To Ferrite or Not?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KD9VV, Aug 13, 2021.

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  1. KD9VV

    KD9VV Ham Member QRZ Page

    Seems to be two camps on this subject.

    Ant: 2M LFA
    Coax: LMR 400...70 Feet.

    There seems to be differing opinions on this.
    Some say they use Ferrite chokes at the feed point, some do not?

    Other than possible RFI from common mode noise; is the full RF still getting to the antenna w/o a RF choke?
  2. NK7Z

    NK7Z Ham Member QRZ Page

    AFAIK, ferrite chokes applied to the outside of coax, have no, to almost no effect on the RF inside the coax. I use chokes at the feedpoint, and at the radio end of all HF feedlines, and no chokes of any kind on my VHF feedlines. All of that said, what are you using the antenna for? If you are just listening to local, repeaters, don't worry about it. If you are hunting DX, then ferrite away.
  3. K6GBW

    K6GBW Ham Member QRZ Page

    They don't effect the transmitted signal. They will reduce the common mode current that can end up on the coax for all sorts of reasons. At VHF it's not as much of a problem but they really don't hurt. If you have beads or a choke that's not doing anything then by all means but it on. Otherwise, if you're not having a problem with noise then you can skip it. It really depends on your situation.
  4. W9IQ

    W9IQ Ham Member QRZ Page

    A common mode choke does affect the transmitted signal. The choke inserts a high impedance in the exterior shield so that the RF current favors the antenna instead of the exterior of the shield. This not only makes the transmitted signal stronger, it preserves the intended pattern of the antenna.

    The degree of common mode current depends upon the antenna and the installation details. There is no downside, other than cost/time, in installing a common mode choke.

    - Glenn W9IQ
    SWL37632 and K0UO like this.
  5. 2E0VSS

    2E0VSS Ham Member QRZ Page

    Choke it choke it choke it..
    Of course
    Good engineering practice..
    Every time
  6. KD9VV

    KD9VV Ham Member QRZ Page

    See what I mean?
    varying opinions at VHF use.
    Posts #3 and #4 are at opposite ends here.
  7. KD9VV

    KD9VV Ham Member QRZ Page

    Always hunting DX. :)
    That said, I have a very low noise level.
    My 2M and 432 are both horizontally polarized.
    Local repeaters I use a small 5 ele short boom vert yagi.
  8. K8JD

    K8JD Ham Member QRZ Page

    Not sure what a LFA is (low freq anrtenna?) for VHF ? some antenas are dsigned to optimumize for coax feed. probbly don't need anything hanging on the coax.
  9. WA4SIX

    WA4SIX Ham Member QRZ Page

    LFA is a Loop Fed Array. Basically, a large horizontal folded dipole for a driven element.
    Usually, designed for a 50 ohm feedpoint.

  10. KD9VV

    KD9VV Ham Member QRZ Page

    Loop Fed Array
    Essentially a full wave loop

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