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Coax cable - capacitance spec clarification please


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A Belden coax spec sheet I was looking at has two capacitance specifications. 

  • Nom. Capacitance Conductor to Conductor:
  • Nom. Capacitance Cond. to Other Conductor & Shield:

 

Which one is relevant to consider for internconnects?

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1 hour ago, audiofeline said:

A Belden coax spec sheet I was looking at has two capacitance specifications. 

  • Nom. Capacitance Conductor to Conductor:
  • Nom. Capacitance Cond. to Other Conductor & Shield:

 

Which one is relevant to consider for internconnects?


which model Belden?

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3 hours ago, audiofeline said:

A Belden coax spec sheet I was looking at has two capacitance specifications. 

  • Nom. Capacitance Conductor to Conductor:
  • Nom. Capacitance Cond. to Other Conductor & Shield:

 

Which one is relevant to consider for internconnects?

 

If it has a "conductor-to-conductor" measurement ... it sounds like it's a 'twinax' cable - not a 'coax'?

 

Like, say, Belden 3079A.

 

In which case ... it's the c-t-c figure that is important - as long as the shield is only connected at one end (so doesn't form part of the signal chain).

 

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2 hours ago, Addicted to music said:

which model Belden?

Belden 9841 "Multi-Conductor - Low Capacitance Computer Cable for EIA RS-485 Applications"

"24 AWG stranded (7x32) TC conductors, polyethylene insulation, twisted pairs, overall Beldfoil® (100% coverage) + TC braid shield (90% coverage), 24 AWG stranded TC drain wire, PVC jacket"

 

Thanks for all comments. 

I'd like to add that I'm not considering buying it or making interconnects from it.  I've probably noticed someone commenting on a forum that they made interconnects from it, and my curiosity was piqued.  I'm just trying to understand the specs. 

 

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18 minutes ago, andyr said:

 

If it has a "conductor-to-conductor" measurement ... it sounds like it's a 'twinax' cable - not a 'coax'?

 

Like, say, Belden 3079A.

 

In which case ... it's the c-t-c figure that is important - as long as the shield is only connected at one end (so doesn't form part of the signal chain).

 

A coaxial cable has 2 conductors.. An inner conductor, and an outer braided conductor. A shielded cable has a conductor and a shield..

A shielded coax cable ie what you use for TV antenna cable, has an inner conductor, an outer conductor, and one or more shields.

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12 minutes ago, audiofeline said:

Belden 9841 "Multi-Conductor - Low Capacitance Computer Cable for EIA RS-485 Applications"

"24 AWG stranded (7x32) TC conductors, polyethylene insulation, twisted pairs, overall Beldfoil® (100% coverage) + TC braid shield (90% coverage), 24 AWG stranded TC drain wire, PVC jacket"

 

Thanks for all comments. 

I'd like to add that I'm not considering buying it or making interconnects from it.  I've probably noticed someone commenting on a forum that they made interconnects from it, and my curiosity was piqued.  I'm just trying to understand the specs. 

 

It's not coaxial cable, it's twin shielded cable..

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27 minutes ago, bob_m_54 said:

A coaxial cable has 2 conductors.. An inner conductor, and an outer braided conductor. A shielded cable has a conductor and a shield..

A shielded coax cable ie what you use for TV antenna cable, has an inner conductor, an outer conductor, and one or more shields.

 

And where does 'twinax' fit in, Bob?

 

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4 hours ago, andyr said:

 

And where does 'twinax' fit in, Bob?

 

Twinaxial Cable has a very close tolerance on the dielectric surrounding each of the conductors, and can be accurately quoted as having a specific impedance, as it is used at high frequencies. In the case of the linked data sheets, up to hundreds of MHz. There is also a dielectric layer between the braid outer conductor and the two centre conductors.

Twinaxila Cable 89207

 

The conductors in twin shielded cable merely have insulation, which is not as tightly toleranced, and can't be claimed to have an accurate impedance per overall, so is not suitable for frequencies much over a a few KHz

Shielded Twin Cable 9841

 

 

Of course you can use Twinax as Shielded Twin cable, but you can't use Sielded Twin as Twinax..

Edited by bob_m_54
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6 minutes ago, bob_m_54 said:

Twinaxial Cable has a very close tolerance on the dielectric surrounding each of the conductors, and can be accurately quoted as having a specific impedance, as it is used at high frequencies. In the case of the linked data sheets, up to hundreds of MHz. There is also a dielectric layer between the braid outer conductor and the two centre conductors.

Twinaxila Cable 89207

 

Aah, OK - understood.  This has a quoted impedance of 100ohm - yet we also have Belden 9272 which is 78ohm???

 

I can understand how precise 'ohmage' specs are vital at MHz ... but I can't see how these two would be different at audio frequencies.  :shocked:

 

6 minutes ago, bob_m_54 said:

The conductors in twin shielded cable merely have insulation, which is not as tightly toleranced, and can't be claimed to have an accurate impedance per overall, so is not suitable for frequencies much over a few KHz

Shielded Twin Cable 9841

 

And then there's Belden 3079A - similar (except solid core).

 

What's the 'evidence' that these can't be good for "much over a few kHz"?

 

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1 minute ago, andyr said:

 

Aah, OK - understood.  This has a quoted impedance of 100ohm - yet we also have Belden 9272 which is 78ohm???

 

I can understand how precise 'ohmage' specs are vital at MHz ... but I can't see how these two would be different at audio frequencies.  :shocked:

 

 

And then there's Belden 3079A - similar (except solid core).

 

What's the 'evidence' that these can't be good for "much over a few kHz"?

 

Data sheets

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11 minutes ago, andyr said:

And then there's Belden 3079A - similar (except solid core).

 

What's the 'evidence' that these can't be good for "much over a few kHz"?

 

10 minutes ago, bob_m_54 said:

Data sheets

 

OK, Bob - attached is the data sheet for 3079A ... where does it say it's no good for audio frequencies?

 

3079A_techdata.pdf

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12 hours ago, andyr said:

 

 

OK, Bob - attached is the data sheet for 3079A ... where does it say it's no good for audio frequencies?

 

3079A_techdata.pdf 180.57 kB · 4 downloads

It doesn't, and actually it would be OK for audio frequencies, depending on the application. ie "Max current 2.3A". It's primarily a High Speed Shielded Pair Data Cable.

 

But you wouldn't use that cable where you would normally use Twinaxial Cable, for example where you want to pump 200MHz over it, for RF applications like in a twin element RF antenna feed..

 

Like I said in one of my previous posts, you can use Twinaxial Cable as a Shielded Twin cable, but you can't do the reverse.

Edited by bob_m_54
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13 minutes ago, bob_m_54 said:

It doesn't, and actually it would be OK for audio frequencies, depending on the application. ie "Max current 2.3A".

 

Thanks, Bob.  (I only use it for my solid-core interconnects; the shield is connected to the RCA barrel at the Source end only - so it acts as a shield ... but is not part of the signal chain.)

 

13 minutes ago, bob_m_54 said:

But you wouldn't use that cable where you would normally use Twinaxial Cable, for example where you want to pump 200MHz over it, for RF applications like in a twin element RF antenna feed..

 

I'm strictly into audio applications.  :smile:

 

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6 minutes ago, andyr said:

 

Thanks, Bob.  (I only use it for my solid-core interconnects; the shield is connected to the RCA barrel at the Source end only - so it acts as a shield ... but is not part of the signal chain.)

 

 

I'm strictly into audio applications.  :smile:

 

Yeah, I come from an Avionics background (29 years in RF, Digital and Analog signals), so as you can imagine there is a big distinction in cable types in that field.

 

And one example of needing to know the difference, is a situation we had where someone drilled into a a cable bundle. He cut through quite a few single conductor wires, and took a chunk out of the side of a Coaxial Cable as well. Through the outer conductor braid, the dielectric and exposing the centre conductor.

 

Someone (ex RAAF Aircraft Elecco) recommended to just do a repair applicable to Shielded Cable repair, for which there was a procedure (about 1 hour max). But I had to explain to him that it was a Coaxial Cable, feeding RF, not just a shielded cable, and there is no repair procedure apart from replacement of the whole cable. About a weeks work..

 

The boss was not happy, and was ready to go along with the "patchup job" for shielded wire. I refused to do the job. When engineering came down to look at the job, and I explained that it was an RF feed, they agreed. No repair, replace the cable..

 

Boss was super p'd off then.. LOL

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