AI4QJ wrote:
If you are considering adding structural supports for your transmission line
it is zero. If you want to understand the physics it is not zero. Photons
have mass (not rest mass but mass), light has pressure that has been
measured and it has momentum.

Quoting Maxwell in 1873: "In a medium in which waves are
propagated, there is a pressure in the direction normal
to the waves, and numerically equal to the energy in a
unit of volume."
--
73, Cecil http://www.w5dxp.com