Gopher(s) ate buried Radio Dog Fence - several thousand feet - need scientific method to locate break

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I installed a 2-3 acre section of undergound electronic Radio Dog Fence several years ago. On those several acres are several thousand gophers.
Today one of those thousand gophers ate one of those thousand feet of wire and I'm getting the fault alarm.
The radio fence manual is rediculous. It says to start at the half way point and patch back to the control unit and keep doing it until the broken section is located. That would take me several years and several thousand feet of patch cords. Beside that I can't pinpoint the flags (gophers ate those too!)
I want a scientific method of pinpointing the break however I don't want to invest $1000's in a TDR (time domain reflectometer).
I do have some old signal generators and I heard there's an AM radio method.
Anyone have any tips at locating the break. The dog doesn't really need it anymore but I want to preserve it for future generations.
TIA
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interesting question
is the wire a single insulated conductor or is it some kind of shielded or multiconductor cable?
do you have acess to both ends and the break is in the middle of the loop someplace?
i guess you could connect an AM signal gen to eac h end and follow it with a portable AM radio
Mark
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All the electronic dog fences I've ever seen use a single insulated conductor.

Obviously he has access to both ends -- they're connected to the control unit.

He already has an AM signal generator connected to each end -- the control unit. All he needs is a radio, as I described in another post.
--
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Doug Miller (alphageek at milmac dot com)
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This should work of course, but it would be best to capture the gopher responsible and make him show you where he committed his crime.
Doug Miller wrote:

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Mike Berger wrote:

.........and sue the little bugger for damages.

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On 8/26/2005 11:29 AM US(ET), Mike Berger took fingers to keys, and typed the following:

Rub his nose on the broken wire? Or, put collars on all the gophers and put them outside the perimeter, so they won't come back onto the property.

--
Bill


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The other day while jogging I passed an electrician truck, with the guy there too retrieving some part -- and I started a conversation.
I was asking about ground-rods, and how you could tell if the cable going to them was perhaps open -- and that led to discussing longer wires underground that might have a break.
He said there was some kind of device they could attach to *one* end and it could tell you how many feet down the wire the break was.
He didn't know how it worked, he said.
My probably-incorrect guess is that it broadcasts some microwave frequency down the wire, and vary the frequency transmitted and see if you can get a resonance -- and do that for several frequencies (relatively prime to each other? -- I make this up as I type it in) and if there is some cheap computer hooked to it, maybe it -- well -- tries to disambiguate how long the resonating part is????
Something like a physicst or musician sending a continuous tone in one end of eg an organ pipe that's got a blockage somewhere, and by tuning the sound for a singing-in-the-shower kind of resonance, dope out where the blockage is?
(Assumes that they're blind, or have no flashlights, and no long pieces of wood to stick down until it hits the blockage ... :-) )
Oh well, it was a try.
David
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On Mon, 19 Sep 2005 05:55:54 +0000 (UTC), snipped-for-privacy@panix.com (David Combs) wrote:

It's called a Time Domain Reflectometer or TDR.

That's surprising, as one has to enter some parameters to get it to work correctly and that generally requires some degree of knowledge of how the process works.

No, the TDR works like radar. It fires a fast rise time pulse down the wire and times the reflection and measures the polarity. The reflection is caused by a change in the characteristic impedance of the line, such as a short or open. The time is the round trip travel time. If the wire's velocity factor is known (one of those important parameters), the instrument can turn the time interval into distance. The polarity of the returned pulse indicates the type of fault - same polarity is a short and reversed polarity is an open. The best instruments even look at the reflected pulse's amplitude and can compute characteristic impedance and indicate if the fault is resistive or not.
A TDR can also check for proper far end impedance termination. A properly terminated line won't reflect anything. The amplitude of the reflection indicates the degree of mismatch.
John
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John De Armond
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There's an interesting method used to find breaks in electric ceiling heating wires buried in the plaster. It would probably work for this too.
I can't go very deeply into how it works since I only understand it well enough to use it.
A radio signal is fed into each end. One end broadcasts a Morse code "N" and the other end broadcasts an "A". At the point of the break, the signals overlap and produce a steady tone. With a receiver and headset I've located breaks narrowed down to the size of a quarter and was able to make invisible repairs working from the top side.
I used the device several times successfully. It was on loan from the local power company.
George Willer
(David

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(David

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Neon John <www.johngsbbq.com> wrote:

Man, that's pretty cool!
(Uh, what's such a thing *cost*? (just wondering))
David
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On Wed, 21 Sep 2005 01:15:52 +0000 (UTC), snipped-for-privacy@panix.com (David Combs) wrote:

Wide range. Little handheld units that display only footage and designed for a specific application might run <$500. A top of the line Tektronix scope-based TDR might run $4k or more. Then there's the "poor man's TDR". A fast risetime pulse generator and a suitable oscilloscope with a delayed timebase or a digital scope. If that kind of equipment is already on hand, the cost is little more than some interconnect cables and the time involved.
One launches the pulse, times the return on the scope screen and then computes the distance manually. In the good old days, with hand prepared lookup tables or slide rule. Nowadays with a programmable calculator or handheld computer like a Palm.
A scope-based TDR can convey all sorts of info beyond an open or short and distance. Any impedance discontinuity causes a reflection. A good unit like the tek can see the impedance upset in even a good connector like a Type N. It can see coax bent too sharply or a staple put in too tightly. It easily sees splices. One can even get a pretty good idea of an antenna's match (SWR) by looking at how little of the pulse is returned.
Stuff that's come along "since my time" :-) includes checking the transmission line characteristics of digital buses on PCBs and the like. Here's some good reading from the Tek site if you're interested:
http://www.tek.com/Measurement/applications/design_analysis/tdr.html
Looks like they're building the TDR function into the higher end digital scopes now. Slick. I still like my old analog Tek TDR though. The eye is still mightier than the DSP for some things.
John
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I've been watching this trace for a while and remember seeing utility guys tracing underground services with a device. Could it be like one of these? http://www.electrical-contractor.net/the_store/Wire_Tracers.htm?source=google It will detect services down to 10'. This site also talks about detecting breaks in underground irrigation systems and specificly about finding breaks."The valve-locator/wire-tracker can be used to trace the wire path, locate valves, locate splices, and locate damaged wires." http://www.igin.com/Irrigation/groundwiring.html For instance this unit uses a number of different frequencies: http://www.professionalequipment.com/xq/ASP/ProductID.863/id.4/subID.72/qx/default.htm Richard
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Sounds like a TDR, they're used a lot in repairing computer networks in large buildings. For something like this though a simple Fox & Hound would probably work fine, we have one at work and it's nothing but a little box you connect to one end of the cable and an inductive probe that picks up a transmitted tone, run it along the wire until the tone goes away. Same can be done with a radio and some sort of noise generator or oscillator. You can't use the fence controller itself as they normally shut down when the loop is open and you need it connected to only one end so the signal will not be present after the break.
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Entirely incorrect. At least with respect to the Invisible Fence brand, anyway. It does *not* shut down on an open loop, you do not need it connected to only one end, and the discontinuity in the signal is *clearly* evident. I just had to trace a break last week; the AM radio showed very strong signals all the way along the wire, which abruptly stopped *directly* over the break.
--
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Doug Miller (alphageek at milmac dot com)
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I've never tried this so don't know if it will work. The AM radio method is to hook one of the spark plug wires from a running engine to the underground wire. The portable AM radio is supposed to get more static as one gets closer to the break.
Dean
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Your FIRST step should be to join the Varmint Hunters Association, and open your place up to them, for a little clean-up.
It's OPEN SEASON for varmints, you'll recall.
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wrote:

Yep. Works great, too. Tune a portable AM radio to the very bottom end of the band, and turn the volume all the way up. Hold the radio close to the ground, and move it back and forth in the area where you think the wire is. When you're over the wire, you'll hear a marked change in the noise on the radio. Once you've found the wire in this manner, keep following it. When you *don't* hear that distinctive sound any more, that's where the break is -- or at least close to it. Go back to where you last heard the distinctive sound (probably won't be more than six feet away) and dig up the wire (typically buried only a few inches deep). Follow it until you find the break. If the other end isn't readily evident within a short distance of the break, you can trace it in the same manner.
--
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Doug Miller (alphageek at milmac dot com)
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I recently had the same problem, though I only had 800' of wire. My instructions told how to put a little coil across the leads, which created a beat pattern. You could then use a radio to listen for it.
It worked, but was a real pita. I got various strong and weak signals all over the place. Of course, in my case I didn't have broken wire; apparently some insulation had worn off and the signal was shorting into the ground, leading to a circuit too weak for the receiver. Maybe with a real break it would work better.
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davefr wrote:

Take one of the collar units off the dog. Hold it (by the collar) at about dog height. Walk to where the fence is. If it chirps then you have signal there so move to a different location. If it doesn't chirp then you're in the right area so move to a different location. Repeat until you have two spots with chirps and a space between with no chirp. Your break will probably be in between the two.
--
"The career politicians are keeping the elevator at the penthouse
floor and not sending it down for the rest of us." - Kinky Friedman
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