Common Mode Filter Choke Question

Discussion in 'Ham Radio Discussions' started by K3KDB, Jul 4, 2020.

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  1. K3KDB

    K3KDB Ham Member QRZ Page

    I would really appreciate any input/suggestions on the need for a CMC. First of all, I have a Zero-Five 43ft HF vertical that includes an Un-Un. I purchased a CMC from and was wondering if there was any need for this with the antenna I'm using. Is this choke better suited for a wire antenna or is there a better place to install it other than right at the antenna? Thanks for any help!
    Last edited: Jul 4, 2020
    KF5KWO likes this.
  2. KK5JY

    KK5JY Ham Member QRZ Page

    The terminology of "choke" vs. "balun" and "unun" and "common mode" are all hopelessly muddy in amateur radio circles.

    The coaxial cable choke is designed to prevent energy from flowing on the outside of the coaxial cable. It's actually a single-wire device, that is designed for trifilar cable (coax) where the third wire is the outside surface of the shield conductor.

    All coax-fed HF antennas should be choked near the feedpoint of the antenna, to make sure that only two conductors of the coax (the center wire and the inner surface of the shield) carry RF current. People will argue with this, but the arguments against what I'm saying are based on incomplete training and unfortunate folklore. Since nearly all amateur HF antennas are compromised in one form or another, the coaxial choke is a very effective, very low-loss, and relatively inexpensive way to make sure that the RF current flows where it should and not where it shouldn't. As such, it should be used on any coax-fed HF antenna (and quite a few VHF+ antennas).
    KF5KWO likes this.
  3. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    An UnUn/Common Mode Choke/Isolator can be used several place. At the feedpoint, it keeps CM from the antenna from traveling down the coax. The next most useful place is at the shack entrance. This keep common mode from nearby noise sources (computers, wall warts, switch mode transformers etc) or strong RF from the antenna that is impress on the outside of the coax.

    An UnUn should not be in the shack between components like tuner, amp, radio, etc. Ferrite beads or toroids are better choices there.
    KF5KWO likes this.
  4. K3KDB

    K3KDB Ham Member QRZ Page

    OK, sounds like I'll keep the choke at the antenna feed point. I do have all components in the shack grounded and used lots of ferrites. Sounds like I'm on the right track...
    WZ7U likes this.
  5. K8JD

    K8JD Ham Member QRZ Page

    What you REALLY need is a tuner with remote control right at the base of the vertical, radials connect to the ground terminal and the coax input connected with a short piece of braid to the bottom of the vertical pipe. . The Z of this antenna can be from 35 Ohms at 60 M to a few thousand Ohms on some other bands ! An UnUn can't transform that range of Z to match the coax and your SWR losses become beyond acceptable .
  6. K8JD

    K8JD Ham Member QRZ Page

    This question should have been posted in the "ANTENNAS...." forum
  7. K3KDB

    K3KDB Ham Member QRZ Page

    Yeah, I was quick to post and just saw how many individual forums there are. I probably would've gotten even more input ; ) The person at Zero Five did explain the need for a really good tuner so I picked up a Palstar AT2K. I'm not really familiar with remote tuners but will check. The SWR on 80M is 2.25:1, 40M is only 1.5:1, 20M 4:1. I mainly ragchew on 80, 40, 10M. I have a separate vertical antenna for 10M, and 6M. This setup is only temporary until I get a place with more room.
  8. NK7Z

    NK7Z Premium Subscriber QRZ Page

    All things CMC can be learned here:
    WZ7U likes this.

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