Here is an alternate view:
"And oh yeah—yes, your Faraday cages DO need to be grounded. If it’s NOT grounded, then the Faraday cage merely becomes a reflector or an amplifier.
http://www.preparednesspro.com/emp-101-part-iv-faraday-cage
EMP 101: Part IV—Faraday Cages
By Kellene Bishop...