Reduce RFI with a Common Mode Choke

Discussion in 'Amateur Radio News' started by VK7HH, Sep 29, 2021.

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  1. COYOTE007

    COYOTE007 QRZ Member

    Can you give me the part number of what you are using please?
    KR3DX likes this.
  2. KR3DX

    KR3DX Ham Member QRZ Page


    Be careful where you look for information. There is a lot of misinformation and bad advice on the internet, and much of it comes from "experienced" hams who should know better. With your background and experience, you should have enough basic knowledge to sort the accurate info from the bad. I'd suggest these websites for accurate info and good advice:,,
    Those guys are knowledgeable and do a good job of explaining and illustrating the data, they don't make unfounded assumptions or arrive at conclusions that violate the laws of physics, like a lot of the information on other websites does.

    The toroids are made by Fair-Rite, they can be purchased here:″-toroid-core/.
    There is a link on that page that will take you to the manufacturer's website where you can view the full technical data.

    Pay attention to the minimum bend radius of the coax that you use to wind your toroids. Foam dielectric coax can't be bent less than about a one foot radius, you need coax with a solid dielectric for tight bends. RG-400 has a solid Teflon dielectric and it's smaller in diameter than RG-8 sized coax. It's expensive, but it's worth the extra cost.

    Good luck with your license and with building your station, I hope to work you on the air in the future. Send me an e-mail if you'd like to set up a sked.

  3. KR3DX

    KR3DX Ham Member QRZ Page

    I forgot to include this in my first reply to you:
  4. COYOTE007

    COYOTE007 QRZ Member

    Thanks mate, I really appreciate your help. I did find that cookbook and haven't stopped reading it.

    Ordered some of those toroids too.

    When I'm up and running I will send you an email and seek you out.

    KR3DX likes this.
  5. COYOTE007

    COYOTE007 QRZ Member


    I undid the choke I made to check the insides. Autopsy revealed no damage to the braid or the foil.

    Slight crushing of the dielectric insulator but no where near as bad as I thought it would be being so soft. So this seems to be usable my only thought is, is 7 1/2 turns enough to work as choke for the rig and is it wound too tight to do its job? Would adding another 2 or 3 or 4 torroids help to improve things with only 7 1/2 turns? If I am going down the wrong path I will use them for the power supply input to the rig etc and get the correct torroids.
    IMG20211020152525.jpg IMG20211101231412.jpg
    KR3DX likes this.
  6. KR3DX

    KR3DX Ham Member QRZ Page

    Hi Mick,

    There are some other things to consider. The center conductor will slowly migrate through a foam dielectric, this process will be accelerated with heat. Most baluns will be at least a little bit warm when they are operating, sometimes they can be quite hot. The migration of the center conductor will certainly cause an impedance change, and it may eventually lead to a short circuit. I would use coax with a solid dielectric, Teflon is preferable over other materials. Winding data for using multiple cores of the smaller toroids is in a previous cookbook on K9YC's website.
    There is a lot of good information in that document, it's worth reading in its entirety.

    Last edited: Nov 1, 2021
  7. COYOTE007

    COYOTE007 QRZ Member

    That is an excellent document.

    A great teaching article.
    KR3DX likes this.
  8. KC3TEC

    KC3TEC Ham Member QRZ Page

    the thing with winding torroids as a choke or transformer! induction wise the windings do not have to be pulled tight.
    looping your coax through a torroid is fine.
    the only reason for drawing them tight is dependent on the space you have for it
    magnet wire is usually drawn pretty tight and spacing observed and adjusted to get the inductance values needed.
  9. AE7XG

    AE7XG Ham Member QRZ Page

    Now I suggest you look into an Ugly Balun(current Balun)
  10. KR3DX

    KR3DX Ham Member QRZ Page

    The "ugly balun" has already been looked into by K9YC, he found that it is a poor performer, under some circumstances it can be worse than no balun at all. I suggest that you look into his documents, there are links to them elsewhere in this thread.

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