Free Space Loss

Free Space Loss

For any type of wireless communication the signal disperses with distance. Therefore,
an antenna with a fixed area will receive less signal power the farther it is from
the transmitting antenna. For satellite communication this is the primary mode of
signal loss. Even if no other sources of attenuation or impairment are assumed, a
transmitted signal attenuates over distance because the signal is being spread over a
larger and larger area. This form of attenuation is known as free space loss, which
can be express in terms of the ratio of the radiated power "Pt to the power p, received
by the antenna or, in decibels, by taking 10 times the log of that ratio. For the ideal
isotropic antenna, free space loss is

Thus, for the same antenna dimensions and separation, the longer the carrier wavelength

(lower the carrier frequency f), the higher is the free space path loss. It is

interesting to compare Equations (5.2) and (5.3). Equation (5.2) indicates that as

the frequency increases, the free space loss also increases, which would suggest that

at higher frequencies, losses become more burdensome. However, Equation (5.3)

shows that we can easily compensate for this increased loss with antenna gains. In

fact, there is a net gain at higher frequencies, other factors remaining constant.