Reduce RFI with a Common Mode Choke

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

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  1. KR3DX

    KR3DX Ham Member QRZ Page

    Your statements apply accurately to low frequency AC chokes and transformers. Things are much more complicated at RF. Take a look at the work that has been done by K9YC, he explains it all. He found that magnet wire is a poor choice for RF chokes.

    A brief quote from K9YC's Choke Cookbook:

    "Enameled copper pairs have much greater loss than other paired lines. This is because the
    magnetic fields produced by currents in very closely spaced pairs used as transmission line cause
    the current to be concentrated in the side of the conductors closest to each other. This
    mechanism, which is strongly related to skin effect, is called proximity effect, and is what causes
    differential current to flow on the inside of the coax shield. Just as skin effect forces current to the
    skin of the conductor, proximity effect forces it to only one half of the skin! Proximity effect rises
    rapidly as the center-to-center spacing approaches the conductor diameter, which is the case with
    enameled wire. As can be seen from the table of measured transmission line data, the enameled
    pairs have significantly higher loss (and greater dissipation) than other paired cables. It’s also
    possible for the enamel to be scraped by the ferrite core during winding, shorting to the core at
    multiple points and significantly degrading choke performance. For both reasons, I no longer
    recommend chokes with enameled wire."
    Last edited: Nov 6, 2021
  2. KC3TEC

    KC3TEC Ham Member QRZ Page

    I agree with you I was just stating that coax does no need to wound tightly to make an effective choke.
    KR3DX likes this.
  3. KG7WPQ

    KG7WPQ Ham Member QRZ Page

    Seems like a good point after looking at the article you linked, and also looking at the article THAT article linked. Maybe it's another case of "This might work for my purposes, but not for yours". Considering there's no one single "Ideal" operating setup, but a large set of variables that are going to change for each operator based on a million different things.
  4. COYOTE007

    COYOTE007 QRZ Member

    I did it to fit in a box but there is no point putting it in a box.

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