Using RF to measure a wire length change

[with2ls]

I need to sense a change in length of a single wire in free space. I only have access to one end of the wire. TDR would be a great option, but this is not a transmission line.

I don't need to measure the length accurately, I just need to know that it changed from one length to another length (imagine a switch part way through it that opens and closes).

It might be possible to treat the wire like an antenna and continually measure S11, which should change if the length of the wire changes. Does that seem feasible?
But, I am a little concerned about this option because impedance is not control before it enters free space (non-rf connectors, multiconductor cable shared with other signals, etc.).

Any other idea or recommendations would be greatly appreciated.

Thanks!

 

[betwixt]

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.

 

[KlausST]

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

 

[vfone]

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.

 

[with2ls]

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).

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.

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.

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.

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.

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

 

[JonathanC]

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.

 

[mtwieg]

this is not a transmission line.

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

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).

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.