Standing EM Wave on conductive bar

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Jon Slaughter 18 years ago

Is is possible to set up a relatively precise standing EM wave on a conductive bar(see below for how precise)?

I am thinking that it could be used to measure the length of such things since L = c/f. Of course this assumes knowing c in the material and the frequency to get an accurate result but I'm just curious as to if it is even possible to do such a thing?

Sounds like it would be very similar to antenna's but here I'm trying to get an accurate result. Maybe simply sweeping the frequency range and finding the resonant peak treating the bar as an antenna would work?

Just looking for some ideas on it as I've been pondering lately how to measure things accurately and this was one idea I was thinking about.

Another similar idea which probably won't work would be to setup a high frequency EM wave on the conductive bar and measure the EM field along the bar counting the peaks. (essentially the intensity plots out a ruler on the bar) Unfortunately for decent resolution one needs to use too high of a frequency. (to get even mm resolution one around a frequency of 100 GHz)

Maybe one could send a pulse like radar/sonar or something and measure the time delay?

Thanks, Jon

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bill.sloman 18 years ago

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If you want to do really precise length measurement, you have to go to really high frequency electromagentic radiation, and use optical interferometry.

Hewlett-Packard (now Agilent) and Zygo sell spectacularly precise bits of gear (though it helps if your optical path is evacuated - speed of light in air depends on atmospheric pressure and can change enough to make a perceptible difference to a really precise measurement).

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-- Bill Sloman, Nijmegen

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Jon Slaughter 18 years ago

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If you want to do really precise length measurement, you have to go to really high frequency electromagentic radiation, and use optical interferometry.

Hewlett-Packard (now Agilent) and Zygo sell spectacularly precise bits of gear (though it helps if your optical path is evacuated - speed of light in air depends on atmospheric pressure and can change enough to make a perceptible difference to a really precise measurement).

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What about my specific ideas? I know there are other methods out there but I am not so much interested in measuring something as I am about how to measure it. I know the idea of linear optical encoders, magnetoresistive devices, magnetorestrictive devices, and a few others but I have not seen any that are based on the ideas I have given and I curious as to how practical they are. (obviously they hold in theory since its just basic physics but I\'m more curious as to how precise in practice for relatively simple implementation)

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whit3rd 18 years ago

Yes, of course. It's called a 'wavemeter'. My favorite variant is a bunch of pieces of copper wire, clipped to different lengths, placed on a paper plate.

After a few seconds in a microwave oven, the one that's nearly one half wave long at 2.4 GHz is ... obvious. And there's a permanent record, too! The wavelength is dependent on both the frequency and the medium, so the paper plate dielectric properties matter.

I've also seen rabbit-ears connected to a light bulb. Same principle, really.

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Wimpie 18 years ago

Hello Jon,

When the thickness of the bar

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Standing EM Wave on conductive bar

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