Another choke question.... tuner power cable

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W3WD, Jun 3, 2021.

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  1. W3WD

    W3WD Ham Member QRZ Page

    Hi guys,

    Power cable extension for AH-4... this is 4-conductor 18 AWG, unshielded.

    I'm trying to completely block CM current from my vertical and plan to run 2 of these chokes on the cable (see attached)-- one just below the tuner and another before the cable enters the shack.

    Present iteration is about 12 turns thru 3 stacked Mix 31's (x2 as indicated).

    Will this choke the current and/or do you have other suggestions? I have plenty of additional toroids (3rd choke in the middle of the cable?) and also some 4" ones.


    Attached Files:

  2. W9IQ

    W9IQ Ham Member QRZ Page

    When you stack two cores, the inductance (or capacitance) of the choke is double and the stray capacitance of the windings roughly doubles compared to a single core with the same windings. The effect is that the resonant frequency of the assembly is halved and the impedance at resonance is approximately doubled. The stacked cores also improves the power dissipation capability of the assembly.

    The capacitive or inductive reactance of the assembly may reduce or worsen the common mode current dependent upon the cable routing, length and frequencies involved. Good common mode chokes for cables focus on maximizing the resistive component of the impedance as this is effective in choking common mode current regardless of the cable conditions.

    It isn't common practice to stack cores for low power applications but this does not mean it cannot be done. Generally though, putting well designed common mode chokes in series is more effective when power dissipation is not a consideration. This would involve placing a wound core right after another wound core on the cable.

    The key to all of this is getting the right number of turns to be most effective for the bands of interest. You can do this experimentally or you can follow the design guidelines in the K9YC cookbook as a starting point. If you would like to read more about the topic in general, Jim's paper "A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing" has more details and it includes the cookbook.

    - Glenn W9IQ
    Last edited: Jun 3, 2021
    WA7ARK and KU3X like this.

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