Faraday cages and cables

agree, the braid on a coax is indeed a form of electrostatic shield, so the volts between inner and outer are not (to first order) affected by the electric fields outside the jacket.

Actually, to define how good (or bad) the shielding is, we often talk about the transfer impedance of a coax (not to be confused with the characteristic impedance or surge impedance.)


The transfer impedance tells you how much the voltage difference between inner and outer is upset if a current that is not part of the wanted signal is flowing on the outer.


Imagine, if you had a very simple wanted signal and  put a 9V battery between inner and outer at the source end, and measured with a meter at the load end, you would also see 9V between inner an outer at the other end, the wanted "signal" is getting there un-affected..

IF however you put say 1 amp down the braid from some other source, then because that causes the braid voltage drop to slope, but not the inner, it is as if a small part of the current outside has got in series with the wanted signal when you measure it - so a transfer impedance of one ohm per meter, would mean a change of 1V (could add or subtract- depends on the direction) the wanted signal, per amp or induced braid current.


For completeness, the characteristic impedance, typically 50 ohms, 75 ohms or other is the ratio of inner  core current to voltage between inner and outer for a length long compared to the measurement time. (and is also the impedance you use as a load to make it look infinitely long, and suppress multiple transit effects)

agree, the braid on a coax is indeed a form of electrostatic shield, so the volts between inner and outer are not (to first order) affected by the electric fields outside the jacket.

Actually, to define how good (or bad) the shielding is, we often talk about the transfer impedance of a coax (not to be confused with the characteristic impedance or surge impedance.)


The transfer impedance tells you how much the voltage difference between inner and outer is upset if a current that is not part of the wanted signal is flowing on the outer.


Imagine, if you had a very simple wanted signal and  put a 9V battery between inner and outer at the source end, and measured with a meter at the load end, you would also see 9V between inner an outer at the other end, the wanted "signal" is getting there un-affected..

IF however you put say 1 amp down the braid from some other source, then because that causes the braid voltage drop to slope, but not the inner, it is as if a small part of the current outside has got in series with the wanted signal when you measure it - so a transfer impedance of one ohm per meter, would mean a change of 1V (could add or subtract- depends on the direction) the wanted signal, per amp or induced braid current.


For completeness, the characteristic impedance, typically 50 ohms, 75 ohms or other is the ratio of inner  core current to voltage between inner and outer for a length long compared to the measurement time. (and is also the impedance you use as a load to make it look infinitely long, and suppress multiple transit effects)