Originally Posted by
Saab Dastard
In theory, a metal shield at the PC antenna on the opposite side to the direction of the desired base station could be beneficial in helping to reduce the strength of unwanted signals - but only from that direction. It wouldn't help with signals from the same direction as the desired base station.
The real problem, though, is that the reflection of the signals coming from the desired base station (and any other extraneous signals) by the metal shield could cause destructive interference at the PC antenna and actually lower the strength of the desired signal!
Basically this is aerial theory. A TV array aerial has a line of 'directors' in front of the actual aerial and a large reflector behind the aerial. The aerial itself is a dipole sized to the central freqency band of the transmitted frequencies. The directors are set at wave-length distance to boost the signal and the reflector behind is similarly spaced to reflect the signal back. As SD says, if that interval is exactly wrong then the reflected signal will be 180 deg out of phase and cancel the signal.
For your router 'transmitter' you can have a reflector buy it must be at the correct distance otherwise it will cancel the signal. A program such as netstumbler will allow you to observe the signal/noise ratio as you move the reflector or as you angle the antenna.