Frequency f=100MHz, so the wavelength= 3*10[sup]8[/sup]/10[sup]8[/sup]=3M
I believe the distance D between your conducting wire grid is much small than the wavelength.
I will assume the wire can behavior like a conducting plate. So I think it can stop most of the electromangetic field if those wire are very good conductors.
However, if the distance D become larger , than I do not think there is a simple formula available.

Are you doing some real project or this is just some test problem?
What is the source for your 100MHz vertically polarized electromagnetic wave?
What is the spatial distribution of the EM wave?