Coil Cable Help

[Tinkerer]

My conclusion from the statements I shared is that a high value resistor connecting the floating shield of the cable to coil ground should be enough to avoid the extra capacitance, shield EMF and provide a path for electrostatic discharge. Something around 300k or more since the capacitance is so small.

Gonna try this. My coax cable has deteriorated the coil decay by about 2us, will see if I can get them back this way.

However the coil shield which is meant to isolate it from the soil's capacitance still must have a low resistance path to ground.

On the system where I tested the resistor, the trimpot was 100 Ohm and gave best results when set at 72 Ohm. It shortened the delay from about 6us to less than 3us.

[Teleno]

Interesting.
However, we also use the cable shield to discharge any electrostatic charge of the coil. Sometimes the electrostatic charge builds up on a detector and then discharges when touching the ground or even grass. I usually test the coil shielding by touching a PVC pipe that I previously electrostatically charge to several thousand volts to the coil. If I don't notice any reaction by the detector, I consider the coil shielding to be good enough.

Does anybody have a better suggestion?

My conclusion from the statements I shared is that a high value resistor connecting the floating shield of the cable to coil ground should be enough to avoid the extra capacitance, shield EMF and provide a path for electrostatic discharge. Something around 300k or more since the capacitance is so small.

Gonna try this. My coax cable has deteriorated the coil decay by about 2us, will see if I can get them back this way.

However the coil shield which is meant to isolate it from the soil's capacitance still must have a low resistance path to ground.

[Tinkerer]

Basically what you do is to increase the isolation between the shield and GND.

I believe it makes sense because a Faraday cage doesn't need to be attached to any potential in order to work. Or so I believe. What would happen if the shield is left floating? There's a patent claiming such a shield https://patents.google.com/patent/US20210375505A1/en

See also this discussion:

"You do not need to be connected to ground to shield something and this is a huge misconception among many people. Enclosing something in metal will stop all EM radiation from getting through

"I would agree with that because the test results for a floating shield still showed attenuation as I suspected."

Interesting.
However, we also use the cable shield to discharge any electrostatic charge of the coil. Sometimes the electrostatic charge builds up on a detector and then discharges when touching the ground or even grass. I usually test the coil shielding by touching a PVC pipe that I previously electrostatically charge to several thousand volts to the coil. If I don't notice any reaction by the detector, I consider the coil shielding to be good enough.

Does anybody have a better suggestion?

[Teleno]

And there's more

"Some cables are built with a floating shield, which has no direct electrical connection to ground. In these applications the shield layer just covers the length of the cable and is cut or trimmed adjacent to the cable’s end. While in theory one would electrically tie the shield to ground, some cables don’t always need this to function and actually work better with this floating ground."

[Teleno]

No, the resistor, actually a 10 turn trimpot goes between the cable shield and the TX or RX ground. Using twisted pairs (2 or3) plus shield.

Basically what you do is to increase the isolation between the shield and GND.

I believe it makes sense because a Faraday cage doesn't need to be attached to any potential in order to work. Or so I believe. What would happen if the shield is left floating? There's a patent claiming such a shield https://patents.google.com/patent/US20210375505A1/en

See also this discussion:

"You do not need to be connected to ground to shield something and this is a huge misconception among many people. Enclosing something in metal will stop all EM radiation from getting through

"I would agree with that because the test results for a floating shield still showed attenuation as I suspected."

[Tinkerer]

Interesting. I guess you placed a resistor between the two ends, correct?

No, the resistor, actually a 10 turn trimpot goes between the cable shield and the TX or RX ground. Using twisted pairs (2 or3) plus shield.

[Teleno]

I obtained good results by critical damping the transient in the shield.

Interesting. I guess you placed a resistor between the two ends, correct?

[Tinkerer]

Speaking from theory, the capacitance of the shielding cable could be eliminated by keeping the shielding at the same voltage level as the transient by means of a low impedance buffer. Hardly feasable if at all given the voltages involved.

Alternatively, the shielding could be disconnected from GND (high impedance state) during the transient and then reconnected to GND once the voltage level drops below a few volts. This could be realized by some sort of voltage dependent resistor, or perhaps even an inductor in series with the shield

Just an idea, I haven't tried or simulated any of this.

I obtained good results by critical damping the transient in the shield.

[KingJL]

What more could you ask for? Capacitance looks good, and it's double twisted-pair-shielded with an over-shielding. I'm always leery of foil shielding but it's worth a try.
Bonus: for CCPI you really want to use shielded twisted pair for the TX. Probably on the differential RX as well.

Concurrent with my thoughts... only concern might be the voltage rating. I am going to place an order and give it a try. Will follow up with the results.

[Carl-NC]

What more could you ask for? Capacitance looks good, and it's double twisted-pair-shielded with an over-shielding. I'm always leery of foil shielding but it's worth a try.
Bonus: for CCPI you really want to use shielded twisted pair for the TX. Probably on the differential RX as well.

[KingJL]

... The S-Video might be your best bet. If you can find 4-way S-Video then you could use 3 cables for TX and 1 for RX.

Not yet located 4-way S-Video. But this... what would be your assessment?

[Altra]

I have had good results with L-COM s-video cables. On ebay you can get a 50ft cable assembly for $19 free shipping. Only thing i dont like is the copper is untinned. It's cheap and can be cut into 10 5ft lengths. On my phone don't know how to cut and paste. Search ebay for
L-COM CCD234MM-50 male male 50ft molded s-video cable.

[KingJL]

The switch would need to be inside the coil, otherwise the coil still sees the cable.
The S-Video might be your best bet. If you can find 4-way S-Video then you could use 3 cables for TX and 1 for RX.

I'll search around and see what I can find in this... it just might solve my problem.

[Carl-NC]

The switch would need to be inside the coil, otherwise the coil still sees the cable.
The S-Video might be your best bet. If you can find 4-way S-Video then you could use 3 cables for TX and 1 for RX.

[Teleno]

Speaking from theory, the capacitance of the shielding cable could be eliminated by keeping the shielding at the same voltage level as the transient by means of a low impedance buffer. Hardly feasable if at all given the voltages involved.

Alternatively, the shielding could be disconnected from GND (high impedance state) during the transient and then reconnected to GND once the voltage level drops below a few volts. This could be realized by some sort of voltage dependent resistor, or perhaps even an inductor in series with the shield

Just an idea, I haven't tried or simulated any of this.