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I wonder about the situation when we apply AC voltage or RF voltage on coaxial cable. Typically, the lateral dimension of coaxial cable is smaller than the wavelength of electromagnetic field in such situation due to relatively low frequency. How this situation reconcile with quantum confinement of electromagnetic field? For optical fiber for example, the core size has to be comparable to the wavelength of light. I wonder this could be related to surface plasmon mode coupled between signal line and shield of the coaxial cable.

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  • $\begingroup$ The key difference is that optical fiber doesn't have a center conductor. Like optical fiber, microwave waveguides also have a minimum signal frequency to permit propagation. $\endgroup$
    – The Photon
    Commented Jun 24, 2022 at 19:45
  • $\begingroup$ Thank you for answering. I understand that microwave waveguides with hollow have minimum frequency. What I wonder is the coaxial cable with a center conductor, and how this helps to confine microwave (or even lower frequency electromagnetic field) smaller than it's wavelength scale. $\endgroup$
    – quasiman
    Commented Jun 24, 2022 at 20:45

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