I was thinking about shielding and its effectiveness.
An ungrounded shield doesn't work well for either inductive or capacitive coupling.
However, if you think about plane wave theory, a PEC surface completely reflects the EM wave (reflection coefficient is -1), so you don't have any power density at the other side of the PEC surface. That seems to be the ideal shield for any EM wave/interference. This should lead to the conclusion that if I wrap around a cable with a PEC shield, no EM waves will be able to either go out or into the shield, be it grounded or ungrounded. Therefore, it should be immune to any interference or coupling.
But it seems not to be the case, why?
I suppose it has to do with the fact that coupling is a near field phenomenon, while plane wave theory usually considers the far field. But how exactly it is different, that I don't know.
Best Answer
A perfectly electrically conducting (PEC) surface would reflect plane waves if it were of infinite extent. If it's got holes, or if it's got edges, the waves will go around it, and even refract around to the back surface depending on the relative size of the plate and the waves.
If you wrap a cable with a PEC shield and do not ground it at one end, that lack of ground connection constitutes a hole in the shielding. If you do not ground it at either end, that constitutes two holes in the shielding. Either way, to one extent or another, EM will radiate.
You can think of this in electrical terms if the wavelength is long with respect to the cable length: in that case, particularly with shielding that's not grounded at either end, the entire shield will be capacitively coupled to the cable innards, and will to some extent follow it. Then the outside of the shield will radiate.