> That's correct. Any good conducting shielding
> connected to ground (earth)
> will strongly reduce radio waves from both sides.

It does if  .........

(1) it has at least TWO independant layers of VERY fine and high quality
mesh
(2) is Soldered or bonded at ALL the seams and has absolutely no holes (eg
no windows and doors)
(3) and all cables passing through (eg mains, phones) have low pass filters

And BTW, earthing makes little difference at VHF/UHF.
  (how many wavelenghts is the earth lead?)

I once set up a screened room which was about 1/2 km away from a phone
tower.

We never could totally eliminate the break-through..

................... Zim