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Building Coaxial transmission line on PCB?

Discussion in 'Electronic Design' started by Geronimo Stempovski, Feb 12, 2007.

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  1. As regulated as a dead short can be. ;-)

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  2. CBFalconer

    CBFalconer Guest

  3. John  Larkin

    John Larkin Guest

    Still got that anal/fecal thing going, I see. I'm sure glad I wasn't
    born with a fetish like that.

  4. John  Larkin

    John Larkin Guest

    Suppose you did an FR4 pcb with a wide microstrip on the top, then
    route out most of the material below the trace, namely route a trench
    under the trace, from the opposite side, say 90% of the board
    thickness. Now we'd have a sort of suspended substrate trace, much
    wider for a given Z, with much lower losses. We could call it

    -------------- copper trace
    ================== ================= epoxy-glass
    ================== air =================
    ------------------ ----------------- copper gnd

    I haven't a clue how to calculate the impedance; field solver, like
    ATLC maybe?

  5. MassiveProng

    MassiveProng Guest

    Well, you're wrong, and that makes the fetish you were born with
    insisting on being stupid, and then showing your ass to the rest of
    the world.
  6. John  Larkin

    John Larkin Guest

    I keep my posterior components discretely covered in public.

    And I'm as smart or a stupid as I was born, so am blameless in that
    respect. Luckily, the world seems to want my brand of stupidity, so it
    all works out. How is your brand of stupidity holding up?

  7. MassiveProng

    MassiveProng Guest

    Like I give a fat flying **** if you read my posts.
  8. It is the imaginary part of the dielectric constant. I believe that
    can be described as a loss tangent, though I haven't done that.
    Yes, heat. You can consider it as electrical friction.
    I don't believe that is true. It may be that coax tends to use better
    dielectrics. Other than that, even if the wave isn't "captured" as you
    say, as long as it doesn't find anything else to induce current into, it
    doesn't contribute to loss. (Well, twisted pair cable is twisted to
    minimize the radiation. Microstrip isn't twisted. Radiation will be
    mostly in the plane of the board, and minimized by keeping the board
    thin.) The very low loss coax cables use a spiral dielectric such that
    most of the space is air and very little is plastic. There has to be
    enough material to hold the center conductor in place.

    The other loss increase with frequency is due to the skin effect, where
    the current travels in a thin layer on the surface of the wire,
    increasing the resistance as seen by the current. I believe FR4
    is chosen for its thermal and strength properties, and ability to bind
    to metal, all not normally required by a coax cable dielectric.
    (And also cost.)

    -- glen
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