• Admin
    150
    avantec created the topic: Characteristic Impedance of Signal conductor

    Trying to find a close form solution for the characteristic impedance of a signal conductor of .125 " with two ground conductors an inch apart on either side of the same diameter in air. Does anyone know how I can calculate it easily.

    Brian Lee
  • Admin
    150
    Desert Sage replied the topic: Characteristic Impedance of Signal conductor

    I found this that might be useful: http://www.fcarc.club/dokuwiki/lib/exe/fetch.php?media=estimating_characteristic_impedances.pdf

    It shows for two wires, Z=276 x Log (D/r), where D is center to center spacing, r is the wire radius. For your configuration I calculate 214.7 ohms. When you add the second ground wire the impedance will be halved or 107.4 ohms.
  • Admin
    150
    avantec replied the topic: Characteristic Impedance of Signal conductor

    Thanks, for response. I used that formula for my transmission line configuration but was uncertain of how to handle the second ground configuration in my other probe. I am in the process of trying to measure it using TDR. Are you sure it is halved? Brian
  • Admin
    150
    madengr replied the topic: Characteristic Impedance of Signal conductor

    If the two wires are wide apart to begin with (i.e. high Z0 and the mutual capacitance <~ self capacitance), then it does not quite cut in half when adding the third wire. If they are already close together (i.e the mutual capacitance >> self capacitance) then it does. Try it with a microstrip calculator, then switch to stripline, keep decreasing the substrate thickness, and it approaches Z0/2.
  • Admin
    150
    Desert Sage replied the topic: Characteristic Impedance of Signal conductor

    This boils down to the assumption used in designing combline filters based upon Cristal's curves. The charge distribution on one half of the conductor is assumed (to first order) to be unaffected by what is happening on the other side. It seems to be a relatively good assumption since I have designed many, many combline filters and never ran into a counter-example.
  • Admin
    150
    madengr replied the topic: Characteristic Impedance of Signal conductor

    What are Cristal's curves? I'll have to look that up.
  • Admin
    150
    Desert Sage replied the topic: Characteristic Impedance of Signal conductor

    "Coupled Circular Cylindrical Rods Between Parallel Ground Planes", Edward Cristal, IEEE-MTT July 1964 pp 428-439. These curves were used mostly to design combline filters but they can be used to design couplers (three rod dual directional for example). This paper is the logical follow up to Getsinger's paper (Coupled Rectangular Bars Between Parallel Plates, Jan 1962, IRE-MTT-10, pp 65-72).
  • Admin
    150
    madengr replied the topic: Characteristic Impedance of Signal conductor

    Thanks:

    ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1125843
  • Admin
    150
    avantec replied the topic: Characteristic Impedance of Signal conductor

    Hi Desert Sage,
    It's me again. I am trying to find out how far the fields extend laterally for the two configurations of probes I had those questions on 7 months ago. Someone from Berkeley Labs wanted to know exactly how far the fields would extend in different mediums, soil and water etc. I remembered in college that about three radius lengths outside of the probes would suffice to encompass about 99% of the fields, but now I am uncertain as I can't find the exact topic anywhere. Do you happen to know if there is closed form solution for this parameter and its dependence on the dielectric constant of the medium? Sincerely, Brian
bold
italic
underline
strike
code
quote
ulist
image
url
mention
reveal
youtube
tweet
Add a Comment

Welcome!

Join the international conversation on a broad range of microwave and RF topics. Learn about the latest developments in our industry, post questions for your peers to answer, and weigh in with some answers if you can!