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I do not understand how a particular parasitic effect is model. I have read that on a coaxial cable there is a problem due to the fact that the external conductor's magnetic field crosses the air around it. This can be represented by a parallel inductance, as shown in the following figure.

enter image description here

But I do not understand the physical phenomenon (and so why we put an inductance in parallel). A part of magnetic field lines go in the air, and so? What does it happen physically?

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  • \$\begingroup\$ Any basic level transmission line analysis is based on the assumption that core and shield currents are just the same and hence cancel any outside magnetic field. The inductance in your sketch is trying to model the effect of any unbalanced current which, flowing either in the core or the shield, will instead generate external magnetic field. It could be arguable wether an inductance is a good choice, sometimes it is, but more generally it can be done by a common mode transmission line with its characteristic impedance \$\endgroup\$
    – carloc
    May 3, 2019 at 18:32

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This is the graph showing the magnetic field induced from current flowing on the outside of a hollow cylinder (coax): enter image description here Source: http://www.ittc.ku.edu/~jstiles/220/handouts/Example%20A%20Hollow%20Tube%20of%20Current.pdf

It also happens current flowing on the outside of coax also has the same behavior as a wire. All wires have inductance. It comes from the magnetic field inhibiting the flow of current. Current flowing through a wire transfers energy to "create" the magnetic field around the wire (or outer conductor of coax), just like wire does in a coil.

If you have current flowing on the outside of the coax, this should be modeled as inductance in a simulator.

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  • \$\begingroup\$ Ok, but I have another question. Why is this phenomenon modeled only with an inductance? I think it should be modeled by a series between the inductance and the air resistance (and all the series is in parallel to the external conductor of the coax). In fact, if we consider only an inductance, we have that at low frequency it is a short circuit, that will mean that current flows only on air, and this seems quite strange to me. \$\endgroup\$
    – Kinka-Byo
    May 5, 2019 at 18:47
  • \$\begingroup\$ You could model the capacitance of air but it would only be around a pF and would be negligible for most analyses, the resistance of the shield could also modeled. Must of the energy in AC transmission lines is found outside the cable, they act as waveguides \$\endgroup\$
    – Voltage Spike
    May 6, 2019 at 4:40
  • \$\begingroup\$ The current in the shield does not for though air, it flows through the conductor and creates the magnetic field \$\endgroup\$
    – Voltage Spike
    May 6, 2019 at 6:13
  • \$\begingroup\$ But if we model this phenomenon by inserting the inductance of the air, like in my initial scheme, there will be a leakage path for current through the air, and I do not understand this \$\endgroup\$
    – Kinka-Byo
    May 6, 2019 at 6:52

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