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How To Measure RF Impedance?

  1. Oct 16, 2012 #1
    I'm trying to determine the best method of measuring impedance from a wire through it's glass coating to determine if the thickness of the coating is correct. At the moment I do not have the actual frequency, I just know it is in the RF range and that my impedance needs to be 50 ohms. I've seen some people talking about putting a known load across the dielectric, but it doesn't make a lot of sense to me. If somebody could point me in the right direction (or explain the aforementioned load across the glass) it would be greatly appreciated! Thanks!

    EDIT: I do not have the ability to hook the wire up into the circuitry it will be used in (and seeing the RF power). I only have the wire.
  2. jcsd
  3. Oct 16, 2012 #2


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    Staff: Mentor

    Welcome to the PF.

    For there to be an "impedance", there needs to be a 2nd conductor (at least for low impedances like 50 Ohms). Is this twisted pair, or coax, or some other type of wire pair? Can you post a picture? What tolerance do you need to determine the impdedance (coating thickness) to?
  4. Oct 16, 2012 #3

    The Electrician

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    Gold Member

    You need to be much more specific. Imagine you were trying to help if you didn't already know anything about the situation.

    How long is the wire? What is its diameter? Is the glass coating like the dielectric coating on the center conductor of a coaxial cable? Is this wire plus its coating intended to be the center conductor plus dielectric of a coax cable? If not, just how will it be used?

    When you say you need to measure the impedance "...from a wire through its glass coating...", is there something on the other side of the glass coating? Something metallic? What exactly is its shape and size? What is the physical configuration with respect to the wire? At what frequency is the impedance being measured?

    Maybe you know all these things because you've been working on the project, but when you ask for help on a forum like this, the people you're asking don't know a thing about your project; you've got to tell them the details.
  5. Oct 17, 2012 #4
    First you have not given nearly enough info. Berkeman is correct to point out that you need to explain what kind of transmission line you are working with, is it a coax, twisted pair or others? You cannot look at it as one wire, that does not mean anything. So first you have to specify what is the structure. Also RF can vary widely, you need to be more specific.

    Assuming your tx line structure can support the frequency you want to work with. If you want to measure the impedance of the structure you have already. Using a network analyzer is the best way.
    Last edited: Oct 17, 2012
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