When same cable isn't same cable...

[nick_f]

Hi All,

In the thread http://www.geotech1.com/forums/showthread.php?t=15383 I was refering to a coaxial cable that is very suitable for metal detectors. The cable has low capacitance, small diameter, flexibile (although it has solid core). Besides that, is very cheap and (good news for Australians), it is widely available through the Reject Shop channels.

At least, this is what I thought when I found it in the store...
The sad and intriguing truth is that they look the same, they even have the same code on them, they are very different.

It happened I went today to Reject Shop (looking for cheap goodies that I could use for building metal detectors) and I came across the same type of cable that I described in that thread. I bought 5m for $3. I decided to measure the capacitance of the whole cable, to have a better precision.
Both my old cable and the new cable, just purchased have about 47pF/m (which is 14.3pF/m). The total resistance measured is about 4.3ohm, which is about 0.9ohm/meter (quite different from the value of the old cable, 0.38ohm/meter).

The biggest difference can be noticed in the latency of the pulse, as can be seen in the next images.

Imagine what would have happened if somebody else would have tried the new cable, after my warm recommendation...

The intriguing thing is what makes them so different??? It is not the capacitance of the cable, and the resistance of the cable has very little influence. Could be their 75 ohm impedance slightly different? Hard to believe it could have such an effect.


Regards,
Nicolae

[bbsailor]

Coax Test

Nicolae,

Here is a simple way to test your cables.

1. Move a straight piece under the coil and reduce the delay and note the distance that the coax might be detected.

2. Bend a piece of coax in a sharp "U" shape and secure the bend with a piece of electrical tape and note the delay and distance that the coax might be detected.

The difference between 1 and 2 above is that by bending the coax, there is a different amount of tension on the shield strands and that could affect the amount of eddy currents that might be developed in the coax shield.

The better PI coax cables can go lower in delay before they are detected. It is even better if they are not detected. Also, minimizing the amount of potential coax movement can eliminate false signals.

bbsailor

[nick_f]

Nicolae,
The better PI coax cables can go lower in delay before they are detected. It is even better if they are not detected. Also, minimizing the amount of potential coax movement can eliminate false signals.

bbsailor

Wow bbsailor, you are the best!
I didn't think of trying to detect the cables, to see how they react. The difference is clear as day from night. The bad cable is detected from about 10cm (no matter if is straight or bent), while the good cable is not detected at all, either straight or bent!
Now I think that the "good" cable is not detected maybe because the delay introduced by my setup (using the same cable to connect the coil) prevents prevents the test cable from being detected. I think a faster detector may detect my good cable as well.

The pulse at TP4 changes a lot with the bad cable. Even my previous good cable that I was using (the high quality audio cable) is still detected at around 2-3cm.
They must be using some different metallic materials (either composition or size, thickness, etc) in the construction of the new cable 3C-2V. Now I know how to test a cable, if it is good or bad, even before connecting it!

Best regards,
Nicolae

[nick_f]

I have some new and intriguing information. I found a video from Woody, and he discloses that new cables of the same type are detected by the detector, while old cables are not detected. Watch his video: http://www.youtube.com/watch?v=3Pt8yk9o-eU
He has also found a method of treatment for the new cables. I suspect what the method is.

Regards,
Nicolae

[Qiaozhi]

He has also found a method of treatment for the new cables. I suspect what the method is.

Is it an Aussie secret?

[nick_f]

Is it an Aussie secret?

Hi Qiaozhi,
At the moment I don't know if my suspicion is right. And since Woody is the original discoverer, I didn't want to disclose his secret
I believe it has to do with the Eddy currents that form in the section of the coax cable. A new cable has the shielding wires not oxidated and they make good electrical contact between them. In time, they oxidate and they appear more like insulated litz wire. If we place the shielding of the cable to a high current source, the wires will become hot and will change colour (which means they will oxidate on the surface). At the moment I don't have a high current source, but I'm thinking maybe an old car battery - connecting to one or two cells (= 2V or 4V - extreme care should be taken, because the cable could melt in a second or two!), or maybe a bank of capacitors (a few farads) charged to a few tens of volt.
If this is not the method, another one I could think of is blowing high pressure oxygen from an oxygen tube through the cable. The oxygen may oxidate the surface of the wires.

Regards,
Nicolae