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