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Large Cavity Oscillators

Discussion in 'General Electronics' started by helix, Dec 15, 2004.

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  1. helix

    helix Guest

    I have an article about a large room being used as a microwave cavity
    but cannot find the dimensions of the room, it isn't available
    online.
    lecture hall, the cavernous Room 26-100, could be turned into a
    low-energy microwave cavity. "He mounted a light bulb on an antenna,"
    recalls Leeb, "and he'd walk around mapping the microwave field
    according to where the light bulb would glow or dim. It was
    terrific."

    http://web.mit.edu/giving/spectrum/...n-learning.html

    I want to calculate the approximate frequency of the primary mode
    generated by the cavity and wonder if anyone has the dimensions for
    Room 26-100?

    I have heard that amateur radio "microwave" enthusiasts have sometimes
    turned metal silos into microwave cavities. Microwave is sort of a
    misnomer as the MIT room cavity could be in the low high frequency
    range. Does anybody know of other examples of "large" microwave
    cavities?

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  2. Robert Baer

    Robert Baer Guest

    The old cars and trucks (ie: no junk under the hood, just the absolute
    essentials) have a broad peak of EMI in the FM band, with vertical
    polarization.
    That means that the space left inder the hood acts as a very low Q
    "microwave" cavity, and the hood gap is the leaky (horizontal) antenna.
    So a rather large room might have a much broader "peak" around the
    10-20MHz region.
    One should not expect any resonance due to all of the lossy materials
    inside.
     
  3. helix

    helix Guest

    I reentered the URL for the MIT article in my original post. For some
    reason part of it was truncated. I had contacted Professor Leeb a
    several years ago and asked him about that item in the article, and
    he stated he had observed the demonstration. I didn't ask him at that
    time about the room size or what mode(s) were being generated.

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    www.GroupSrv.com
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  4. Robert Baer

    Robert Baer Guest

    Higher frequencies can create multi-modal patterns.
     
  5. helix

    helix Guest

    Robert Baerwrote:
    Higher frequencies can create multi-modal patterns.
    Any electromagnetic container will generate multiple modes, but am
    interested in the lowest frequency that might be generated in that
    room.

    I suspect the floor of that building is concrete and wonder how that
    satisfied the "boundary" conditions necessary to generate a resonance
    in a cavity.

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  6. Robert Baer

    Robert Baer Guest

    If it is fairly "moist" it might act as a lossy "reflector"...
     
  7. helix

    helix Guest

    [/quote]Robert Baerwrote: If it is fairly "moist" it might act as a
    lossy "reflector"...[/quote]

    Interesting. I have heard that one can "paint" a surface with a
    material like "aquadag" or "electrodaq", which are carbon based
    paints, to make non-conductive materials equivalent to metallic
    enclosures. If you have a standard crt monitor, a major portion of
    the interior and exterior of the glass envelope is "painted" with one
    of the carbon paints for shielding and conductive purposes.

    The MIT article didn't state how the "cavity" was excited. I am having
    trouble finding a internet site that discusses the various methods of
    exciting a cavity resonator.

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    Posted at:
    www.GroupSrv.com
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  8. Robert Baer

    Robert Baer Guest

    Robert Baerwrote: If it is fairly "moist" it might act as a
    lossy "reflector"...[/quote]

    Interesting. I have heard that one can "paint" a surface with a
    material like "aquadag" or "electrodaq", which are carbon based
    paints, to make non-conductive materials equivalent to metallic
    enclosures. If you have a standard crt monitor, a major portion of
    the interior and exterior of the glass envelope is "painted" with one
    of the carbon paints for shielding and conductive purposes.

    The MIT article didn't state how the "cavity" was excited. I am having
    trouble finding a internet site that discusses the various methods of
    exciting a cavity resonator.

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    Posted at:
    www.GroupSrv.com
    *-----------------------*[/QUOTE]

    Be advised one can use more conductive paints: nickel, copper or
    silver.
    Carbon based paints are rather lossy.
    Nickel Carbonyl paints seem to be even more lossy and formulated for
    EMI suppression.
     
  9. beast

    beast Guest

    Quote: "Lewin, says his former student, figured out that MIT's biggest
    lecture hall, the cavernous Room 26-100, could be turned into a
    low-energy microwave cavity. "He mounted a light bulb on an antenna,"
    recalls Leeb, "and he'd walk around mapping the microwave field
    according to where the light bulb would glow or dim. It was
    terrific."


    He must have mounted the light bulb in the center of the room, and
    map the reflection of the light on the walls , using a light meter,
    such as those used by photographers, get a good idea of the EM
    patterns inside the room.

    The resonnant frequency calculations would be the same as those used
    to calculate small cavities. I wonder if an EM beam penetrating such
    enclosure would go out the same direction it came in?
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    Posted at: http://www.GroupSrv.com
    Check: http://www.HotCodecs.com
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