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Balun, which output is ground without opening it?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by 2E0EZE, Aug 28, 2018.

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  1. 2E0EZE

    2E0EZE Ham Member QRZ Page

    Antenna rebuild time:

    My Western HF 10 having been rebuilt after eight years now has a new feeder IE 450 ohm as it should.
    The 450 ohm feeder then goes to a 50 ohm coax connection via two posts at the Balun.

    I need to know which is ground and which is center at the Balun posts since I have to slope the antenna up one end which I would prefer to be connected to center core of coax and the lower end to ground (outer braid) of coax.

    Can anyone suggest an easy way to tell the polarity please with respect to the coax?

    * Western HF 10 is a dipole with two coils toward each end with 450 ohm feeder then a balun then coax to the shack.

    Regards
    Sean
    2e0eze
     
  2. G0JUR

    G0JUR Ham Member QRZ Page

    if its a current balun your multi meter will id which is which.
    although it wont make much difference which way round its connec ted your not feeding dc into it.
     
  3. W2WDX

    W2WDX Subscriber QRZ Page

    You're going from balanced to unbalanced. There is no ground per-se on the balanced side. That's the point of a balun. It doesn't matter. Whatever connection on the unbalanced side is referenced to ground is determinant, and only for that side (+/gnd). The balanced side is equal amplitude (+/-) on each pole.
     
    KX4O likes this.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    If your Balun is working, then for RF signals, neither of the output terminals is referenced to the coax shield. One will be driven positive, while the other will be driven negative with an equal amplitude but opposite polarity, at the same instant.

    Say you connect a 450 Ohm non-inductive resistor between the two balun outputs, and drive 1Vpp into the 50 Ohm port from a RF signal generator (assuming that the generator frequency is between the lower and upper frequency limit of the balun). If you connect a two channel oscilloscope (assuming it has the required frequency response at the RF freq in question) to the two output ports of the balun, using 10X probes, with the ground clips of the probes connected to the shield of the coax, then you will see 1.5Vpp at a phase angle of 0 degrees at one of the two balun ports, and 1.5Vpp at a phase angle of 180 degrees at the other.

    Note that the voltage step-up through the balun is 1:3 which makes the impedance step-up 1:9, i.e. it steps 50 Ohms up to 450 Ohms.

    If you believe that the balun works as intended, then it makes absolutely no difference as to which balun terminal is connected to which antenna leg. That is the one of the primary reasons that the balun exists. The other is to make the required impedance step-up.
     
    2E0EZE and KU3X like this.

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