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Reduce RFI with a Common Mode Choke

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

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  1. G0KDT

    G0KDT Ham Member QRZ Page

    Agree With your findings Jim.

    I have made Peter’s 1:1 common mode choke and his 4:1 and 9:1 baluns testing as he does in the videos. They certainly test out ok. I have also wound coax common mode chokes which have tested out. Frankly I have not noticed any difference between the two common mode chokes.

    As to the coax wound chokes, I have to say that I have seen two designs. Some with a cross over and some without I don’t follow the logic or reasons for having a cross over in winding a coax choke.
    M1WML and NJ3T like this.
  2. K6SSH

    K6SSH Ham Member QRZ Page

    I saw statements that cross winding reduces capacitance added by coils being next to each other. But I have another question here...

    According to and other resources common mode can be caused by two things: abrupt change in line like end of dipole (can be bad wording but hope you got the point) and induced CMC which is "spill" from outside surface of shield to outside and it depends in first place on impedance of shield which depends on cable length (pretty good described in article). BUT! I've seen tons of choke designs where you have coax open right before balun what doesn't make sense if all above is true because inside-outside surfaces of the shield connected before choke!
    That being said original post/video makes a lot of sense to me because in this case we have coax running through.
    Am I missing something?
    M1WML likes this.
  3. G0KDT

    G0KDT Ham Member QRZ Page

    I have tried following Steve G3TXQ and K9YC’s notes (Cookbook) on chokes and transformers. Clearly lots of effort has/had been put into them but some things, sad to say, simply are not explained well or in a way that does not leave me with as many questions as I had before I started to read them.

    Frankly I have yet to find an article that explains the ‘Black Art’ of chokes in a way that is readily digestible. Splitting a coax winding as part one way, then cross over part another for instance simply seems to setup a way for one part winding to cancel the other’s effect.
    K6SSH likes this.
  4. WB4OFT

    WB4OFT Ham Member QRZ Page

    Last edited: Oct 21, 2021
  5. WB4OFT

    WB4OFT Ham Member QRZ Page

    Just makes it easier to place the core and cable in a box with connectors on the opposite sides. The cable starts on one side and ends on the opposite side.
    G8HGN likes this.
  6. N5SMO

    N5SMO Premium Subscriber QRZ Page

    Minimizes impulse noise picked up by the coiled braid, opposite phases cancel each out.
  7. KC3TEC

    KC3TEC Ham Member QRZ Page

    a magnetic core such as that in a transformer and toroids build up a magnetic field, collapse of that magnetic field induces current in the conductor, that impulse can be thousands of times higher ( Basic magneto theory) for a very short duration, but still long enough to damage sensitive electronics.
    just ask any industrial electrician what happens when power is lost on a 3 phase motor!

    to explain it better a typical magneto coil on a lawn mower has a primary with several hundred wraps if wire and a secondary with way fewer number of windings.
    one end of the primary and one end of the secondary are tapped together and run to the points ( or electronic ignition module)
    when the magnet passes the core it generates an intense flux field when the points are open,
    when the points close however the secondary is shorted to ground causing the flux field to collapse.
    this collapse induces a current of several thousand volts on the primary tap to the spark plug.

    toroids tend to be self shielding outside of their own outside diameter but still generate a magnetic field within the core.
    whenever you transmit even at low power you are building up a magnetic field, when you stop transmitting the field collapses. circuitry in your radio disconnects the transmit function but does not disconnect the shielding/ ground and that conductor can back feed your radio and cause degraded performance, rfi, and or damage your radio.
    split or reverse winding halfway through cancels the magnetic field that can generate in the core! while it cancels the field it still resists the current on the outside conductor of the coax.
    after all what this accomplishes is choking out or minimizing the current coming down the shielding.

    non split winding is primarily a transformer function regardless of the toroid in many instances specific mixes and number of windings are used depending on the frequency.
  8. COYOTE007

    COYOTE007 QRZ Member

    You cannot get any more than 7 turns of LL400 through a -31. upload_2021-10-31_12-1-56.jpeg upload_2021-10-31_12-1-56.jpeg
  9. KR3DX

    KR3DX Ham Member QRZ Page

    That depends upon the size of the toroid. Mine are 4 inch O.D. by 3 inch I.D. by 1 inch long. I can get over 25 turns of RG-400 through them. I'm not sure what you meant by "LL400". Is that the same diameter cable as RG-400, or is it the same diameter as LMR-400? LMR-400 is much larger in diameter than RG-400.
  10. COYOTE007

    COYOTE007 QRZ Member

    Low loss RG-213 style coaxial cable

    LL400 - RG8 Type Foam Low Loss Solid Centre/Foil/Braid

    Diameter 10.29mm

    Attenuation db/100m

    100Mhz 3.9db
    500Mhz 9.1db
    1000Mhz 13.8db
    1800Mhz 18.3db
    2200Mhz 20.6db
    2500MMhz 22.3db

    Im not an amateur yet but going for my licence next month. I'm basically using this forum to improve my knowledge and make some new friends along the way, no substitute for experience and there is plenty of that here. If I ask a question that seems to be very basic, please forgive me, its only experience I am missing. I was a missile tech in the Australian Air Force. IR, Torpedoes, Semi Active radar homing missiles. Its trying to restart my brain after 30 years of being out of the service. Its hurting a bit.

    I am starting a new station from scratch and as far as antennas go when I put one up, if it needs a choke at the antenna end then I want to it now and not have to struggle with it later.

    Thank you in advance for any help you can send my way.
    KR3DX likes this.

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