Physics question

Victoria,

Take a look at this link to the classic "Bill Brown" paper, which is one
of the original works on beaming power from space. It contains the
basic equations and parameters for the SPS beam spreading and antenna
design.

http://engineer.tamu.edu/tees/csp/wireless/70dec5.htm

In particular, look at Section II-C of this paper.

II. UNIQUE PROPERTIES AND BASIC PRINCIPLES OF BEAMED MICROWAVE
POWER TRANSMISSION

C. Free Space Transmission

Regards,

Charles R.