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

[David Combs]

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.

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

 

[Neon John]

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.

It's called a Time Domain Reflectometer or TDR.

He didn't know how it worked, he said.

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.

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????

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

 

[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

 

[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?
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

 

[123go]

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.

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]

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????

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]

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.

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.

 

[David Combs]

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
---
John De Armond
See my website for my current email address
http://www.johngsbbq.com
Cleveland, Occupied TN

Man, that's pretty cool!

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

David

 

[Neon John]

Man, that's pretty cool!

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

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

 

[[email protected]]

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

 

[Tim Fischer]

http://www.canadapets.com/RFbreak.htm

 

[Doug Miller]

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.

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

 

[Angrie.Woman]

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

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]

http://www.canadapets.com/RFbreak.htm

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.

2. Wrap the boundary wire around the choke leads

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

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

5. 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