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Thinking of radio waves as light

Discussion in 'Electronic Basics' started by John Smith, Oct 23, 2003.

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  1. John Smith

    John Smith Guest

    Radio waves, being on the em spectrum, is light. So, can I imagine being
    able to see radio waves and imagine seeing that light shining through
    windows? And does it shine through thin walls, at not at all? Many thanks.
     
  2. John Larkin

    John Larkin Guest

    Radio wavelengths are long, so the view will be fuzzy. It will shine
    through most walls very well, but metal (wires, pipes, even nails for
    the shortest wavelengths) will reflect, and even insulators diffract
    and reflect a little.

    Given a small board full of patch antennas and a lot of DSP, it should
    be feasible to make a short-range UWB (baseband impulse) imaging
    radar, maybe packaged sort of like binoculars with little LCDs inside.
    That would be a great consumer product and a hell of a stud finder.

    John
     
  3. Dana

    Dana Guest

    Only low freq radio will have long wavelengths. From 800MHZ on up they get
    pretty short.
     
  4. John Smith

    John Smith Guest

    That's intensely interesting. I'm trying to get a feel for why the radio
    goes out when I stand in certain parts of the room, mostly near the large
    window. I thought, well if radio wave are like light then maybe they are
    shining through the windows and maybe I'm casting a shadow. I'm having
    trouble seeing how this light can shine through walls merely because there
    is more of a delay between one pulse of energy, a wave, and the next. I know
    that from watching welders that high frequency light casts sharper shadows
    at a further distance than regular light... but I'm not certain why that is,
    either. Refracting less? But, why?

    Also, if you wouldn't mind: What is a patch antenna? What is a DSP? What
    is a "base band impulse"? I know, I'll look it up, but I want to see your
    answers, too. Many thanks.
     
  5. John Larkin

    John Larkin Guest

    800 MHz is still 0.375 meters, not very good for, say, shooting a game
    of pool. Even x-band radar is still 3 cm, damned fuzzy compared to
    normal vision.

    John
     
  6. Jim Large

    Jim Large Guest

    800Mhz... That's 37.5 cm. Shorter than some
    wavelenghths, longer than others. In fact it's
    around eight hundred times longer than the middle
    of the AM broadcast band, and its around five
    hundred THOUSAND times longer than the longest
    visible wavelengths. Don't be surprised if it
    behaves a little differently from visible light.

    -- Jim L.
     
  7. John Larkin

    John Larkin Guest

    What often happens is interferance. If the waves take multiple paths
    from the transmitter to you, the paths can get 1/2 wave (etc) out of
    phase and cancel. Move a few feet, and you can find a spot where they
    reinforce. Just like optical interferance fringes.
    Light will shine through materials thinner than a wavelength, even
    thin metal films. Ditto radio: the dimensions are just all much
    bigger.

    Your body is too small to cast much of a shadow at AM and FM
    frequencies... the waves just flow around you, like a popsicle stick
    poking up on the beach.
    That's true, but probably not actually visible. An arc welder is just
    a very small, intense source so it makes nice shadows.
    Oh, OK.

    A patch antenna is a little, well, patch of copper on a
    printed-circuit board, designed to be an antenna. Cheap.

    DSP = Digital Signal Processing. You'd need a lot of number crunching
    to turn a bunch of radar echoes into a 3D image.

    A baseband impulse is just a very short, sharp pulse, not modulated
    onto a carrier wave, just blasted into space. UWB (ultra wideband
    radar) often uses impulses as the transmitted/received signal.

    Read up on this stuff; it's cool.


    John
     
  8. Jim Large

    Jim Large Guest

    Interference. Have you ever seen a laser
    speckle pattern? The same thing happens on
    a larger scale anywhere you have radio waves
    bouncing around in an enclosed space (i.e.,
    a room). There are strong spots where the
    various reflection paths are in phase so
    that they reinforce one another, and there
    are weak spots where they are out of phase
    and they cancel out.

    You are not casting a shadow. You *WOULD*
    cast a shadow (a faint one) if you were out
    in space, and the transmitter were far away.
    You don't cast a shadow in your room for the
    same reason you don't cast a shadow on the
    ground on a cloudy day. On a cloudy day,
    the light is coming from all directions.
    The radio waves in your room are coming from
    all directions too because they're bouncing
    off the walls of your house and all of your
    neighbors' houses, nearby hills, etc.
    The wave crests are not "pulses of energy."
    I don't know what they are. I don't know
    if anybody *REALLY* knows what they are.
    All I know, and all that most garden-variety
    physicists know is that whatever electro-
    magnetic energy is, it seems to obey (ON
    AVERAGE!) the same mathematical laws that
    govern waves like on the surface of a pond.

    My understanding of why certain wavelenghts
    pass through some materials and not through
    others is pretty weak, but I know that it
    has to do with the fact that electrons in
    the material are confined to certain "energy
    levels," and they interact with photons of
    different wavelengths (i.e., photons of
    different engery) only when the difference
    between two allowable energy levels for the
    electron is compatible with the photon
    energy.

    What is "allowable" and what is not
    allowable depends on the the atoms in the
    material and on the way they bond to one
    another, and it's so far over my head it
    isn't funny.
    Ah, shadows again.

    That's a simple geometry problem.
    A welder's arc is a very compact, very bright
    source of light. A compact source casts
    sharper shadows than a larger source, and the
    fact that the source is very bright makes the
    shadow very noticible.

    -- Jim L.
     
  9. John G

    John G Guest

    The AM Broadcast band is about 500 to 1600 kilohertz and that is 600 metres
    to around 185 metres which is quite a bit longer than 37.5 centimetres not
    shorter.
    But it is mny times longer then visible light.
     
  10. Jim Large

    Jim Large Guest

    Ooops! I meant to say "shorter," really!

    -- Jim L.
     
  11. Rich Grise

    Rich Grise Guest

    It would easily be visible, if you have eyeballs the size of a high-rise
    apartment building. ;-)

    Cheers!
    Rich
     
  12. Radio waves are electromagnetic radiation as are light waves. But
    each is in a completely different part of the spectrum, so saying
    "radio waves .. is light" (sic) is not true.
    Radio waves can penetrate walls like light penetrates glass. But not
    all wavelengths of light can penetrate, as not all wavelengths of
    radio waves can penetrate walls.

    --
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  13. In John G typed:
    All this talk about objects thinner than a wave being transparent makes
    me wonder about antennas. So then why does an antenna not have to be
    more than a small fraction of a wavelength?

    And what about the principle of shielding as in the door of a microwave
    oven? The rule there seems to be that the screen is opaque because the
    holes are smaller than a wavelength, but that measurement is
    perpendicular to the wave. Why?
     
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