By Ian Darney

The screen of a coaxial cable does not form a barrier to electromagnetic interference. It neutralises the effect of that interference.

Transient current flows along the core of a coaxial cable and creates an electromagnetic field which radiates out through the screen into the environment. In doing so, it induces a voltage in the screen which causes a current to flow in the opposite direction to that in the core. The screen current creates one field which radiates into the environment and another which radiates energy back to the core. The fields emanating from the core and the screen into the environment neutralise each other.