# Does grounded sheet metal reflect or absorb radio waves?

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.

• This is way more than one question. You're practically asking for an entire E&M course. See Ben Crowell's answer for the infinite sheet case. Google "refraction" for the case of your refrigerator (whose size you did not specify, and size in wavelengths matter). If you take your "distance from source" question and turn it into a question about plane waves striking an infinite sheet head-on, and what's happening at various distances, that's probably compact enough to answer reasonably. Dec 23, 2019 at 16:19
• I was just wondering if placing a radio remote control on my fridge will affect signal quality and in what way (improve or worsen). Dec 23, 2019 at 17:41
• I just wanted to understand how it works, rather than getting a yes or now answer. Dec 23, 2019 at 17:42
• @lolmaus-AndreyMikhaylov: I would suggest editing the question to explain the motivation for the question, rather than only giving that explanation in comments.
– user4552
Dec 23, 2019 at 18:39

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.