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

[Art Schwartz]

[Neon John]

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

[David Combs]

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

[spudnuty]

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?

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." instance this unit uses a number of different frequencies:

[George Willer]

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

[c_kubie]

LOL@"Rent-a-Coyote"

I was going to suggest some pelet rifle, night scope, and some bait. When done, make small crucifes and make an example of the dead gophers.

[123go]

hence the old saying "you are on the beam" (when aircarft pilots were not veering off to the left or right of the proper flight path).

With a receiver and headset I've located

[James Sweet]

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.

[Doug Miller]

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.

[maradcliff]

You can buy an electric signal tracer at many of the larger hardware stores. They are under $50 for most. You hook they device on the end to the wire. Then you follow the path of the wire with another handheld device and either listen to a tone or watch for flashing LEDs. These are made for wiring in a building. Dont know if it will work underground.

Mark

[Neon John]

Wide range. Little handheld units that display only footage and designed for a specific application might run

[Tim Fischer]

[David Combs]

Man, that's pretty cool!

(Uh, what's such a thing *cost*? (just wondering))

David

[Doug Miller]

As previously posted... a portable AM radio is the only thing you need.

[Angrie.Woman]

When we did it, we also used a coil that we got from Radio Shack. We put the coil across the trassmitter, tuned the radio to the pulse, and walked.

A

[Angrie.Woman]

Method #2: This method requires you purchase an "RF-Choke" from Radio Shack (Catalog item # 273-102) and use an AM Radio. Once you have these, follow these procedures:

1. Disconnect the boundary wire from the terminals on the transmitter.

1. Wrap the boundary wire around the choke leads

2. Connect the choke leads to the terminals on the transmitter. The choke has now completed the loop as far as the transmitter is concerned.

1. Turn the range adjustment knob up 1/4 to 1/2 turn.

2. Take the transistor radio and set it to AM 600. Stand outside the structure where the twisted wire exits and listen for the pulsating static of the transmitter.

Gently swing the radio back and forth across the front of your body and follow the wire out to where the loop begins. Pick either direction and continue until the pulsating stops for a 4-6 ft area. In this area is your break.

That's what we did. We got pretty good at it!

A