How can I accurately measure RF impedance on a wire with a glass coating?

[BMOC]

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.

 

[berkeman]

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.

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?

 

[The Electrician]

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.

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.

 

[yungman]

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.

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.

 

[alphysicist]

There are a few different methods for measuring RF impedance, but one commonly used approach is the Reflectometer method. This involves sending a known RF signal through the wire and measuring the reflected signal, which can then be used to calculate the impedance.

To do this, you will need an RF signal generator, an impedance bridge, and a detector. The first step is to connect the RF signal generator to one end of the wire and the detector to the other end. The impedance bridge is then connected between the RF signal generator and the wire.

Next, the RF signal generator is set to the desired frequency (in this case, the RF range) and the impedance bridge is adjusted until the reflected signal is at its minimum. The settings on the impedance bridge can then be used to calculate the impedance of the wire.

In terms of the glass coating, you could try using a known load across the dielectric as you mentioned. This would involve connecting a known resistor in parallel with the wire and measuring the impedance again. If the impedance changes, it could indicate an issue with the thickness of the coating.

Ultimately, the best method for measuring RF impedance will depend on your specific setup and equipment. It may be helpful to consult with a colleague or expert in the field for further guidance.