An innovative development described recently in Interference Technology magazine (click to see article) provides a way to couple data and power through metal shielding. This has been tested by the developers through steel up to 15mm thick, and could be very useful in a multitude of monitoring-type and other applications which could benefit from the simplicity and cost saving possibly afforded by being able to pass signals and power through the walls of vessels, pipes, other enclosures and large barriers.
Of course it always has been possible to pass electromagnetic signals and possibly a little useful power through most forms of metal shielding, provided, with an appropriate system, that the often considerable power loss involved could be tolerated and/or a low frequency could be used.
Shielding does not form a perfect barrier to electromagnetic disturbances; it merely attenuates very strongly, ideally to a point of insignificance for the relevant frequencies in the context of the intended shielding purpose. And the attenuation encountered is dependent on frequency; in simple terms, increasing with higher frequency and (importantly) tending to zero at very low frequency.
The technology described apparently mitgates the attenuation encountered through steel (a very common containment material) by modifying its properties, namely by magnetically saturating an area of the wall. This reduces the effective magnetic permeability of the steel in that area, thus increasing the skin depth for any given frequency and thereby reducing the attenuation. Thus it is possible to significantly improve the practical frequency range available for coupling through a thickness of steel, and so for many walls more commonly encountered.