.".
    I use this with an IC-7000, MA-1000 amp (400-500W) & a High Sierra 1800 screwdriver antenna, in my 2008 Ford Ranger pickup. No heating or CM issues, whatsoever from 160M to 6M (Modded the MA-1000 for 6M). I tried several other chokes, but when I added the amp, this is the only one that worked.

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  7. VK4FUQ

    VK4FUQ Ham Member QRZ Page

    G'day all, to be completely honest I'll say that I'm not sure about all this as I have built baluns (choke and voltage), with rather different ferrite permeabilities but my favourities, with excellent performances from 3 -30 MHz have used 61 mix ferrite (125 permeability) material. So there you go! Regards, Felix vk4fuq.
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  8. VK2TIL

    VK2TIL Ham Member QRZ Page

    Dr Sevick had a wide range of ferrites, from several manufacturers, available for his experiments.

    The materials and suppliers are listed in Transmission Line Transformers; ui values were between 40 and 2700.

    It appears that he obtained the ferrites gratis, probably aided by his position of respect in the electronics community.

    His work focused on measurements of transformer bandwidth, power-handling and efficiency; it's unsurprising that he found, in general, that materials with ui between 40 & 300 gave the best results on those criteria.

    He was, however, very impressed with CMD 5005, produced by Magnetic Ceramics; this had ui of 1400.

    His focus was not on choking impedance and he did no measurements of this property.

    Sevick's work was done in the latter half of the 1980s when ferrites were not easily obtained by mere mortals; most companies then, as today, dealt in very large quantities and had no interest in amateur experimenters.

    Perhaps even Fair-Rite did not "retail" at that time; it's my understanding that Amidon Associates was set-up by Bill Amidon to provide experimenting amateurs with a source of small quantities of Fair-Rite products.

    Fair-Rite, at that time, did not have as many materials as they have today.

    My oldest (1995) Fair-Rite catalogue lists only three materials with ui between 125 (#61) and 850 (#43); #83 (ui 300) was a "square-loop" material designed for power supplies and available in toroid form, #44 (ui 500) was only available to special order and #33 (ui 800) was only available in slugs or rods.

    Today's focus is on choking impedance, due largely to the great increase in noise/RFI since Sevick's day, but his work still repays careful study for those interested in ferrite transformers; I've had a few "light-bulb moments" when reading his work.
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  9. W9IQ

    W9IQ Ham Member QRZ Page

    The design of the choke is not based upon 'ui' per se but rather upon the complex permeability the ferrite at the frequency of operation. The goal of a good choke is to develop as much real resistance as possible and whatever capacitive or inductive reactance is present is almost inconsequential. So it is u' and u'' (the real and imaginary permeability respectively) along with ui at the frequency of operation that is of the most interest.

    If this question was asked many years ago, mix 43 material would have been the best choice for the HF bands. But since then, mix 31 material has been developed and it will equal or outperform type 43 material as a choke on 160 through 6 meters. The question of mix 31 vs mix 43 material for a choking applications came up on another forum and I posted this chart to clarify the relative resistance that can be obtained from the two ferrites:


    - Glenn W9IQ
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  10. W9IQ

    W9IQ Ham Member QRZ Page

    Just to clarify my earlier comment, ui is not frequency dependent as it is typically specified at a very low frequency. So my statement may have been better worded as:

    So it is u' and u'' (the real and imaginary permeability respectively) at the frequency of operation along with the ui that is of the most interest.

    - Glenn W9IQ
    Last edited: May 31, 2021
  11. VK2TIL

    VK2TIL Ham Member QRZ Page

    Like W9IQ, I have attempted to demonstrate the relative values of u' (the real part of the complex permeability that contributes to reactance) and u" (the imaginary part of the complex permeability that contributes to loss) in graphical form.

    I re-drew the Fair-Rite permeability data on semi-log scales;

    31, 43 & 61 - Real Part.gif

    31, 43 & 61 - Imaginary Part.gif

    Whilst the inevitable self-resonance of the choke will alter the above simple relationships, the superiority of #31 is evident.

    Attached Files:

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  12. EA1DDO

    EA1DDO Ham Member QRZ Page

    Have you measured those chokes?

    I've done it and my results match G3TXQ's tables.
    61 mix shows best performance at VHF bands.
    43 mix is best from 21 Mhz up to 50 MHz.
    31 mix best from 1.8 up to 14 MHz.

    That is what I can measure with my VNA.

    73, Maximo
    Last edited: Jun 1, 2021
  13. VK4FUQ

    VK4FUQ Ham Member QRZ Page

    G'day all, a couple of things that I don't really understand! If 61 mix (ui 125) is best at VHF frequencies then why is it used in every ferrite rod coil antenna in every AM/MW radio?

    In Sevick's 'Transmission Line Transformers 3rd Edition, there was no real discussion on the different ferrite mixes for different frequency ranges, but only discussion and loss profiles for the different ferrite material permeabilities with his measurements suggesting that ui's below around 290 were best! Sorry I just don't get it! Regards, Felix vk4fuq.
  14. VK2TIL

    VK2TIL Ham Member QRZ Page

    " ... with his measurements suggesting that ui's below around 290 were best ..." as might be expected in a book entitled Transmission Line Transformers.

    Transformers are not chokes; the requirements are quite different.

    #61 ferrite has almost zero loss (u") up to about 7 MHz; it therefore has high Q over that frequency range and
    is very good for efficient tuned circuits such as BC/low HF radios.

    The aim of a choke is to introduce loss, ie resistance; #31 & #43 have greater ui than #61 and, following the basic physics of ferrites, have greater loss/resistance at lower frequencies.

    There seems to be a widespread misunderstanding of Dr Sevick's work; he studied transformers, not chokes.

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  15. 2E0VSS

    2E0VSS Ham Member QRZ Page

    Ai6RW, Richard Wheeler Choke Balun
    Or Anything Jim Brown or Owen Duffy promotes...


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