PROJECT: Tracing a buried coax aerial cable

[Lesolee]

To set the scene, the TV at my mum’s house was reported to drop out due to a weak signal, apparently worse during bad weather. There was a two way 6dB amplified splitter, so I upgraded to a 16dB amplified splitter. Not good enough. The aerial was on the roof, having replaced the original aerial an indeterminate number of years ago. The aerial socket, when removed, showed two other coax cables which had been cut. The original aerial wire to the loft space was chased into the breeze block wall but I couldn’t find where it ran using an ordinary metal/pipe/wire finder. Just going straight up from the socket to the loft, and removing the loft board didn’t reveal the cut end.

I googled wire tracers, but they seem to be when you touch the cable to identify it. I needed something with more range to locate the original wire in the walls and under the loft boards.

The idea was to quickly build up something with stuff that was to hand. I tried a quick experiment using a signal generator onto a coax cable outer, using a paint stripper as a capacitive sensor into a scope. Looked easily possible, so I knocked up the circuit below, based on running the source at 3kHz. This seemed to give a loud and easily detectable sound from the headphones, as seen by just powering the headphones directly from the signal generator.

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On the bench it oscillated at around 40kHz when the headphones were plugged in. I tried two 1nF caps across the 33K feedback resistor with no success. But since that was inaudible I just let it go and carried on. 

The sensor is conductive adhesive copper tape on the bottom of a piece of wood, and wrapped around the ends to both stop it peeling and to give an easy connection point. The tape on the top edge (as shown in the picture) is the ground reference ( to be touched and held by the operator).

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The left side tape is the sensor. The middle area is the ground reference, which sits under the board as a grounded shield. The battery box was left over from an LED lighting strip (which would never have worked from battery power anyway). It hangs over the edge of the wooden block to allow access to the on/off switch on the underside of the battery compartment.

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The “insulator” under the circuit board is a piece of cardboard from an A4 writing pad. I covered it with a single layer of black electrical tape to make it look as if I had thought about insulation, at least a little. The big capacitor is 2µ2 and had been in a damp shed for 40 years. The bit of wood holding the circuit board was stuck on with quick-setting epoxy, and had a groove cut in it with a saw to hold the circuit board edge.

The picture shows alkaline batteries, but high drain batteries are not required. The current drawn was 7 mA with no tone, and 8 mA with, so the battery life should be fine with cheaper batteries.

The two blue blobs are the 1nF capacitors which were supposed to be across the 33K feedback resistor. Maybe if they had been fitted in the correct place they might have actually prevented the 40kHz oscillation! 

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I went back to mum’s house, armed with a signal generator, cables and the tracer. It didn’t make any sense. I apparently traced the aerial feed to the next door TV, and the feed to the next door TV to the external aerial feed. It took a while to sink in that my idea of how the cables were wired up was now how they actually were. The TV aerial socket had been rewired to be the connection to the other room. What I took to be the feed to the other room was in fact the aerial feed. How it ever worked was a mystery since the aerial was connected to the amplifier output!

Anyway, the tracer did its job, the wires were plugged in correctly and it all worked. The old 6dB splitter could not be used as it had insufficient bandwidth, and therefore lost one of the digital multiplex channels. All working. 

But how could I leave this circuit oscillating?    It is an insult to my engineering skill! That is the subject of the next post …

[EDIT]: added current consumption data

[Lesolee]

I designed a new circuit with a narrow band amplification response. That would fix any spurious oscillations!

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It just oscillated at 3kHz instead, but only when the headphones were plugged in (seen on a scope). It didn’t make any sense. I grounded the input sensor to the ground pad. Still oscillating. More decoupling. Still oscillating. 

I looked up the specs on the opamp and the PSRR at 3kHz should have been ok. There was no measurable signal on the power rail with a 1:1 connection and 20mV/div (highest sensitivity on the scope).

So it only oscillates when the headphones are plugged in, so maybe putting a resistor in series with the headphones would decrease the load enough t make it stable. Bring on trusty R-box. Viewing the put in spectrum analyser mode (for maximum sensitivity) the oscillation was totally gone with 40 ohms extra resistance. Success! I used 100R to give some margin.

My intention is to have wired the headphones (Altai HP110A) up such that the outer screen (if there is one) goes to ground. I have connected the large part of the jack plug to ground, and the other two connections in parallel. But the impedances don’t work out well that way. It measures 8 ohms to one side and 20 to the other. If I take the middle of the three jack plug connections as the ground then the other two are 21 ohms and 29 ohms which makes more sense.

But it works, and it doesn’t oscillate, so honour is satisfied.      It may never be used again anyway!

I should also mention the range. At 1V ptp the range on the bench was about 1 foot. At 10V ptp it was at least 1 metre. The signal generator used had a grounded output, so it was only necessary to connect the signal wire to the searched-for coax outer. For an un-grounded generator you would need to ground the 0V end of the signal source.

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[IDEngineer]

Cool project, thanks for sharing!

During a remodel a few years ago, they sheetrocked over the coax and low voltage box and could not find it. No bulge in the sheetrock, etc. After making certain nobody was touching anything, I shorter the other end's center and shield and connected them to one phase of the 120VAC, turning off all other power in that area. Then we were able to trace the coax with a traditional electric field probe like those carried by every electrician. Worked perfectly, cost nothing, and saved redoing all the drywall that would have been damaged exploring for it manually.

Obviously great care must be exercised doing this. Standard disclaimers, etc.

[LaserSteve]

 Lesolee,

 You get my HERO of the WEEK award for doing all that just for your Mum.   

 I had just bought a commercial tracer to sort sixty unmarked  cables going up and down three stories at work, so I know what is involved.
 
Steve

[Lesolee]

This is effectively Part 2 of the project. Having previously hooked up the aerial connections to the external antenna on the chimney, and having found that this connection is intermittent, I still had to find the cut coax in the loft. I have been clearing the loft (and garage, and sheds) of old crap for months now. I even had the “pleasure” of having to get 5 live TV tubes (CRTs) down from the loft and dispose of them.

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I energised the cut end of what I supposed was the original coax downlead from the loft. The signal generator output is earthed (grounded) so it was only required to attach a croc clip to some of the shield braid. The signal generator was set at 2V ptp 1kHz sine. This signal was plenty big enough to trace the cable up the walls, downstairs. The cable then moved more towards the centre of the house close to the chimney. I had to move a whole heap of light boxes sideways by a few feet in the loft and there was a signal again close to the chimney. The loft boards are tongue and groove so I had to lift more than a few until I found some coax. Hurray!

But wait a minute there where 3 cables which looked like coax, but not the big aerial coax I was expecting.  Two dissimilar types were joined with some sort of nasty insulating tape covering. That means three distinct cables disappearing under yet more loft boards. All three measured “hot” at 3kHz with no discernible amplitude difference. Not good. 

I reconfigured the generator to inject signal relative to the shield (ie signal on centre and ground lead to shield. Obviously the coax was not then “traceable” but I could see if this was connected to the required coax. No. No connection. Evidently these dubious coaxes ran next to the target coax for sufficient length to pick up the trace signal. 

Remember what I was expecting to find was a cut-end of a coax cable. I didn’t expect there to be a signal wire poking out of the screen which might have been picked up. Everything pointed to the chimney, so I looked around where the loft boards parted to let the chimney through. There were a couple of tiny pieces of wood, each held with a couple of screws. Drill-driver in hand I buzzed out a few screws and slid the wooden pieces out in a correct sequence, given that they were  also tongue and grooved. SUCCESS! There was a little cut-end tucked in under the wood with no indication at all. What sort of F*CKWIT does that? I now have a 4 inch tail to work with, so a quick coax female plug and I will be good to go.

Needless to say I had to restore four other roof boards to their correct locations. I also got a felt pen out and marked where the buried cables are. What those other random coax-like cables are doing is anyone’s guess.

It means I should be able to fit a loft antenna, hook it up to the down feed, put in a double coax socket downstairs so I can pull the aerial feed into a powered splitter to drive both TVs. But that is for another day, or probably many more days (at a rate of one trip per week). I think the hardest part is done.