RG-62 as a vibration probe.

There are tests occasionally done of that sort, though. I recall Belden makes a "low microphonic" type which claims something like 20dB less noise under their testing conditions than their regular model. Maybe today's regular coax is yesteryear's low noise...

Tim

-- Deep Friar: a very philosophical monk. Website:

I was approached today for a general question. Apparently our business has an old customer that got a few thousand feet of RG-62 in the past for making vibration/movement detectors and using rungs of RG-62 coax as the sensing probe.

This customer purchased a few more thousand feet recently and for what ever reason it no longer has the characteristics it once had and causing the devices to no longer work as they should. I guess the sensitivity changed dramatically?

Now, this last lot of coax of course passes all the R.F. transmission line test as it should but they are using it differently. I'm thinking it must be the spiral thread that supports center conductor.

I was able to find a piece of old stock and compared it to the last lot that was made and all I can find that is different is the old stock was loose in the center where the thread wrap is against the tubing wall sitting loose, with the new stock it is rather snug on the tubing wall.

So the question was put to me as to how the customer was using this as a probe of this nature and all I could think of at the time was maybe a capacitor probe attached to a front end of a JFET. The loose movement inside causing variable charge values.

It's not in our business to have this kind of customer support and the customer does understand that this coax was never designed for such systems in the first place, other wise, our Q.C. and manufacturing specs would specify a test for it.

What's your take on this? Has RG-62 been used for vibration and movement probes that you know of ?

Jamie

Two techniques to use cable as a sensor. One is high impedance amp and measure the piezoelectric voltage. The other is to build what amounts to a time domain reflectometer, where you sense the impedance change where the cable is deflected.

I'm not 100% sure of this, but I think the TDR technique was eventually upgraded to fiber optic cable. The idea is you want to make your perimeter detection difficult to detect. Every measure has a countermeasure, which fortunately keeps a lot of engineers in the dough!

Sandia Labs did a lot of research on these kind of things.

I built an amphometer years ago using coax. IIRC it sensed the change in capacitance.

Years ago, I worked on systems that used plastic tubes, filled with water, with acoustic sensors at the end. Two tubes, about 6 ft apart made the boundary. If both tubes got the same signals the system ignored it, as it was outside the area. If one tube was significantly higher than the other, the system then knew which side the intruder was on, and sounded the appropriate alarm, alerting the guard of the attack location and direction (someone breaking in or someone breaking out...)

No metal, very hard to detect and very hard to defeat. Tubes were buried a few inches below the soil, nothing was allowed to grow in the area as that would cushion the footsteps.

ion

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The vibration microphone is based upon the triboelectric effect - the bane of analog cables. All cables display the effect to some degree. 'Microphone' cables are PURPOSELY made poorly. The center conductor is so loose, you can grab 1 inch sticking out of a 20 foot length and simply pull out the whole conductor! Plus, usually made with teflon insulation which has a huge surface electron work function, which greatly enhances the triboelectric effect.

I thought millivolts came out of this stuff, laid an 8 inch piece on the bench, attached a scope to it, and tapped the side of the cable lightly got an 8 VOLT SPIKE out of it!!!!

Where to get? Can't remember. Perhaps Stellar Security, Sunnyvale, CA.

But, *IF* you can't pull that center conductor out of a 100 ft piece, it isn't microphone cable.

Actually, it's the multiple reflections as the cable is moved creating sidebands, invented by one of the Parker twins. Don't know if he got credit or not.

Nothing was allowed to grow there to prevent the roots from pulling at the soil during a rain/wind. Root motion looks like someone walking around, too.

Back in the '90s, my company was working on making fiber optic weigh stations for trucks, where the truck would roll across a rubber strip with the fiber inside. They measured the deflection of the fiber and could determine the vehicle weight from that. Worked terribly! The rubber changed characteristics drastically as a function of temperature. In winter, you could drive a bulldozer over it and not get any signal... 8-)

Charlie

Fibre bend sensors are horrible even in good conditions. The calibrations aren't stable and the response is usually badly nonlinear.

Cheers

Phil Hobbs

I seem to recall the original nuclear testing in Nevada used coax reflections to measure the shock wave speed away from the "epi-center". ...Jim Thompson

Yep, that was what they finally discovered, after throwing a lot of money at it for about two years... ;-)

Charlie

That's a good approach if you need spatial resolution, for sure. They did a lot of cool stuff in the bomb projects, as well as a lot of really strange stuff. The megaton race stopped after the Soviets detonated the "Tsar Bomba", which was supposedly 100 megatons. All that did was to make a normal (!) megaton-sized mess--the extra yield just lifted a section of atmosphere out into space.

Cheers

Phil Hobbs

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think I saw a tv show about it was supposed to be 100megatons but they turned it down to 50 to reduce the fall out so it turned out very "clean" compared to the bang it made

wonder how much mess all those trials made compared to Chernobyl/ fukishima

-Lasse

I seem to remember something about the fiber along railroad tracks some how connected the track that they can not only transfer data, but via the fiber being bend by the passing train they can also measure where the train is along the line

-Lasse

What is that? My dictionary does not have that word (amphoteric is the closest).

Rumor has it that large parts of Russia are contaminated with nuclear waste. Probably much worse than what was caused by Chernobyl and Fukishima together.

Ummm...what do you mean by "rungs"?

=20

=20

The original coax perimeter protection used a "leaky" cable bistatic radar = mimetic. The cable is laid as a parallel pair, RF is burst into one cable a= nd received by leakage coupling into the second. An intrusion changes the c= oupling in amplitude and phase allowing relatively precise determination of= the location of the intrusion. This is a good system for large area protec= tion where it is not enough to know only that an intrusion has occurred, bu= t, because of the extent of the area, it is also necessary where it has occ= urred.