But even that has a weakness...
...in that the Bad Guys can join the network and learn the realtime "shield nutation frequency" (just love that TrekBabble™), providing them with realtime calibration.
Like you said elsewhere, this is just what we can come up with in a couple of hours of idly shooting the bull. Looking at the big picture, I figure the NSA wouldn't have applied for a patent, with necessary disclosure, if they either didn't feel pretty confident they could make it work despite countermeasures...or they don't think it'll be very effective, and they just don't care if the whole world knows. Beyond that spot down the slippery slope of speculating about the minds of spooks I dare not go...