By "radio waves" I mean waves used for radio transmission appliances, e. g. within 1 MHz to 10 GHz.
If the size of the metal sheet makes a difference, please answer for a infinite sheet and a fridge.
If the distance from the emitter to the sheet matters, please provide an answer for several wavelength, half wavelength and flush on the surface.
If you would like to provide a more general answer including different length and materials, please do so.
Additional concerns are also appreciated, e. g. interference, ungrounded case, etc.
Best Answer
A perfect conductor is a perfect reflector of electromagnetic waves. This follows from conservation of energy, since the wave can't exist inside the conductor. It doesn't matter if the conductor is grounded.
Real sheet metal is not a perfect conductor, so you have a skin depth, which is $\delta=(\sigma fk)^{-1/2}c$. Plugging in the conductivity of aluminum, for instance, and a frequency of 1 MHz, we get half a millimeter, which is a lot less than the thickness of a typical piece of sheet metal. For higher frequencies it will be even lower. So there will be very little transmission of the wave at these frequencies.
However, there will be a certain amount of absorption, so not all the wave energy will be reflected. Estimating this is something I know less about, but I believe that in this frequency range it can be estimated using the Hagen-Rubens equation, which gives for the fractional absorption $\alpha(1-\alpha)$, where $\alpha=2\sqrt{f/k\sigma}$. Even for the highest frequencies you refer to, we have $\alpha\sim 3\times10^{-4}$, so absorption is negligible for most purposes.