Compensation of electromagnetic distortion caused by metal mass

A shield for electromagnetic radiation comprising:

• a first layer having a high magnetic permeability;
• and
• a second layer having a high electrical conductivity,
• at least one of said first and second layers having a plurality of apertures formed therein, said apertures being operative to reduce the effect of the shield on the electromagnetic field outside the shield,
• wherein said plurality of apertures comprise a plurality of generally "U"-shaped slots,
• wherein said shield has a generally cylindrical shape of a given length,
• wherein said first layer comprises an inner portion, and said second layer comprises an outer portion of said generally cylindrical shape,
• wherein each of said "U"-shaped slots comprises a lengthwise slit and two circumferential slits in said second layer,
• wherein said circumferential slits are generally perpendicular to said lengthwise slit and communicate with the ends of said lengthwise slit, and
• wherein said two circumferential slits are separated from a nearby lengthwise slit of an adjacent "U"-shaped slot by a solid conducting material surface.