Triax

[0xDEADBEEF]

Can anyone tell me what triax cable is good for. As far as I understand it, it is unbalanced so you cannot use it for differential transmission, and I don't understand how there is supposed to be a difference between single and double shielding.

[Bob S]

I have used triax coaxial cable in two different situations. 1) When I have serious grounding problems, the outer shield is used to ground equipment together, and the inner shield is used to carry a single point ground, like at a pre-amplifier. Also, wrapping a few turns of coax around a hi-mu toroid will help. 2) When I need to feed a signal back from an amplifier to a coax shield from a sensitive pickup to reduce the effective cable capacitance, I can feed it back on the inner shield, and use the outer shield for ground.

[davidrit]

In coax, the shield serves multiple purpose like signal reference/return, earth ground for safety, electromagnetic shielding. With triax you can separate the signal reference from the other functions when you can't tolerate the noise they introduce.

They might help in cases where you care about other cable problems like triboelectric charge depending on cable materials and construction too.

[dlgoff]

Evidently it is use for video signals.
http://www.newark.com/productimages/nio/standard/4277167.jpg
http://www.newark.com/belden/8233a-0101000/video-triax-cable/dp/29M7273

[f95toli]

The reason why triax is sometimes used from video is -as far as I remember- that some devices (presumably things like CCTV cameras) are powered using the same cable as for the signal; i.e. one if the shields is actually carrying a current.

Anyway, if you want to know more about triax I would suggest getting (it is free) the "Low Level Measurements Handbook" from Keithley; they use triax for a lot of their instruments and as far as I remember they explain the reasons quite well in that book.

[0xDEADBEEF]

[...]
Anyway, if you want to know more about triax I would suggest getting (it is free) the "Low Level Measurements Handbook" from Keithley; they use triax for a lot of their instruments and as far as I remember they explain the reasons quite well in that book.

Thanks. That book is floating around somewhere here, I'll give it a try...

[0xDEADBEEF]

That was easy, they are used for gate circuits to measure high impedance sources.