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routing coax through strong DC magnetic fields

Discussion in 'Electronic Design' started by Tracy Hall, May 27, 2005.

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  1. Tracy Hall

    Tracy Hall Guest

    I understand that coaxial cable does not radiate much energy because
    symmetric opposing current sheets in the outer skin of the center
    conductor and the inner skin of the shield essentially balance each

    Suppose I route coax through a strong DC magnetic field, such as in an
    MRI, with the magnetic field perpendicular to the cable axis. Will the
    current distribution in the center conductor then become biased "up"
    with respect to the field (right hand rule), and the current
    distribution in the shield then become biased "down," thereby
    destroying symmetry?

    If so, will the coax then radiate and become lossy?

    Can anyone point me to an analysis of this problem?


    Tracy Hall
  2. Tim Wescott

    Tim Wescott Guest

    I think the signal in the coax and the DC magnetic field will obey
    superposition and that the coax will stay shielded as well as it would
  3. Ken Taylor

    Ken Taylor Guest

    Can't point you to any studies or the like, but coaxes in our klystron amp's
    pass by the klystrons with no ill effects noted.

  4. ChuckC

    ChuckC Guest

    no, the coax fields still cancel at a distance (small)

    But most coax is between 90% and 98% covered, not 100%, so there can be
    leakage into or out of but not at DC, there is no effect on the coax (which
    is high frequency)
  5. Hal Rosser

    Hal Rosser Guest

    If you <<shake>> the coax real hard
    in the dc magnetic field,
    you probably still won't affect the coax on hf or vhf.
  6. Contrary to the common perception, an average coax
    with a wire screen is loyy anyway.

  7. W9DMK

    W9DMK Guest

    Dear Tracy,

    No, I don't believe that the "biasing" up and down that you refer to
    will cause any destruction of symmetry, because everything is linear.
    So long a linearity prevails, there will be no effect whatsoever due
    to the DC field, in my opinion.

    Bob, W9DMK, Dahlgren, VA
    Replace "nobody" with my callsign for e-mail
  8. Dave

    Dave Guest

    analyze this way...
    if the current in the coax is AC then each half cycle the electron flow
    reverses so the forces would reverse, and since the actual electron velocity
    is relatively low there would not be enough time for the electrons to really
    move before the current reversed and they had to go the other direction. so
    i would say that it would be unlikely to have any effect on coax carrying
    AC. now, if you go to very low frequencies where the electrons have a
    chance to move you might be able to measure something, but since the
    wavelength increases with the lower frequency the relative size of any
    imbalance becomes smaller. and essentially if it did completely separate
    you still end up with balanced currents but in a twin lead arrangement and
    you still have currents that cancel at any distance away from the line.
    what would be more interesting is to calculate the torque on a piece of
    twinlead carrying dc in a strong field.
  9. Tim Shoppa

    Tim Shoppa Guest

    Suppose I route coax through a strong DC magnetic
    I think you're distorting the Hall Effect here. One side of each
    conductor will have a positive charge and the other side a negative
    charge, and there will be a potential across the shield and across the
    inner conductor. But at the ends of the cable these effects are gone.
    No. Superposition of fields. Even for an AC magnetic field.

    If you have an alternating AC magnetic field you can induce a current
    in both the shield and inner conductor, but these will match. If
    you've got ground loops you may misinterpret this as a signal. Don't
    have ground loops.

  10. Phil Hobbs

    Phil Hobbs Guest

    To leading order, the superposition argument is right, but it assumes
    that the current distribution in the coax is unchanged by the applied DC
    field, which is not true in detail.

    The Hall effect operates in metals, so an AC current in the shield will
    indeed cause an AC voltage across the diameter of the coax shield,
    outside as well as inside, and this will radiate as an electric dipole.

    However, the Hall effect in metals is so small that it's difficult even
    to measure it, and the electric dipole radiation from a source very
    small compared to a wavelength is weak. The resulting leakage is
    therefore (miniscule)**2, far smaller than that caused by the poor
    shield coverage and poor shield continuity.

    The shield conductors are just laid on top of each other, and since
    their contacts are neither gas-tight nor self-wiping, they are somewhat
    noisy and unreliable--we don't rely on that sort of contact elsewhere in


    Phil Hobbs
  11. Jon

    Jon Guest

    A constant (DC) magnetic field has no effect on either the current
    distribution or the voltage in a coaxial (or any other type) of cable.
  12. Ed

    Ed Guest

    I assume you only ask, in theory? First thought coming to my mind is
    what is going to happen with that coax near the MRI when it is turned on!

    Ed :^)
  13. Tracy Hall

    Tracy Hall Guest

    Thanks, all, for your helpful responses.

    The general consensus appears to be that a tiny electric dipole will be
    induced that oscillates transverse to the cable at the same frequency
    as the carrier. It might possibly leak a tiny amount of energy to the
    area immediately surrounding the coax, but probably not a significant
    amount compared to that which is already lost due to imperfections in
    the cable.

    I'm not planning any installation in an MRI, but I do have an
    application where the cable is routed through some fairly strong
    permanent magnetic fields, and I wondered if I needed to magnetically
    shield the cable to avoid substantial losses. Sounds like I don't need
    to worry.

    I suppose I could actually run a test, but it's a lot easier just to
    freeload off all your experience!

    Tracy Hall
  14. John Smith

    John Smith Guest

    I would read the manufacturers data, attempt to get a 100% braid (silver
    plated copper tubing for a shield would be GREAT! <grin>), and limit
    exposure to the fields in as much as it is possible..
    But hey, I am a natural paranoid--but that trait kept me alive when I was
    younger and carrying on with women of questionable character in cheap
    bars... <grin>

    Warmest regards,
  15. Ian Stirling

    Ian Stirling Guest

    I suppose that for largish low frequency components in the signal, you
    might also get the cable bits physically moving, both together, and in
    relation to each other, which might screw up the impedance a little.
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