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REALLY long wavengths.

  1. Nov 21, 2008 #1
    We all know you create E&M waves by spinning a dipole. Wether it's an AC current in an antena to produce radio waves, or an electrion falling to n=1 to produce an x-ray, moving charges create EMR.

    That being said, theoretically, one could charge a macroscopic object, oscillate it extremely rapidly, and produce EMR. Has anyone done this experimentally? It's a rather silly experiment, but what would be the low frequency/long wavelength detection limit for radiation?
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  3. Nov 22, 2008 #2


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    ...in order to pick up the signal on a radio you would have to shake the macroscopic object back and forth over a millions times a second. That's sort of difficult with a *macroscopic* object.
  4. Nov 22, 2008 #3
    As far as detection of radiation goes: as a rule of thumb, you need an antenna roughly the size of a wavelength to detect radiation (maybe a quarter wavelength, but that is the order of magnitude). As the wavelength increases beyond a few metres (frequencies < maybe a few MHz), it starts to get difficult.
  5. Nov 23, 2008 #4

    Thats not entirely true. If we take a radio signal broadcast at 100 MHz then it would have a wavelength of 3m. Yet there are those very tiny radio's only a few centimeters long which manage to pick up the signal quite clearly? These are only around 1/1000 the length of the wavelength. I would say that it is less than this is where it gets difficult.

    Then again this is only for radio where a loss of quality is accepted. So it only works if you can lose some of the data.
  6. Nov 23, 2008 #5
    Submerged submarines receive signals as low as 76 Hz (ELF) and it has been proposed that huge wire dipoles could be floated in space in order to communicate over interstellar distances at a carrier frequency of 0.1 Hz. Bit science fiction as half wave dipoles would be 1.5 million km long and the data rate would be very very slow.

    Short antennas don't receive well and portable radios working at 150 kHz - 1.5 MHz (AM bands) use ferrite rod aerials which are also not efficient.

    The lowest frequency you are likley to receive at home is the 60 - 80 kHz (5000 - 3750 metre) signal for a radio controlled clock. These use ferrite rod 'magnetic...loop' aerials.
    Last edited: Nov 23, 2008
  7. Nov 23, 2008 #6


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    ELF and VLF transmitters are used by the Navy to communicate with submarianes at sea.
  8. Nov 23, 2008 #7
    You mean pulsars?
  9. Nov 23, 2008 #8


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    *who* means what?
  10. Nov 23, 2008 #9
    I don't know about you, but my ears are nowhere near a half wavelength in dimension with respect to what "radiation" they can detect. :smile:

    Look for info on "radiation resistance" to see why small apertures (<<wavelength) are not efficient antennas.


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