Can somebody help

I have got a transmitter card, a power supply , two fans and an
amplifier.
The transmitter card accepts data from a computer and processes and
trasmittes the required signal based on the data from the computer.
The amplifier is used to amplify the signal generated by the
trasmitter card. And the power supply gives the necessary voltages for
the Amplifier, transmitter card and the two fans. The fans are used
for cooling mechanism.
All this electronic stuff is placed in a metal case.
When the metal case is open the system workes according to the
specification but when the metal case is closed it doesn't work at
all. Can some body help me why?

 

[JeffM]

fissehaye@ gmail.com wrote:

I have got a transmitter card, a power supply ,
two fans and an amplifier.[...]
All this electronic stuff is placed in a metal case.
When the metal case is open
the system workes according to the specification
but when the metal case is closed it doesn't work at all.

http://www.google.com/search?q=cache:_WsLLzPQTDkJ:en.wikipedia.org/wiki/Faraday_cage+shield.the.interior.from.external.electromagnetic.radiation
It also prevents radiation inside the cage getting out.