Cable tracing through a building

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 installe d 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 u sing 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 i t 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 wond er 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 practica l) 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 analyze r. Of course I realize that the superior shielding of the quad cable is wor king 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 suggestio ns for finding this cable? Thanks, Lenny

Reply to
captainvideo462009
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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...

An EM door buzzer could work too as a noise source - or a dog fence:

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Reply to
John Robertson

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

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Reply to
Leif Neland

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.

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Reply to
Jeff Liebermann

Quad means 4 coaxes in a jacket?

Drive the signal across two of the coax shields. That should radiate enough for a tracer to pick it up.

Jon

Reply to
Jon Elson

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)

Reply to
amdx

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

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

I just noticed Jeff's idea of using an RF frequency, modulated it seems wit h 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, bu t with one exception. I have a function generator, but I don't know if it w ill cover this band. I also have an AM signal generator that unlike my boa t anchor Marconi, is portable. I can internally modulate it at 400HZ. I und erstand the procedure you've outlined reversing the shield, levels, etc, so why couldn't I use a standard AM signal generator?

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

Reply to
captainvideo462009

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

Reply to
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.

Reply to
William Sommerwerck

I'm not certain, but I suspect that if you simply connect one end of the cable to a lead near the noisy contacts that the wire would then act as an antenna.

Also, the 15KV would only puncture the insulation if it formed a path, so if you connect all the conductors/sheilds to the same point source and that risk vanishes, and don't ground the other side of the coil - it is simply a spark gap transmitter at this point.

Now I haven't tried any of these ideas, just recall them from old electronics and wiring books I've read over the years.

YMMV

John :-#)#

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Reply to
John Robertson

Ummm, I said function generator or maybe RF generator. In case you haven't heard, spark transmissions are banned by the FCC.

It would melt the insulation and cause a short as the center conductor migrates towards the shield on corners.

Clip leads.

Most function generators will go to 1MHz.

Good enough.

I didn't know you had an RF generator. Whatever you have. Just don't use a 100 watt HF transceiver.

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Reply to
Jeff Liebermann

Quad means quad shielded or 4 shields. The idea is to reduce cable TV leakage and ingress.

Quad shielded is usually used on the poles for CATV. The in-building distribution stuff is usually double shielded (foil+braid). They will be labeled RG-6A/U for double and RG-6A/UQ for quad.

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Reply to
Jeff Liebermann

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

Reply to
captainvideo462009

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.

Matt Roberds

Reply to
mroberds

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.

Reply to
RobertMacy

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Reply to
Adrian Tuddenham

That looks more like "spontaneous bovine combustion".

Reply to
William Sommerwerck

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

Reply to
hrhofmann

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

Reply to
dave

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

Reply to
William Sommerwerck

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