Owen Duffy wrote:

Frank,

The existence of a tall conductor provides a measure of protection to
nearby structures. The zone is often defined by a "ball" of radius equal
to the height of the tall structure that is rolled on the ground to rest
against the tall structure, and structures between the contact with the
ground and tower, and under the ball are considered protected by the tall
structure, ie that lightning is much more likely to strike the tall
object. In this way, a tower often protects the nearby equipment hut from
direct strikes.

I believe that the latest research indicates that the "rolling ball"
isn't as accurate as once believed (or necessarily better than the older
"cone of protection"). But the difference might be in the noise floor
for most ham installations.