Odd Coax Cable Behaviour

I have a roll of old coax I've used for making assorted cables. Recently, I've had a couple of short cables (used for DC control applications) that show resistances on the order of 100kohms between centre conductor and shield, low enough to upset the operation of the equipment with which they were used.

Taking a careful look at this I notice this old cable is marked "ITT SURPRENANT DIV RG-174/U L.N.". I take it that the "L.N." must mean I've stumbled across the sort of low noise coax used with accelerometers and the like. It would seem that the process of soldering connectors to the cable, or this plus the application of a DC voltage to the cable for some time, is causing the dielectric to become conductive on occasion. Other cables I have made up from the same RG-174/U are fine, however. Cutting the connector off the cable appears to restore a >20 Mohm resistance, too.

Is anyone familiar with this sort of cable (this one has a uniformly black dielectric) ? Any ideas as to what might be going on and how I could avoid the low-resistance problem ? (other than buying some new, normal cable....I have lots of the old stuff I would like to use up)

Steve

Reply to
Steve Kavanagh
Loading thread data ...

Flux + humidity + time = conductivity, sometimes. This is why you clean boards ever so carefully before use. It could be the soldering itself, not the dielectric at all.

--
Tim Wescott
Wescott Design Services
 Click to see the full signature
Reply to
Tim Wescott

True...but in one case the resistance was there within a few hours (or even minutes...I don't recall).

Steve

Reply to
Steve Kavanagh

Thanks, John. Indeed, upon closer inspection the dielectric has a black, conductive layer over a very small diameter layer of ordinary clear polyethylene. With care and attention it can be cut back independently. I've done so on one cable that was giving me trouble...we'll see if it stays OK.

I suppose it won't matter much on low frequency signal cables or low impedance DC stuff, though I imagine the high frequency attenuation may be pretty high and there might be impedance issues at RF.

Regards, Steve

Reply to
Steve Kavanagh

Thanks, John. Indeed, upon closer inspection the dielectric has a black, conductive layer over a very small diameter layer of ordinary clear polyethylene. With care and attention it can be cut back independently. I've done so on one cable that was giving me trouble...we'll see if it stays OK.

I suppose it won't matter much on low frequency signal cables or low impedance DC stuff, though I imagine the high frequency attenuation may be pretty high and there might be impedance issues at RF.

Regards, Steve

Reply to
Steve Kavanagh

I read in sci.electronics.design that Tim Wescott wrote (in ) about 'Odd Coax Cable Behaviour', on Fri, 7 Oct 2005:

Yes: the dielectric is coated with graphite. Check carefully; usually there is a very thin surface film that carries the carbon, and you can cut that back at the termination so you don't get the leakage resistance.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
 Click to see the full signature
Reply to
John Woodgate

Hi,

There is a type with a black carbon-loaded inner sheath designed to reduce noises due to cable movement. It possibly absorbs charges due to local changes in cable capacitance (the parametric amp principle) but there I'm only guessing. Maybe the black stuff has migrated or perhaps it was there all the time and your previous projects were not so choosey.

Cheers - Joe

Reply to
Joe McElvenney

I read in sci.electronics.design that Steve Kavanagh wrote (in ) about 'Odd Coax Cable Behaviour', on Fri, 7 Oct 2005:

It should do. I have some that I acquired many years ago and it's still OK. Not the same spec as yours, but similar.

I guess IT&T bought the original company, because it wasn't ITT France that offered it to me.

No, it's OK at RF. The carbon layer is just an addition to the shield. I think this type of cable was originally developed in France, and I was offered some more than 30 years ago as 'Cable Ecran Haute-Efficace' - 'EHE cable' [Cable with high-efficiency shield].

The versions with no copper shield but just a drain wire aren't too good; the drain wire creates an asymmetry in the structure that causes the efficiency of the shield to diminish.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
 Click to see the full signature
Reply to
John Woodgate

I think you have got cable with a partially-conductive black coating on the outside of the dielectric. This is in contact with the screen and it is there to prevent charges building up on the outside of the dielectric and causing electrical noise when the cable is moved.

The coating must be carefully stripped back a short distance at the plug to prevent a partial short circuit between the screen and the centre conductor. If you haven't been doing this, it would explain your low resistance readings.

Are you sure this is really co-ax? Most of the older low-noise cables of this type were audio cables with an unspecified impedance and may well give trouble in digital or HF circuits due to being unsuited to the terminating impedances.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
 Click to see the full signature
Reply to
Adrian Tuddenham

It has been over 20 years, but the "LN" which stands for "low noise" (as mentioned), means that the insulator is impergnated with some carbon, to drain of piezoelectric, triboelectric and other induced voltages from bending and/or crushing. You either live with the characteristics, or use standard coax when the LN could cause problems.

Reply to
Robert Baer

Triboelectric it is--as in rubbing a balloon on your hair and sticking it to the ceiling. Cable flexing and vibration causes changes in capacitance and triboelectric charging. If you have a significant bias voltage between shield and centre conductor, the V*delta C noise is usually much worse than triboelectricity, but you can get rid of it by getting rid of the bias voltage. Triboelectric noise occurs even when the dc voltage across the insulation is zero, so it needs a special cable rather than a circuit mod. Putting the amplifier at the transducer works even better, usually.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.