Common Mode Choke Question-Inverted L

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N1LOU, Dec 12, 2019.

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  1. W1VT

    W1VT Ham Member QRZ Page

    I had a tiny bit of RF feedback on 75M when running a 500W amp--I asked for a critical report on my signal and someone said it was there but not objectionable.
    I fixed it by winding the mic cord through an FT-240-43 core perhaps 10 to 15 turns.

    I also winding my digital cords through ferrite toroids.
    If the cords are thin I'll winding multiple cords through the same toroid core to save money.

    Zak W1VT
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    I would start with this order:

    AC Wall-outlet
    rig and its DC powersupply
    short coax through wall
    long coax run
  3. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    The antenna length is close to resonant on 160m. So I suspect the feedpoint impedance on both 80 and 20m is likely very high and the tuner is struggling to make a match. In that case there will be a lot of RF coming back through the tuner, down the coax shield to the shack.

    Chokes might help some but the root cause is likely trying to drive an anti-resonant antenna. It would be better to pick a total length that is non resonant, so the tuner can make a reasonable match on all bands of interest.
  4. N1LOU

    N1LOU XML Subscriber QRZ Page

    Fair point, I have been following a lot of W4RNL's writings in building this. I think he recommends a length closer to 90'
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    I disagree. There is mutual coupling between the inverted L conductors and the long coax run to the shack. That is the source of the common mode current on the coax. You want to prevent that common mode current from entering the shack and flowing through the rig and computer into the duplex wall socket/house wiring.

    Putting the choke right at the antenna base/tuner does nothing to prevent the mutual coupling from inducing the CM current onto the coax; the mutual coupling is inevitable due to the field around the antenna. Creating the shunt path into the earth/series Choke impedance at the station end of the coax prevents the CM current from flowing in the rig/computer/house wiring.
  6. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Before pulling the antenna down to change the length, you could experiment by adding some series reactance at the antenna feed point to allow the tuner to get a good match, and see if that reduces the problem. The voltage might be quite high though so make sure the reactive component is away from other conductors and can handle the voltage - air coil or capacitor rated for at least a kV or more.
    Last edited: Dec 12, 2019
  7. KK5JY

    KK5JY Premium Subscriber QRZ Page

    Respectfully, you don't know that. The only conduction path from the inside to the outside of the coaxial cable is via the point I mentioned... at least in a system where the cable and connectors are properly installed.

    While there may also be an induction path such as that you described, it makes no sense to address that prior to addressing the conduction path, which has much more capacity to send power down the outside surface of the coaxial cable.
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    You didn't tell us what you think the source of CM current on the coax is.
  9. KK5JY

    KK5JY Premium Subscriber QRZ Page

    Actually, I did. And it's not really "common mode."

    Again, there's only one point in a properly-installed coaxial cable feedline where the current can conduct from the inside of the cable to the outside of the shield. That point is the antenna end of the coaxial cable. Placing a coaxial choke at that end of the coax forms a mono-filar choke, that attenuates the current flowing between those two surfaces by choking the outer surface.

    That doesn't mean that you were wrong about the possibility of an induction path, as well. The problem is that most people don't have a way to measure which they are getting, and the conduction path has much more capacity for generating I3 current on the feedline shield surface.

    So the diagnostic method for solving "RF in the shack" that has the highest likelihood of success with the minimum amount of effort is to choke the coax cable at the antenna end first, then if RF issues persist, choke the cable much closer to the "shack end," preferably outdoors if possible.
    Last edited: Dec 12, 2019
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    My contrarian view is that most Marconi antennas are lacking a ground-plane, counterpoise, radials, whatever you want to call it... Running the coax along the earth radially away from the base of the Marconi just adds yet another radial wire to the mix. Naturally, Kirchoff's Law dictates that the sum of all the radial currents must add to the current in the vertical on either side of the feedpoint.

    It is better to let the coax shield act as the "last" radial, allowing the CM current to exist along the coax shield from the base of the antenna outward to a point where it is cut-off completely by a good CM choke... We can argue all day if the current in the radials is there because of a field, or because of Kirchoff.

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