# Large Cavity Oscillators

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

1. ### helixGuest

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?

*-----------------------*
Posted at:
www.GroupSrv.com
*-----------------------*

2. ### Robert BaerGuest

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. ### helixGuest

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.

*-----------------------*
Posted at:
www.GroupSrv.com
*-----------------------*

4. ### Robert BaerGuest

Higher frequencies can create multi-modal patterns.

5. ### helixGuest

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.

*-----------------------*
Posted at:
www.GroupSrv.com
*-----------------------*

6. ### Robert BaerGuest

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

7. ### helixGuest

[/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.

*-----------------------*
Posted at:
www.GroupSrv.com
*-----------------------*

8. ### Robert BaerGuest

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.

*-----------------------*
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. ### beastGuest

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?
*---------------------------------*
Posted at: http://www.GroupSrv.com
Check: http://www.HotCodecs.com
*---------------------------------*