Re: [EE] measuring the magnetic field from a small trace carrying a < 50uA load?

The trick there is that a transmitter is wired to the pipe, i.e. the pipe is a crude antenna.  For plastic pipelines (like natural gas) they run a wire with the pipeline in the trench and bring it up attached and hopefully lableled for testing.  

I improvised this once with an am radio on the end of a stick, 100ft of wire with styrofoam ball floats/drag, and a bit of running water in a drain pipe (to pull out the wire being fed in to a cleanout).  For a transmitter I just used one signal generator to sweep another across the AM band, the broad band fm is similar to am from the receivers point of view.  It worked well within 2-3 feet for a line buried about twice as deep, giving a better idea along most of the length (we were just trying to avoid it with a satelite tv coax).  I think normally higher frequencies are used.  This was off hand in the forest trying to trace the line to a septic tank.  Considering the amount of clay it did OK.  I wound up just running it across the driveway in a small steal pipe.  That packed clay and sand is possibly more annoying than concrete to get through, and it goes deep.  If you're going to try to dig in a glacial valley, you really won't like it!

The real problem was that a glacier had packed clay and sand into something a ditch witch had trouble dealing with, as did a human with a pick axe and shovel.  And this was a narrow trench.  I'm sure the preexisting well was very challenging.  Not to mention this was near Colorado Springs, home to the U.S. missile command (and the air force academy, space command, etc., right next to look out mountain!) so I'm sure it was noticed, those guys are always hunting for stray transmissions even miles and miles away from the base.  At least it should have been clear that my signal was not carrying any information, and it was somewhat brief, I'm sure if I'd done it for days I would have been talking casually at least to some one, if it wasn't too week to even be detected which it probably was (short term).  I've actually seen air force guys in a pickup with a camper shell on it and various antenna on the roof they use for tracking signals down, non-descript pickup but it does have air force plates, and two guys in air force uniform inside (I was at a gas station, basically in the middle of no where in Wyoming and far from bases).




"Human beings are the only creatures on earth that claim a god, and the only living thing that behaves like it hasn't got one.  Does the world belong to no one but you?"  The Lobster in "The rum diary"


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On Monday, January 6th, 2025 at 4:28 PM, Chuck Olson <[email protected]> wrote:

> Martin -
> 
> I remember during a summer job in college working with a ditch digging
> crew - it was mostly standing around watching the guy with the
> excavator work and then waiting for the sod delivery.
> 
> But before the excavator could dig there was a gas company guy out
> marking the pipeline - he had on headphones attached to a search
> thingy - but thinking about it now, how did he get a signal from a gas
> pipelne ? I'll have to do a search on that :)
> 
> Chuck
> 
> On Mon, Jan 6, 2025 at 4:40 PM Martin McCormick [email protected] wrote:
> 
> > Chuck Olson [email protected] writes:
> > 
> > > Martin -
> > > 
> > > You are correct, I should have said something about moving the head
> > > across the conductors, cassette tapes moved less than 2 inches per
> > > second so the head wouldn't have to be moved super fast ?.
> > > 
> > > The thing I've noticed with Hall effect devices was that they are
> > > really noisy, 50 uA probably not going to give much of a signal above
> > > the noise.
> > > 
> > > Chuck
> > 
> > I figured it was something like that plus one would have
> > to have some sort of low-pass filter because 60 or 50 HZ magnetic
> > fields are going to be common due to all the transformers and
> > motors in our world.
> > 
> > I was once part of a group of people who dug a hole for
> > the base of an amateur radio tower and the neighborhood in
> > question had buried power and telephone lines. A fellow from the
> > power company came out and had a portable detector which was
> > probably inductive but had a speaker so you could hear the hum of
> > the 60-HZ magnetic field around the underground power cable. He
> > marked the ground across the yard, following the buzz so we knew
> > where not to dig.
> > 
> > Too bad the telephone line which was also buried near
> > didn't radiate much because we only found that out when we came
> > across one heck of a tree root and, after the shovel cut it, the
> > man's wife who had been on the phone and wasn't too keen on this
> > whole deal came out to inform us we had cut their telephone wire.
> > That root had a lead shield and 4 black wires. Ah, what they are
> > doing with recombinant DNA these days.
> > 
> > We patched the cable temporarily so the phone worked
> > again and, I guess, AT&T fixed it right a few days later. I'd
> > much rather accidentally cut a phone wire than 200 amps at 240
> > volts phase to phase. An analog telephone line pulls a few
> > milliamps DC when a conversation is going on plus a little more
> > if it rings but in the year 1975 when this all happened, there
> > wouldn't have been any ADSL carriers or anything much on that
> > line to radiate through the few feet of soil to make precise
> > location easy. When a phone is on the hook with no conversation,
> > there is 0 DC current being drawn. It would have taken a pretty
> > good metal detector to find that line. I digress.
> > 
> > Martin
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