 but air comes between the satellite and Earth station antennas. In contrast, a line-of-sight is usually absent in mobile terrestrial communications: transmitted signals are usually reflected from or travel through physical objects before they arrive at a receiver antenna, as illustrated in Figure 10.7. The physical objects blocking or reflecting the signal absorb much of the transmitted energy and therefore the received signal is significantly weakened. While signal loss for line-of-sight channels can be theoretically derived from the dis- tance and frequency of operation, path loss in terrestrial channels is a function of terrain and building topology, and an exact formula cannot be derived. Instead, a number of empirical models have been developed based on path loss measurements in a real physical environment. These models calculate path loss as a function of frequency of operation, the height of transmitter and receiver antennas and the type of environment (dense urban, urban, suburban or rural). A broad summary of these models can be found in electromagnetic wave propagation textbooks and references such as given in [2]. In this text we use the simplified extended Hata model for urban areas. This model applies best to a frequency range of 2-3 GHz, and transmitter/receiver antenna heights of 30 and 1.5 m. We apply the model for a range of frequencies and environments as illustrative and not necessarily as an accurate calculation of path loss under all circumstances. Path loss expressed in deci- bels is calculated from the following formula, where d represents the distance between the transmitter and receiver in kilometres, and f represents the frequency of operation in megahertz. In contrast path loss for a line-of-sight (also known as freespace) channel is given by: L(FreeSpace)=32.4+20log(d)+20log(f) As might be expected, path loss for a similar propagation distance is much higher in a typical mobile terrestrial compared with line-of-sight channels. Example 10.1 illustrates this. Figure 10.7 Signal propagat