Quad and Tri shield coax

[Ralph]

Can anyone tell me the difference between Quad and Tri shield coax?
I know that it has an extra layer of shielding, but as all the shields touch
I can't see what difference it makes.

 

[Ralph]

I understand that, but if they are all touching wouldn't they act as one
layer?

 

[Fred Ferd]

I understand that, but if they are all touching wouldn't they act as one
layer?

Wouldnt 5000 meters of layer of lead act as if it just a film 0.1mm thick ??

NO, the thickness sets the impenetratability for EM.

Yeah I thought the benefit of extra shielding would be improved by having
the layers insulated from each other, but thats not so. maybe because it
increases capacitance to do that.

 

[Terry Given]

I understand that, but if they are all touching wouldn't they act as one
layer?

yes, but a thicker layer. *IF* they are in good electrical contact.
Often they are not, and the contact resistance is a lot higher than the
longtitudinal resistance of the shield.

A good reference is:
Cable Shielding for EMC, A. Tsaliovich, Chapman Hall, ISBN 0-442-01425-2

the maths is a little bit crunchy, but can safely be ignored - his
waffly stories are excellent, and the graphs of shield transfer
impedance (for a variety of shield combinations) carry a wealth of
useful data.


maybe. depends on the execution of each shield, eg weave coverage, braid
angles, foil pitch & overlap etc, blah blah blah. Many ratshit shielded
cables exist, and many multiple-shield cables are not worth the money.

Cheers
Terry