On Mar 21, 3:25*pm, Bill wrote:
On Mar 21, 4:59*pm, joe wrote:



If it is this article,http://www.fas.harvard.edu/~scidemos...sm/SkinDepth/S...
Then more was given.

It was an experiment in skin depth.

Strictly speaking the page describes a demonstration from page 321 of
this book:

G. Bekefi and A. H. Barrett, Electromagnetic Vibrations, Waves and
Radiation, *(MIT Press, 1977)

http://www.amazon.com/Electromagneti...ion-George-Bek...

http://mitpress.mit.edu/catalog/item...d=7576&ttype=2

A reviewer comments, "MITonline offers the course based on this book
for free. The course is great! This book is a less engaging, but a
more comprehensive version of A.P. French's original text for the
course. There are many gems in the book, such as insights into
microwave oven fundamentals, that are not present in basic texts.
Beware the softcover binding, it needs a gentle hand. "

http://www.amazon.com/Electromagneti...ion-George-Bek...

What does the book point to, waves or particles? We all have plenty of
books and all cannot agree on the subject on radiation! In other words
they have placed their own interpretations of the observations seen
with the double slit experiment as over ruling of all and cast in
stone. This is what the physics forum sponsered by Scientific American
said to me as they banned me because of my temerety in challenging
their position.In addition they stated that it is not possible outside
the physcics spectrum to challenge anything which thus puts all in the
spectrum of crackpots. Dr Davis of MIT provided the mathematics
that confirmed the presense of particles, but mathematics was not
considered a reputable answer compared to the majority argument that
the mathematical aproach was illegal.
So waves hold the majority in the books but by its very presence all
understanding of radiation has been stymied for the last hundred
years by the resistance to change without any accumpanying facts and
where physicists refuse to review to re examine what they state is now
"cast in stone." New books are written every year via plagarisation
for personal profit where the professor orders purchase of such books
for his pay off. Not one has come out with a review of radiation and
why progress has been stymied. And that goes for Radcom and QST who
have no real interest in advancing the hobby of its members.
Regards
Art

On Sat, 20 Mar 2010 21:49:42 -0700 (PDT), Art Unwin
wrote:

If on places a am/fm radio inside a box made of thin aluminum foil the
radio will be able to hear am broadcast band but not the fm band.
(Experiment by Harvard in Boston)
Intuition tells me that when using a perforated plate the lower the
frequency then the smaller the perforations in the shield to create a
blocking effect. This would, I believe, opposes the progression of
skin depth with respect to frequency.
The books state for a mesh shield the perforations should be less
than 1/10 of a WL which on the surface opposes the results obtained by
the box experiment! Is the difference involved with calculations
changes for sheets that are thinner than skin depth such as circuit
board traces or something else?
Where has my intuition gone wrong in opposing the books?


Shielding and grounding is a secret science, so secret and of military
importance that Don White Consultants would not teach foreigners
(damned furriners) how to shield a building from EMP.

w.

"Helmut Wabnig" hwabnig@ .- --- -. dotat wrote
...
On Sat, 20 Mar 2010 21:49:42 -0700 (PDT), Art Unwin
wrote:

If on places a am/fm radio inside a box made of thin aluminum foil the
radio will be able to hear am broadcast band but not the fm band.
(Experiment by Harvard in Boston)
Intuition tells me that when using a perforated plate the lower the
frequency then the smaller the perforations in the shield to create a
blocking effect. This would, I believe, opposes the progression of
skin depth with respect to frequency.
The books state for a mesh shield the perforations should be less
than 1/10 of a WL which on the surface opposes the results obtained by
the box experiment! Is the difference involved with calculations
changes for sheets that are thinner than skin depth such as circuit
board traces or something else?
Where has my intuition gone wrong in opposing the books?


Shielding and grounding is a secret science, so secret and of military
importance that Don White Consultants would not teach foreigners
(damned furriners) how to shield a building from EMP.

The aether waves were, are and will be a secret. But we all want to guess
like the things are.
Fortunatly the papers by Stokes, Kelvin and many others are available on
line.
Maxwell, Lorentz and Einstein are for teaching.
S*

Szczepan Bialek wrote:



The aether waves were, are and will be a secret.

Only to gibbering idiots like you.


--
Jim Pennino

Remove .spam.sux to reply.

On Mar 20, 11:49*pm, Art Unwin wrote:
*Intuition tells me that when using a perforated plate the lower the
frequency then the smaller the perforations in the shield to create a
blocking effect. ...* The books state for a mesh shield the perforations
should be less than 1/10 of a WL which on the surface opposes
the results obtained by the box experiment! ... Where has my intuition
gone wrong in opposing the books?

If reality will help shape your intuition, then you may be interested
in this paste-up from an IEEE paper linked below.

Note that the spacing of the conductors in the mesh forming these
cavities is much greater than your intuition says is required "to
create a blocking effect."

http://i62.photobucket.com/albums/h8...y_Radiator.gif

RF

On Mar 22, 6:20*am, Richard Fry wrote:
On Mar 20, 11:49*pm, Art Unwin wrote:

*Intuition tells me that when using a perforated plate the lower the
frequency then the smaller the perforations in the shield to create a
blocking effect. ...* The books state for a mesh shield the perforations
should be less than 1/10 of a WL which on the surface opposes
the results obtained by *the box experiment! ... Where has my intuition
gone wrong in opposing the books?

If reality will help shape your intuition, then you may be interested
in this paste-up from an IEEE paper linked below.

Note that the spacing of the conductors in the mesh forming these
cavities is much greater than your intuition says is required "to
create a blocking effect."

http://i62.photobucket.com/albums/h8...Cavity_Radiato...

RF

Interesting. thank you.It would appear that they are using approx .1
lamda sections but the beam pattern appears to be what is expected
other than the differential between E and H
as shown on my page.
Ofcourse it doesnt show how it is fed which can make a lot of
difference.The size of the squares really determine whether the
currents slides across the surface or follow a paths per a normal
radiator. I certainly would not feel comfortable using such openings
as protection against lightning.

On Mar 22, 8:23*am, Art Unwin wrote:
I certainly would not feel comfortable using such openings
as protection against lightning.

Real comfort is produced by real (proven) knowledge, not intuition.

The purposes of the gridded cavity are (1) to isolate its radiating
elements from coupling into the nearby supporting tower, (2) to
minimize coupling into adjacent cavities on the same level and those
installed above and below, (3) to minimize the windload of the
installed antenna compared to using solid cavities, and (4) to create
predictable, unit radiation patterns that can be used together to
generate specific, directional radiation patterns for the array that
meet given specifications in the azimuth and elevation planes.

The cavity, and all of its components operate nearly at earth ground
potential in the low r-f spectrum, where induced energy from nearby
lightning strikes is greatest.

Arrays of these cavity antennas have been in operation at the top of
the Sears Tower in Chicago and many other "tall-tower" sites for more
than 25 years now, with zero lightning damage.

RF

Richard Fry wrote:
On Mar 22, 8:23 am, Art Unwin wrote:
I certainly would not feel comfortable using such openings
as protection against lightning.

Real comfort is produced by real (proven) knowledge, not intuition.

The purposes of the gridded cavity are (1) to isolate its radiating
elements from coupling into the nearby supporting tower, (2) to
minimize coupling into adjacent cavities on the same level and those
installed above and below, (3) to minimize the windload of the
installed antenna compared to using solid cavities, and (4) to create
predictable, unit radiation patterns that can be used together to
generate specific, directional radiation patterns for the array that
meet given specifications in the azimuth and elevation planes.

The cavity, and all of its components operate nearly at earth ground
potential in the low r-f spectrum, where induced energy from nearby
lightning strikes is greatest.

Arrays of these cavity antennas have been in operation at the top of
the Sears Tower in Chicago and many other "tall-tower" sites for more
than 25 years now, with zero lightning damage.

RF

Do you have any web references to gridded cavity antennas?

My intuition says your statements are correct. It is likely not wrong.

Thanks.

tom
K0TAR

On Mar 22, 8:56*pm, tom wrote:

Do you have any web references to gridded cavity antennas?

A good source is the paper from which I posted a few "fair use" clips,
and was published in the Sept 1979 edition of the IEEE Transactions on
Broadcasting. I'm not aware of any web link to it, and it is a
copyrighted work. But probably your local public library would be
able to access it through inter-library services, and provide you with
a copy.

This design was developed by Harris Corporation, and has evolved/
improved over the years especially in the crossed-dipole element used
to excite the cavity.

The design is now available through several manufacturers. The first
link below shows a description of it from the 2004 catalog of
Dielectric Communications, who bought the design from Harris. The
picture shows an array of 12 layers of 3-around cavities that was
built by Harris as a master FM antenna (9 stations) for the Senior
Road Tower Group in Houston.

The second link below shows the measured axial ratio for an
omnidirectional version of the antenna.

http://i62.photobucket.com/albums/h8...vity_Array.gif

http://i62.photobucket.com/albums/h8...xial_Ratio.gif

RF