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Voltage induced into coax by external electric field.

  1. Oct 12, 2011 #1
    I want to verify the mechanism to which a varying E can induce voltage into a shielded coax.

    According to Maxwell

    [tex] \nabla \times \vec E = -\frac{\partial \vec B}{\partial t} \;\hbox { and }\; \nabla \cdot \vec E = \frac{\rho_{free}}{\epsilon}[/tex]

    From this, with varying E, you induce varying charge density onto the outer shield which create a varying current onto the shield. But the inner conductor is partially shielded by the shield don't see as much E so the current induced is not as much. Therefore there is a difference in the two currents which create the voltage into the coax.

    At the same time, with a 200V/m varying E there MUST be B associated with the E by the Maxwell

    [tex] \nabla \times \vec B = \mu\vec J +\frac{\partial \vec E}{\partial t} [/tex]

    But B induce equal current in both the inner conductor and the outer shield. This is common mode and don't matter.

    Is this the mechanism?
  2. jcsd
  3. Oct 13, 2011 #2
  4. Oct 13, 2011 #3
    Thanks for the reply. My theory is the copper shield and the inner conductor are both none magnetic and [itex]\mu[/tex]=1. I am using:

    [tex]\int_S \vec B \cdot d \vec S= \mu I[/tex]

    and this is true for both conductors and this is common mode and don't effect the coax. I don't look at this as shielding. As the article said, the shielding don't even work because it is non magnetic material.


  5. Oct 15, 2011 #4
    There are three common things I've seen:

    1) Shields are typically 90+% shielding and not 100%. Thus, a nearby influence can get in.
    2) If the source and destination of the signal have a source of noise current, this travels along the outside of the coax, which isn't of itself an issue. However, at the ends of the coax, a potential can arise when the center portion seperated from the shield. This happens a great deal when taking high frequency oscilloscope readings.
    3) Many pieces of equipment simply don't like being joined at the hip when inteferring noise is present. Try connecting them on the same AC outlet.
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