Page 125 - Intro to Space Sciences Spacecraft Applications
P. 125
Introduction to Space Sciences and Spacecrajl Applications
112
Figure 5-5. Link budget. A certain minimum amount of power must be
received for successful communications.
because the power would be spread out throughout the 47c radians of the
sphere around the isotropic radiator. Notice that this is an inverse square
relationship which describes the loss of power of any electromagnetic sig-
nal propagating through space.
The gain of the antenna concentrates the power in the boresight direc-
tion and represents a multiplicative factor for the power in that direction.
At a distance of R along the boresight direction from a parabolic antenna,
the power would be:
The above relationship gives the power per unit area at range R. The
amount of that power that falls on the receiving antenna depends on that
antenna's effective antenna area A,, the expression for which was given in
equation 5-6.
Relating this result to the form of equation 5-5 for the receiving anten-
na gain (GR), we can write:
