Cable tracing through a building

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We have a piece of RG6 Quad in one area of a building and we can't seem to find the other end of it. I thought that I knew where it went as I installed all these cables, but it's just not there. I have a small tone generator and an inductive probe that work well on unshielded wire, but attempts at using this on shielded cable have always been a problem. I've been thinking of trying to set my old HP200CD audio oscillator to 1KHZ and really crank it up and feed it into the cable. The 200CD will output a respectable signalacross 600 ohms, (naturally I'd be using it unterminated here), and I wonder if I'd be able to pick it up through the cable then?

The only other thing I can think of,(and this is probably not very practical) is to inject a high enough RF frequency into the cable at a level where it would egress. Perhaps I might be able to see this on my spectrum analyzer. Of course I realize that the superior shielding of the quad cable is working against me here too.

I know that there must be equipment designed to do this, but I'm not reallyin a position to make a purchase like that. Does anyone have any suggestions for finding this cable? Thanks, Lenny

 

[Leif Neland]

John Robertson sendte dette med sin computer:

Use a spark generator tied to the shield and an AM pocket radio...if you have
an old Model T Ford ignition coil (OK, my junk box, what's in yours?) that
makes a great noise generator. The AM radio will happily track the wire
antenna...

I guess he wants to use the cable again, without having the insulation
burnt by sparks

But I'm using the same trick when finding breaks in the loop for my
automatic lawnmower.

I can usually hear the spark even if it is underground.

I'm using an electric cattle fence generator, around 6kW.

Leif

 

[amdx]

You need to use a higher frequency in order to get any kind of range.
I suggest you find a function generator that covers the AM broadcast
band. Make sure it's modulated so you can hear the signal on an AM
portable radio. Connect the RG-6u to the generator with the coax
connections BACKWARDS as in generator ground to the coax center, and
generator hot to the coax shield. That turns your RG6/u shield into
an antenna. You may have some problems if the other end of the coax
is grounded, but that just means cram in more signal. Then, fumble
around with the portable radio until you get signal. Also, it might
be useful to check if there's any DC on the coax, just in case it's
connected to a DBS receiver.

One problem with this method is that you can easily end up with too
much range. I used this method once to locate some RG-6/u in a large
house. I thought I had the signal in one room, only to find that the
cable was really going through the wall in the adjacent room. Play
with the signal levels.

There are other frequencies that will work, but the BCB seems to be
the easiest. If your portable radio has a loopstick, you can play
direction finder. I've had marginal success with DF, but it might be
worth a try.

If all else fails, just apply 117VAC to the coax. Whatever device
explodes is the other end of the coax.

This won't help the OP, but it is a technique I have used to find
buried coax several times and it has worked every time.
With a portable AM radio, position the radio on it's backside and
rotate it until you find a null in the audio. This is how you want to
lay it on the ground, now as you move it over a coax in the ground the
radio signal will be disturbed and the null will be lost and audio will
appear. I have followed 40ft of coax with this method. I love it, saw it
somewhere on the net.
Mikek



Tune a portable AM radio to a *station that when you rotate and find
the null, (all audio disappears)

 

[[email protected]]

I really like the AM radio idea in conjunction with a noise source, and while I don't have a Ford coil around I'm sure that I could come up with a coil and some type of interrupter to do the spark. However as was mentioned inthe thread, using 15KV I don't want to blow a hole through the cable either. That might make a few nice unwanted notch filters for God only knows what frequencies...

The EM door buzzer sounds like a pretty non destructive idea, but how wouldI connect it to the cable?

I just noticed Jeff's idea of using an RF frequency, modulated it seems with some sort of waveform on the AM band. I especially like that idea and in fact was just considering throwing it out there when you posted it Jeff, but with one exception. I have a function generator, but I don't know if it will cover this band. I also have an AM signal generator that unlike my boat anchor Marconi, is portable. I can internally modulate it at 400HZ. I understand the procedure you've outlined reversing the shield, levels, etc, sowhy couldn't I use a standard AM signal generator?

The answer to the other suggestion posted about using two shields is also interesting, however the term "quad" does not describe four cables but rather a description of the shield on one cable. Lenny

 

[William Sommerwerck]

"Leif Neland" wrote in message

I'm using an electric cattle fence generator, around 6kW.

Did you mean 6kV? You aren't trying to grill the beef on-the-hoof!

 

[William Sommerwerck]

Is the destination end "dangling" -- that is, exposed and not connected to
anything?

If so, just stick a battery on the other end and start measuring.

 

[[email protected]]

I have gotten a lot of great ideas guys. I'll give it a shot. Thanks for all the help. Lenny

 

[[email protected]]

We have a piece of RG6 Quad in one area of a building and we can't
seem to find the other end of it. I thought that I knew where it went
as I installed all these cables, but it's just not there.

Don't discount the possibility that somebody cut it short and shoved
the end back into the wall, or up above the suspended ceiling, or
whatever. If they shoved it back in the wall and that wall has since
been painted or otherwise remodeled, there might not even be a hole in
the wall anymore.

I have a small tone generator and an inductive probe that work well on
unshielded wire, but attempts at using this on shielded cable have
always been a problem.

Try putting the -/black lead of the tone generator to earth ground (like
powerline ground at an outlet) and the +/red lead to the *shield* of the
RG6. Then use the inductive probe like normal.

Matt Roberds

 

[Bob F]

Don't discount the possibility that somebody cut it short and shoved
the end back into the wall, or up above the suspended ceiling, or
whatever. If they shoved it back in the wall and that wall has since
been painted or otherwise remodeled, there might not even be a hole in
the wall anymore.


Try putting the -/black lead of the tone generator to earth ground
(like powerline ground at an outlet) and the +/red lead to the
*shield* of the RG6. Then use the inductive probe like normal.

I might be a good idea to test for continuity to ground at the unknown end
before some of the suggested tests to find out it somethink is connected to it.

 

[RobertMacy]

...snip...
I might be a good idea to test for continuity to ground at the unknown
end
before some of the suggested tests to find out it somethink is connected
to it.

some usually gets through even if 'grounded'

However, better if possible to inductively couple to the shield, less load
for your source. Or, just tap into the shield about a foot away from the
grounding.

 

[Adrian Tuddenham]

"Leif Neland" wrote in message

Did you mean 6kV? You aren't trying to grill the beef on-the-hoof!

http://www.coalcanal.org/other/images/Cattle.jpg

 

[William Sommerwerck]

I'm using an electric cattle-fence generator, around 6kW.

http://www.coalcanal.org/other/images/Cattle.jpg

That looks more like "spontaneous bovine combustion".

 

[[email protected]]

Lenny, please be sure to tell us what you end up doing and if you are successful.

 

[William Sommerwerck]

"dave" wrote in message

At least get the receiver part. Use a sweeping or warbling tone
for fast positive ID.

My point in suggesting DC is that there's no inductive coupling to confuse
identification.

 

[josephkk]

We have a piece of RG6 Quad in one area of a building and we can't seem to find the other end of it. I thought that I knew where it went as I installed all these cables, but it's just not there. I have a small tone generator and an inductive probe that work well on unshielded wire, but attempts at using this on shielded cable have always been a problem. I'vebeen thinking of trying to set my old HP200CD audio oscillator to 1KHZ and really crank it up and feed it into the cable. The 200CD will output a respectable signal across 600 ohms, (naturally I'd be using it unterminated here), and I wonder if I'd be able to pick it up through thecable then?

The only other thing I can think of,(and this is probably not very practical) is to inject a high enough RF frequency into the cable at a level where it would egress. Perhaps I might be able to see this on my spectrum analyzer. Of course I realize that the superior shielding of thequad cable is working against me here too.

I know that there must be equipment designed to do this, but I'm not really in a position to make a purchase like that. Does anyone have any suggestions for finding this cable? Thanks, Lenny

You might try using 10 kHz and exciting both core and shield against
ground. It might work better.

?-)