Frank wrote: > At 03:57 pm 19-01-05 -0800, you wrote: > >> I put in the radio to demonstrate the Faraday Cage effect > >but to my disgust the radio sounded as loud as ever. In > >fact the 12x6 lid, which had folded edges soldered at the > >corners, was a close fit but not air tight. Presumably the > >long cracks were sufficient to allow penetration of the AM > >signal. > > > >That doesn't sound right. How did you ground it? > > > > Ground it??? I didn't know you were supposed to ground it! 8-( > [Curls up in embarrassment] > > You can see why I failed Applied Electricity in my first term at > UCL .
MC: In the case cited, grounding of the can is irrelevant. Small AM radios have a ferrite stick as the antenna, which responds to the magnetic portion of the EM fields and grounding is not needed. They also have automatic gain control circuits which try to make local, strong stations and distant weak stations sound equally loud. Thus if you were in a urban area with nearby transmitters, your box migt be a effective shield, giving, say 100 x attentuation of the incoming signal. The radio would just up its internal gain to make the output equally loud. Yes, the signal gets in through the gaps between the lid and the box, illustrating my earlier point that you have to think watertight and airtight when discussing shielding. Mike Carrell
