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

On Mar 23, 6:31*am, Richard Fry wrote:
On Mar 22, 8:56*pm, tom wrote:

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.

CORRECTION, that was the Sept 1976 edition.

Richard Fry wrote:
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

Thanks. Gives me plenty to start from.

tom
K0TAR