John Passaneau wrote:

The support leg is just that a leg. You can make it any length that
works for you.

John W3JXP

A J pole, or ground plane for that matter, couples into the support
structure and outside of the feedline, making those conductors part of
the radiating system. This alters the radiation pattern and can result
in further coupling to anything near the support structure or feedline.
The amount of coupling can be anything from insignificantly small to
very large -- with the support and/or feedline radiating more than the
"antenna" --, depending on the length and orientation of the entire
conductive path to Earth ground.

Roy Lewallen, W7EL