Antennas and real refflection

Hey guys,

I need some help with my project, I got 2 antennas(length between 140m) and i got two-ray model over ideal conductive flat earth and i have to do propagation loss for perfect reflection and for different real lands.
I already did for perfect reflection and i have no idea how to count this real lands. For example asphalt is almost perfect reflecion, isn't it?
Can u give me examples and some tips?
My calculates for perfect reflection.

anteny.rar

Its done in Mathcad