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Common Mode Choke Question-Inverted L

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

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  1. N1LOU

    N1LOU XML Subscriber QRZ Page

    I recently changed antennas from a ladder line fed doublet to a base fed inverted L. The L is 100' long, 50' vertical and 50' horizontal. I have a remote tuner at the base of the L, and 75' of LMR-400 from the tuner to the shack. The antenna has a ground rod and roughly 40-50 radials of varying length (length is limited in some directions by topography). The longest radials are 120' long the shortest are maybe 30' long. I am getting RF in the shack when I run at 100 watts on SSB phone. Interestingly, when I run at 30 watts on PSK31, I don't seem to get any RF, or at least it is not noticeable. I am finding that the RF is messing with my computer, and I lose CAT control after I transmit the first time on phone. I am going to put a common mode choke on the system (something like an MFJ-915). The question I have is do I put the check at the entry to the shack or at remote tuner? Or both?
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    My recommendations, in order:

    Step 1. Bond coax shield to a ground rod just before the coax comes through a wall into your ham shack. That alone might do it... (This step has an electrical code implication, my recommendation has to do with curing RF in the shack).

    Step 2. Coax Common Mode Choke (8 to 12 Turns of Teflon coax wound through FT240-31 core) inserted in-series with the coax between the ground rod and the antenna. Place the CMC near the ground rod. Teflon coaxes include RG141-2, RG302-4, RG400-5.

    Step 3. If you still have RF in the shack, move the CMC to the shack-side of the ground rod.

    These steps will likely lower your receive noise level, too...
    Last edited: Dec 12, 2019
    KU3X likes this.
  3. N1LOU

    N1LOU XML Subscriber QRZ Page

    This piece I don't understand. there is no coax between the ground rod and the antenna. Coax runs from the remote tuner to the shack. The remote tuner is connected by 6 gauge ground wire to the ground rod and radial system, as well as the inverted L itself...both via binding posts
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    I am suggesting adding a ground rod to bond the coax shield to near the shack end of the coax run. Then you have a choice of placing the CMC either upstream or downstream from that new rod.

    The ground rod under the antenna is not doing much except possibly providing a static drain...

    The first step is to shunt the coax shield CM current into the earth at the shack end of the coax before it enters the shack. The second step is to add a high series impedance to block the coax shield CM current. The two steps in combination are the most effective.
    Last edited: Dec 12, 2019
    WB5YUZ likes this.
  5. SM0GLD

    SM0GLD Ham Member QRZ Page

    I used a CG-3000 tuner for the same application. I put the choke on trx side of the tuner.
    Just remember to add a choke on the power cable too.
  6. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    On what band(s) are you getting the RF in the shack?
  7. N1LOU

    N1LOU XML Subscriber QRZ Page

    Gotcha now. Thx
  8. N1LOU

    N1LOU XML Subscriber QRZ Page

    Power is fed to the tuner through the coax via a bias tee. I am guessing that the one common mode choke should cover it.
    SM0GLD likes this.
  9. N1LOU

    N1LOU XML Subscriber QRZ Page

    20-80 mostly. 20 and 80 are the worst. On 20, the PC goes nuts, and the usb com port drops right out. On 80 I get these weird tuner glitches.
  10. KK5JY

    KK5JY Ham Member QRZ Page

    The best place to initially place a coaxial choke is at the antenna end of the coaxial cable, which would be at the remote tuner in your case.

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