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Are Coax (RG ?) really sheilded?

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LeastDrag93066(at)aol.com
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Posted: Thu Jun 22, 2006 9:47 am Post subject: Are Coax (RG ?) really sheilded?

Hi All,

 

Someone on this list must know the real answer to this question.

 

Is a coax cable a shielded cable?

 

My thinking:

A twisted pair of conductors inside a braided shield grounded at  one end 

would be a reasonable definition for a shielded cable.

 

A coax cable is a center conductor (power/signal) with an outer  braided 

conductor grounded at both ends.

The outer braid would provide some shielding of the inner  conductor.

However, it seems to me that the outer braided conductor would  couple with 

any AC power source and induce this into the ground side of the  signal.

 

I seem to remember good things to do.

Like running the coax by itself.

And always crossing a coax at right angles across other wire runs.

 

But I'm still learning, so please help me.

 

Regards,

Jim Ayers

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pitts_pilot(at)bellsouth.
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Posted: Thu Jun 22, 2006 10:17 am Post subject: Are Coax (RG ?) really sheilded?

LeastDrag93066(at)aol.com wrote:



 	  Quote: 	
		  



Hi All,

 

Someone on this list must know the real answer to this question.

 

Is a coax cable a shielded cable?



Is this a trick question???   'coax' is short for co-axial, meaning that 
	


one conductor surrounds the other with a dielectric medium to keep them 

separated.  You COULD think of coax as being a single shielded wire, but 

certainly coax is more than that.



 	  Quote: 	
		  My thinking:

A twisted pair of conductors inside a braided shield grounded at  one end 

would be a reasonable definition for a shielded cable.



My thinking too.  The twisting is done to 'cancel' induced signals from 
	


one conductor to the other.  Take my word for it.  It's too complicated 

for me to explain ..... even if I really did remember it all!!!



 	  Quote: 	
		  A coax cable is a center conductor (power/signal) with an outer  braided 

conductor grounded at both ends.



Only if YOU ground the braid.  Sometimes the braid is 'floating', or 
	


ungrounded, grounded at one end only (prevents current flow in ground 

loops) or grounded at both ends.  It all depends on the application.



 	  Quote: 	
		  The outer braid would provide some shielding of the inner  conductor.

However, it seems to me that the outer braided conductor would  couple with 

any AC power source and induce this into the ground side of the  signal.



This is a typical application for coax.  The shield keeps the 
	


'offensive' signal (doesn't have to be AC, it can still be pulsed or 

varying DC etc.) from being inductively coupled to the inner conductor. 

 It also keeps the signals on the inner conductor from being inductively 

coupled to their neighbor.



 	  Quote: 	
		  I seem to remember good things to do.

Like running the coax by itself.

And always crossing a coax at right angles across other wire runs.



In a theroetical world, those are two good things.  However, in the real 
	


world (well, depending on the application, of course) these aren't 

always practical solutions.  The signal levels we're going to encounter 

aren't usually high enough to cause major problems ...... as long as 

everything stays normal.



The reason I said that, it's possible for alternator whine (that's AC on 

the battery + buss caused by failing diodes in the alternator) to be 

coupled into the audio inputs, or even into the internal amplifier 

circuitry, be amplified and appear in your headsets and in your 

transmitted audio.  Same is true of ignition noise.  



 	  Quote: 	
		  But I'm still learning, so please help me.



Ah, aren't we all???? 
	


Linn

do not archive



 	  Quote: 	
		   

Regards,

Jim Ayers

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Erich_Weaver(at)URSCorp.c
Guest

Posted: Thu Jun 22, 2006 2:56 pm Post subject: Are Coax (RG ?) really sheilded?

yes, coax is shielded.  Contrary to common opinion however, shielding is of

no use for preventing interference from inductive coupling.  High current

in a wire adjacent to coax can still present a problem.  Coax IS good at

preventing electrostatic interference.  Venture over to the aero-electric

list or get a copy of the Aero-Electric Connection from Bob Nuckols to get

the full skinny.



Erich Weaver

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n320wt(at)yahoo.com
Guest

Posted: Thu Jun 22, 2006 4:29 pm Post subject: Are Coax (RG ?) really sheilded?

Jim, It's simple enough to understand as it applies to aircraft construction. Coax is used for RF connections to carry radio signals between radios and antennas. Shielded wire is for any thing that could create noise like the alternator or power to a strobe power supply. Shielded twisted pair is for signaling sources where data is being used such as communication for an EFIS installion. I also use twisted 2 and 3 conductor for the headphone and mike jack wiring. I was a comm/nav systems specialist on C-130-H for the WVANG in a former life.Hope this helps.  



BRIAN ALLEY (N320WT) 

CARBON FIBER COMPOSITES

101 Caroline Circle

Hurricane, WV 25526

www.carbonfibercomposites.net

304-562-6800 home

304-395-4932 cell



How are you going to win by a nose if you don't stick out your neck?

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