Measuring common mode current

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W4LKR, Aug 5, 2018.

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  1. W4LKR

    W4LKR Ham Member QRZ Page

    I'm interested in adding a balun to a portable dipole and doing some before and after measurements to confirm there is a reduction in whatever common mode current was present on the coax shielding before the balun was added.

    There are some kits and plans online for building a clamp-on RF ammeter, but I have enough trouble finding time to build an antenna, so I'm not looking to DIY the meter itself.

    Will something like this work for this application?
  2. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Check out MFJ:)
    I see they have a couple of clamp on types.
    If your dipole is a low impedance feed and balanced you might need something pretty sensitive.
    I use mW power meters and a home brewed transformer because the currents are low in my applications. I am only looking for relative changes for the most part as well. My tranformer is just a toroid with a number of turns directly connected to a BNC connector and to a low power HP sensor and meter. I also have a pair of transformers mounted on a copper-clad PCB for testing on balanced lines. 73
    Last edited: Aug 5, 2018
  3. WR2E

    WR2E Ham Member QRZ Page

    Highly unlikely, those are made for 50/60 Hz. It probably won't respond at all for RF. If it does respond it won't be accurate.
  4. N3DT

    N3DT Ham Member QRZ Page

  5. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    K7TRF likes this.
  6. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    BTW, the toroid is large enough to pass thru a BNC connector on RG-58.
  7. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Why a VNA? I just need a scalar measurement. 73
  8. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    I believe you have confused measuring coaxial air choke Impedance and external Common Mode Current. Neither an antenna analyzer like the AA600, nor a VNA can measure common mode current on the outside of a coax shield/braid.
  9. K4VLF

    K4VLF Ham Member QRZ Page

    I always use a $12 rad shack field strength meter. Just tx a continuous waveform, and hold the FS meter near the coax.

    To prove that ferrite beads work very well, measure on the TX output cable, and then on the antenna side of the bead.

    If the bead shows improvement (low RF field), then move the bead as close to the antenna, and check RF field again. on the TX output cable.
  10. AI3V

    AI3V Ham Member QRZ Page

    It's much better to move the meter along the entire length of the transmission line and graph peak and null amplitude and position, before adding choke and after.

    There is a very high probability you are only moving the common mode current peak.

    K0OKS likes this.

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