Newnes Radio and RF Engineering Pocket Book, 3rd Edition

7

where d and λ are in metres, or:

Free space loss (dB) = 32.4 +20 × log

10

d + 20 ×log

10

f

where d = distance in km and f = frequency in MHz.

The free space power loss, therefore, increases as the square of

the distance and the frequency. Examples are shown in Figure 1.5.

With practical antennas, the power gains of the transmitting and

receiving antennas, in dBi, must be subtracted from the free space loss

calculated as above. Alternatively, the loss may be calculated by:

Free space loss (dB) = 10 log

10



(4πd)

2

λ

2

×

1

G

t

× G

r



where G

t

and G

r

are the respective actual gains, not in dB, of the

transmitting and receiving antennas.

A major loss in microwave communications ...