Transmission lines and permeability

Discussion in 'Feedback and Suggestions' started by Unregistered, Oct 30, 2008.

  1. Unregistered

    Thread Starter Guest

    The chapter on characteristic impedance indicates that the permittivity
    (dielectric constant) is important for the impedance and velocity in a
    transmission line, but doesn't mention permeability.

    As I understand it, there are lines with high permeability materials
    around the center conductor to increase the inductance.
  2. steveb

    Senior Member

    Jul 3, 2008
    Your question is good because you obviously understand that wave speed and impedance depend on both permittivity and permeability. Generally, characteristic impedance is important for wave transmission in vacuum, air, and other dielectric materials; in free space, or in waveguides and transmission lines. Traditionally, transmission lines do not use magnetic materials, so permeability is that of free space. So, the chapter is just noting that the permittivity is what is changed using different dielectric materials.

    Magnetic materials are used in inductors as core materials to increase inductance, or on wires as ferrite beads to attenuate unwanted signals. As far as I know, they are not used on transmission lines because they would induce loss.
    Last edited: Oct 30, 2008
  3. Dave

    Retired Moderator

    Nov 17, 2003
    Thanks for the feedback. Is there a specific part of the TM-line sections that you are referring to?

    I agree with what steveb has said; the permeability is a factor, but materials with permeabilities appreciably different from that of free-space are not typically used in physical TM-lines - it is omitted (for want of a better word) from the discussion for simplicity and clarity of the core message even tough the relative permeability is a factor in the series inductance and hence characteristic impedance of the TM-line.

    If anyone has information counter to this then please feel free to post it up and we can investigate further. I also helps if we have the link to the relevant section in the e-book so we can consider it in context of the material.