Where does the common mode go with a dummy load?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AI3V, Jun 15, 2019.

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  1. AI3V

    AI3V Ham Member QRZ Page

    Consider 2 cases:

    First case, a simple coax and center fed halfwave dipole.

    Coax directly connected to the dipole.

    Second case, replace the dipole with a resistor (dummy load).

    Where did the "common mode" go?

  2. WR2E

    WR2E Ham Member QRZ Page

    Is this a test Rege?

    I'm not gonna answer, wanna see what answers you get, if any.
    K0UO, KU3X, NH7RO and 1 other person like this.
  3. NE1U

    NE1U Ham Member QRZ Page

    In an ideal world a dummy load perfectly terminates. "Perfect" suggests the coax does not leak thru the shield and the dummy load is exactly the output impedance of the tx ... thinking 50 Ohms "real" but a perfect termination is whatever it is.

    Again thinking perfect coax feeding the dipole, the shield is connected to half the dipole. A dipole is balanced. Coax is unbalanced. Without going a lot of detail that others will be into, this is where common mode is created. Without a balun of some kind, current will flow on the shield and this is common mode.

    For what it is worth, long long ago and far far away as a kid before wet behind the ears, I had an inverted Vee driven with 2-wire power cord. Balun? WTF is a balun in those days. I used this with a 6146B tx and then followed with a 813 with a roller inductor that could tune to anything. Yes, my flourescent desk lamp changed intensity when I transmitted but the house wiring was also under sized (I know that now) so there is no way to know if it was RF in the shack or I2R drop on the house wiring.
    Last edited: Jun 15, 2019
    K2CQW likes this.
  4. AI3V

    AI3V Ham Member QRZ Page



    NH7RO and WR2E like this.
  5. AI3V

    AI3V Ham Member QRZ Page

    Lets assume 100% braid, a perfectly balanced , resonant dipole, and the dipole impedance=coax impedance=dummy load impedance.

    You may vary the length of the coax as you please.

  6. WB5WPA

    WB5WPA Ham Member QRZ Page

    I think I have Rf energy flowing out the bottom of my SB-220, over the chassis, and onto the coax cable ... I took the cover off to work on it, never put it back on- it didn't really shield the bottom of the chassis anyway!


    /mild sarc
  7. KL7AJ

    KL7AJ Ham Member QRZ Page

    There is no common mode. The current on the outside of the inner conductor exactly balances the current on the INSIDE of the outer conductor.
    K9AXN, KD6RF, WG7X and 3 others like this.
  8. AI3V

    AI3V Ham Member QRZ Page

    But I am assured by many, many posts that the outside of the coax shield is a "third wire" and cannot possibly be connected directly to the balanced dipole without significant current flow-that varies with coax length- and the only possible way to eliminate it is to use some sort of balun/choke device.

    Yet this is clearly not the case with the resistor.

    So I ask again, where did it go?

    Or, perhaps, its not there on the dipole....o_O

  9. AI3V

    AI3V Ham Member QRZ Page

    Here is a typical thread:


    From the thread:

    Last edited: Jun 16, 2019
  10. NE1U

    NE1U Ham Member QRZ Page

    I was assuming heliax, so no braid.

    I am not completely comfortable with the wording in this, but close enough for hams. The bazooka balun might be the easiest to understand although it is the most difficult to implement.
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