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Light from antena?

Discussion in 'Electronic Basics' started by [email protected], Jul 30, 2005.

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  1. Guest

    If light is part of the same spectrum and waveform as radio waves and
    radio waves can be transmitted through an antena then couldn't visible
    light? If a sine wave driver was adjusted to run in the visible
    spectrum and then connected to an earth ground and needle point antena,
    could visible light be seen coming of off the needle point?
  2. John Fields

    John Fields Guest

  3. Guest

    Around the antena? By the way, has this ever been tried before?
  4. John Fields

    John Fields Guest

  5. Nick Funk

    Nick Funk Guest

    I believe it is called a light bulb!
  6. "John Fields"
    I recommend using a "generic" isotropic point radiator for best results.
  7. Jamie

    Jamie Guest

    i would like to see the set up capable of emitting that spectrum! :)
    but to answer you question, you would see light but only at the
    initial take off point and normally with in the first wave length or
    half wave length..
    i am taking this from memory while in school many years ago and
    back then it was just more theory on some parts of the instructors
    since the thought of generating RF at the wave length was far fetched!
    don't remember all of the physics behind it but i seem to remember
    that you should see only a surface glow effect due lost and wave length
  8. You would need a sinewave oscillator of frequency in the 428-750
    terahertz range. A halfwave dipole would have a length of .2 to .35
    micrometer. If you want to make an oscillator along the line of a Hartley
    or a Colpitts or an Armstrong, you need an inductor wound with a wire a
    fraction of a wavelength long (less than .2-.35 micrometer), and all other
    parts much smaller still - submicroscopic.

    As for semiconductors that oscillate at such frequecies nowadays: We
    have laser diodes.

    A halfwave dipole antenna in a laser beam will produce a sinewave
    electrical signal - but have fun finding a way to detect AC at frequencies
    that high, or building an antenna too small to see with any ordinary

    - Don Klipstein ()

  9. The basic idea is sound, but consider that FM radio has a wavelength of
    several meters, long wave radio of several km. That makes building
    antennas (which have 1/4 or 1/2 of that length) relatively
    straightforward. Wavelength for visible light is 400--700 nm, light
    antennas are therefore a subject of nano-technology.
  10. Eric R Snow

    Eric R Snow Guest

    Why antennas made that are 1/4 or 1/2 the wave length? Is it because
    the energy emitted is a sine wave? Or am I totally off base. I'm just
    curious and have little electrical knowledge.
  11. Don Bruder

    Don Bruder Guest

    Physical size/convenience and radiating properties.

    A quarter or half-wave antenna can send/receive the tuned frequency just
    fine, in literally half or a quarter the space of a full-wave antenna
    for that frequency. Which, in the case of low frequencies, might
    literally mean hundreds of miles. (See also "Project Seafarer", later
    renamed "Project ELF" (for "Extremely Low Frequency") and the plans they
    had for turning a strip of Michigan's upper penninsula into an
    ultra-jumbo antenna for talking to the nuke subs back in the 70s.)
  12. Rich Grise

    Rich Grise Guest

    Actually, it's not. It's a different mechanism.

    What you really need is an atom-sized klystron! ;-)

  13. I do recommend getting from your library and reading relevant sections
    of the "ARRL Handbook" and the "ARRL Antenna Book".
    ARRL stands for "Amateur Radio Relay League", and editions of these
    books 50 or probably more years old remain valid in this area. Antenna
    theory known as of WWII covers this.

    - Don Klipstein ()
  14. No, it doesn't. ARRL stands for "American Radio Relay League". I
    have a copy right here by my desk.

    Link to my "Computers for disabled Veterans" project website deleted
    after threats were telephoned to my church.

    Michael A. Terrell
    Central Florida
  15. Bob Myers

    Bob Myers Guest

    A short-form, somewhat-simplified explanation is as follows:

    Consider what happens on a quarter-wave long length of transmission
    line, open at the far end. Or better, at multiples of a quarter-wavelength
    back from the open end on such a line. The "standing wave"
    pattern on such a line, resulting from the total reflection of the
    forward signal by the "open," has a voltage maximum (and
    obviously, a current minimum) at the open end, and conversely
    a voltage minimum (and current maximum) at a quarter-wavelength
    back (with this pattern repeating from there).

    Now, think about a half-wavelength antenna - a simple "dipole"
    - as being nothing more than such a quarter-wave section of
    line, "opened up" such that one conductor points this-a-way and
    the other conductor points that-a-way (in the opposite direction).
    Our first guess might be that the standing-wave situation described
    above remains in effect - the tips of this "opened-up" structure are
    points of high voltage (and opposite polarity, with respect to one
    another), while the center is a low-voltage/high-current (read: low
    impedance) point. This is precisely what you want for making EM
    radiation - an electric field created "across" the structure (i.e., between
    high-potential ends), plus a magnetic field created "around" it and in the
    correct phase relationship. So what we've just described ought to be just
    fine for launching EM radiation, while providing a reasonably decent
    impedance at the point we wish to drive (the center of the structure).
    So, with at least very minimal math, this is a workable model or
    visualization as to why half-wave dipoles work well for transmitting
    radio waves. (And fortunately, what works well for sending such
    signals works just as well for receiving them.)

    A quarter-wave antenna results from noting that the two pieces of
    the above structure are really just quarter-wave-long mirror
    images of one another, and either of them will work in the same
    manner if the other is replaced by a "ground plane" normal to the
    remaining element and passing through the center point of the original
    dipole structure. This plane, from the standpoint of determining the
    fields produced by the remaining element, really DOES act like a
    "mirror," although obviously nothing is really emitted by the "mirror
    image" viewed as being on the other side of that plane.

    In reality, various factors make for neither the half-wave dipole or
    the quarter-wave derived from it exactly a half- or quarter-wave
    long, relative to the wavelength of the signal in question in free
    space. They're generally going to turn out being a bit shorter, to get
    the proper "resonance" and a decent (resistive only, in the ideal case)
    feedpoint impedance. But they're close enough that these names
    work just fine.

    Bob M.
  16. Bill Bowden

    Bill Bowden Guest

    Why antennas made that are 1/4 or 1/2 the wave length?

    It's a natural length that works best without extra parts.
    Shorter antennas less than 1/4 wavelength are sometimes
    used where the antenna needs to operate at different frequencies.
    But they need to be tuned with extra inductors and capacitors
    to keep the whole arrangement resonant at whatever frequency
    is used.

    The basic idea is to make the antenna look like a resistor
    to the transmitter, and a 1/4 wavelength looks pretty close to 50 ohms
    without extra parts.

    Think about it. If the transmitter is connected to a resistor,
    all the energy is converted to heat. Now if you connect
    the transmitter to an antenna that acts the same as a resistor,
    and the antenna doesn't get hot, the energy must have
    gone out into space, which is what you want to do.

    But, if the antenna is not the right length, and looks like a
    capacitor or inductor, the energy will flow back a forth between
    the transmitter and antenna and probably make the transmitter
    hot, and not radiate much energy, other than heat.

  17. Eric R Snow

    Eric R Snow Guest

    Thanks Don,
    I'm going on vacation and will try to get those books before I leave.
    Nice, relaxing, reading.
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