Time Capsule / AirPort Extreme Wi-Fi Signal
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.