Using RF to measure a wire length change

TDR might still be an option but what lengths of wire are you considering. Could be miles/Km or very short.
Another possibility if the length isn't too long is to sweep frequency and look for resonance.

Brian.

Hi,

to measure "change of length" .. it means "change of electrical characterisitics".

But electrical characteristics also depend on meachanical parameters.
The electrical charactereistic may change if the wire is in free air .. or in water, or just is laying on a conductive surface, even if properly isolated.
The electrical characteristic will change if the length is just sraight or if it is coiled
The electrical characteristic may change if the wire is in free air in space or in a highly reflective room.

Thus you first need to decide which parameters can be seen as fix and which parameters are variable.

Klaus

I do have a simulator that have a wire over dielectric model. Set the wire to 10m length and 10m above the ground, dielectric air, I get S11 = -1dB for a short to the ground at the end of the wire, and about -20dB when I use a 50 ohms resistor. The simulation frequency is set to 100kHz. For longer wires have to decrease the frequency, which makes hard to find a proper VNA.

I appreciate everyones' responses.

A few other details I neglected to mention:
1. The total wire length (before breaking) can be up to 12 feet (4 meters). It will also be coiled (sorry, that was an important detail).
2. The wire will be moved around and near various materials (metal, plastic, wood).

betwixt said:

TDR might still be an option but what lengths of wire are you considering. Could be miles/Km or very short.
Another possibility if the length isn't too long is to sweep frequency and look for resonance.

Brian.

Click to expand...

See my answer regarding wire length above. I tried running it though a TDR, but couldn't get any conclusive results. I'll looking for resonance with a VNA again.

KlausST said:

to measure "change of length" .. it means "change of electrical characterisitics".

But electrical characteristics also depend on meachanical parameters.
The electrical charactereistic may change if the wire is in free air .. or in water, or just is laying on a conductive surface, even if properly isolated.
The electrical characteristic will change if the length is just sraight or if it is coiled
The electrical characteristic may change if the wire is in free air in space or in a highly reflective room.

Thus you first need to decide which parameters can be seen as fix and which parameters are variable.

Click to expand...

You're definitely correct. See me responses above, what I know is fixed is it will always be in air. Basically the wire contains a switch the opens when part of its mechanical apparatus comes in contact with something.

In general, environmental factors are also a concern of mine, but I wanted to see if I could get something working in a fixed environment first.

vfone said:

I do have a simulator that have a wire over dielectric model. Set the wire to 10m length and 10m above the ground, dielectric air, I get S11 = -1dB for a short to the ground at the end of the wire, and about -20dB when I use a 50 ohms resistor. The simulation frequency is set to 100kHz. For longer wires have to decrease the frequency, which makes hard to find a proper VNA.

Click to expand...

I don't have a way to ground the wire at the end of it.

if you have an VNA you maybe you can measure the phase or group delay at a specific freq and then transfer it back to the actual length. Measuring S11 doesn't work for your case.

with2ls said:

this is not a transmission line.

Click to expand...

Certainly every conductor has transmission line behavior (at a high enough frequency), even in a vacuum.

with2ls said:

1. The total wire length (before breaking) can be up to 12 feet (4 meters). It will also be coiled (sorry, that was an important detail).
2. The wire will be moved around and near various materials (metal, plastic, wood).

Click to expand...

If the wire passes near materials or media with differing conductivity/permittivity/permeability, then that will have a confounding effect on the "length" measurement. Same for the path of the wire deviating from a straight line. Coiling the wire will dramatically change its apparent length.

Basically unless you can constrain many aspects of the wire (its path and/or the media nearby) it's impossible to determine its length.