Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I Large wavelength and "fitting into" small spaces

  1. Jun 20, 2017 #1
    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?
    Woolsey_Bridge_oblique_view.jpg
     
  2. jcsd
  3. Jun 20, 2017 #2

    berkeman

    User Avatar

    Staff: Mentor

  4. Jun 20, 2017 #3

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    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.
     
    Last edited by a moderator: Jun 20, 2017
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Large wavelength and "fitting into" small spaces
Loading...