Placement of Common Mode Choke

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WB2IOL, May 15, 2019.

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  1. WB2IOL

    WB2IOL XML Subscriber QRZ Page

    I have 3 hf antennas: End-fed half-wave, 40 meter inverted V and a dipole for 20 all switched through a tuner. Can the CMC go between the amp and the tuner so I only need one? Or do I need 3, with one between each antenna and the tuner?

    Thanks, Elan WB2IOL
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    You can place chokes many places in your system, but if you do as you suggest and place a shared choke between the amp and your tuner then any common mode on the feed lines will still arrive at your shack and if the common mode RF is high level you'll still have potential for RFI and noise pickup problems. It also means any common mode on the dipole and inverted-V won't be choked at the feed point so antenna patterns may change relative to a well choked feed point.

    Best to choke common mode as early in the system as possible. in the case of the antennas you've listed:

    - Choke the Dipole and Inverted-V right at the antenna feed point if possible

    - Where you place the choke on the end fed depends in part on the antenna, how it's designed and whether you have any counterpoise at the matching transformer. I'd make sure I had at least a short counterpoise wire at the matching transformer and try choking the coax very close to that transformer. If that throws the SWR out of whack then move the choke back down the feed line, basically allowing some or all of your coax shield to work as counterpoise to the end fed antenna. Depending on the specific design you might get away with choking right at the feed point or you could do as some, including Palomar Engineers, recommend and place the choke back near where your coax enters the shack. Just know that common mode on the feedline means your coax is acting as part of the radiating (and receiving) antenna so keeping that as close to the feed point as possible is typically a good idea.
    KQ0J and WB5YUZ like this.
  3. WB2IOL

    WB2IOL XML Subscriber QRZ Page

    Thanks for the info--I think I'm getting the picture. The inverted V and the dipole each have 1:1 current baluns. The EFHW has the transformer with an attached 20' run of ground braid tied to a dedicated ground rod. My understanding is that the baluns will keep the coax from radiating. So then the CMC should be on the EFHW near the entry point to the shack.
  4. AI3V

    AI3V Ham Member QRZ Page

    For best results the "choke" needs to be at the lowest impedance point in your antenna system.

  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    First, I have found if you are going to use a ground wire on a EFHW (the kind with a 1:49 transformer), the lengths to avoid are odd multiples of a quarter-wave on any frequency you plan to use the antenna. 20 ft seems to avoid resonance on any ham band. If no ground wire is used (only coax), then avoid those same lengths as measured from transformer to first place that the coax is connected to ground.

    If you have CM on the coax from any antenna, I find that a two step approach is needed. First ground the coax shield just before it comes into the shack (entry-panel with ground rod). Then, if needed, add a CMC to the coax between the entry-panel and the antenna, located close to the entry-panel-end of the coax. Never put a CMC right next to the transformer on either an OCF or EFHW antenna.
    Last edited: May 15, 2019
  6. W5LZ

    W5LZ Ham Member QRZ Page

    For the best results put a choke at the feed point of each antenna. Maybe another one at the "shack" end of their feed lines.
    That's he 'quick-n-diry' way to do it. It's also probably the most common way.
  7. AI3V

    AI3V Ham Member QRZ Page

    If you have common mode?

    Then do this, and if this Dosn't work do that?

    How come this antenna Dosn't just work, you know, like a center fed halfwave dipole (1/4 l on a side)?


    P.S. I have a altoids tin here if you want to do a experiment. :)
  8. AI3V

    AI3V Ham Member QRZ Page

    So lets say you have a 7000 ohm impedance at the feedpoint, and you install a "very good" 5000 choke.

    How much would you calculate the "common mode" reduction to be?

    Now suppose you place that same choke at a 50 ohm point, think it would work a bit better?

    AG5CK likes this.
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    And it is the wrong way for either end-fed, or off-center-fed antennas, both of which depend on a small common-mode current along their feedlines for resonance. Putting a choke at the feedpoint of a balanced dipole, or a vertical is a good thing to do.
    Last edited: May 16, 2019
  10. N8QZH

    N8QZH Ham Member QRZ Page

    Place the CMC at the feed point of the offending antenna.
    It an antenna is causing CMC on the feed line associated with that antenna, then it needs a choke at the feed point,.
    First step is determining if you even need a choke for a given antenna. Most wire antennas will benefit from at least a 1:1 current balun choke.
    To answer your question, NO don't place it as a common point such as you described, That wont help and will cause trouble everywhere...
    WB2IOL likes this.

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