Large wavelength and "fitting into" small spaces

[snate]

As far as I know radiowaves can't penetrate such kinds of metallic constructions because their wavelength is too long to "fit in the gaps". But I don't understand what does the wavelength have to do with being able to penetrate though these gaps or fitting into small spaces. Can someone explain it please?

 

[berkeman]

Do you know what a typical wavelength is for AM radio (in the US)? The frequency is around 1MHz, so the wavelength is around 300 meters.

Does your AM radio in your car work as you drive across a bridge like that one?

Maybe your question is better answered with a shielding structure like this:

http://www.fsindustries.com/more_info/wireway_security_room/images/security_cage_drawing.gif

 

[sophiecentaur]

As far as I know radiowaves can't penetrate such kinds of metallic constructions because their wavelength is too long to "fit in the gaps". But I don't understand what does the wavelength have to do with being able to penetrate though these gaps or fitting into small spaces. Can someone explain it please?

It's more a matter of degree than absolute screening. Radio waves can penetrate into all sorts of small spaces and it is incredibly hard to reduce interference levels to 'undetectable' levels, even inside a so-called screened enclosure. Imagine a single hole in a perfectly conducting and massive sheet of metal. An incident wave will cause currents to flow all over the sheet and that will mostly cause the wave to be reflected back (like a mirror). But the 'missing currents' in the hole will cause currents flowing around the edges of the hole and that behaves like a small aerial, radiating signals through to the other side of the sheet and also back out again.

For an antenna to radiate well, it needs to have dimensions of the order of a wavelength (a half wavelength dipole or slot will radiate well). A much smaller hole will radiate only a small amount of energy and a hole of just a couple of cm will let through a very small amount of a 300m wavelength wave. The same is true for a whole mesh of holes and the wires (or metal bridge frame) will act like a pretty good reflector and, hence, a good screen, as long as the holes are a small fraction of a wavelength.

A steel framed building may reduce mf radio signals to an unusable level but cell phone signals (much shorter wavelengths) will rattle around the inside quite happily as they reflect off the large steel components and go straight through the gaps. Microwaves in an oven are screened perfectly adequately (but not perfectly!) by the mesh in the door; small holes / short wavelength.