Time Capsule / AirPort Extreme Wi-Fi Signal Regard maximizing the Time Capsule or AirPort Extreme Wi-Fi signal, just for my curiosity and knowledge, from the hardware standpoint why is not recomended to place the Time Capsule or AirPort Extreme in areas surrounded by metal surface on three or more sides?
Radio waves (your WiFi signal) are blocked/reflected by metal surfaces.

If you surround the antenna with metal surfaces on 3 sites you will get very limited reception.

Using that factor one can build a directional antenna that works in one direction and magnifies the signal. 

If you have a TV dish that is how it works, it is a directional antenna that reflects the signal to a receiver mounted in the focal point.

So if you want to boost your Airport signal you would use a directional antenna. As it is right now it has a signal that is 360 degree.