Coax cable shielding - full or semi?

[Quantumplate]

Back to basics today.
So i get that the outer cylinder acts as a Faraday cage for the signal carrying core. But  the cable carries pure a.c. from an antenna say, so it's only semi-shielded right?
And if so, wouldn't you need full shielding for such tiny signals?
 

[David Hess]

The shield leaks both ways to a certain degree, and some cables includes double shields, or a foil wrap inside the shield to lower the leakage.

This becomes very apparent when using more than about 80dB of attenuation and the signal leaks around the attenuator.  Double shielded cable solves this.

[Terry Bites]

The shield is the return conductor carrying our precious signal every bit as much as the core. So it ought to be shielded of course.

The impedance along the shield conductor is assumed to be zero. Its assumed that the shield is tied to the signal reference potential via a series impedance of zero. And so, any current flowing along it the length of the shield creates zero emf. It redcuces the core condcutor to "universe at large" capacitance to zero.  Therefore it's a perfect gift from heaven and needs no shielding it's self.  I think that's what the thinking is.

I reality emfs are induced, signals leak in and out of the shield in spite of your best efforts. Often a shield is good enough to carry out both signal retun path and shielding duties, and often it ain't. Welcome to the craft!

Knowing how to apply shields, earths, guards and bonding is a neglected cornerstone of EE.

[jonpaul]

So, triax, double Shielded are well known but seldom needed.

99% of RF, instruments, audio use ordinary coax single shield RG174/U, RG58/U, RG59/U

read up on coax, shields, Faraday shields, see

Belden Cable app notes, especially Steve Lampen
Noise reduction techniques by Ott
SYNAUD CON site of Jim  Brown and Bill Whitlock

Jon

[Terry Bites]

Triax has been alive and well in pro TV cameras analog and digital since the begining of colour.
Long high bandwith cables are a must. You can shove AC power down a triax too. Very handy.

You often see it in very low level and very high impedance amplifiers and measuring instruments.
My B&K mics are triax.
It does great job in reducing that leakage and capacitance.
The inner shield is driven from a unity gain buffer connected to the core. Slightly less than unity in higher impedance systems- unless you want an oscillator.
See Keithly (Tek) Low Level Measurements Handbook attached.

[EPAIII]

Manufacturers like Belden and Alpha Wire publish specs on the shields of their co-axial cables. Get a catalog or check out their web sites.

[Nassau Cable]

Hello. It is not necessary to have complete/full shielding, but if you want, you can find good options in the market.

[David Hess]

So, triax, double Shielded are well known but seldom needed.

99% of RF, instruments, audio use ordinary coax single shield RG174/U, RG58/U, RG59/U

Even when I do not need the extra shielding, I like to use double shielded cable to make patch cables because the double shielding makes the connector attachment more rugged and reliable.

[radiolistener]

there is no ideal shielding, it depends on the frequency and interference type (magnetic, electric or plane electromagnetic wave). But things even a little bit more complicated, because there is also effect of common mode currents on outer braid of coax cable. Also note that coax cable impedance is out of specification at low frequency.