(l = 4km 103), we get the free-space loss of a radio wave in decibels as follows: 20 log 10 /km dB (4.4) We see that the loss or attenuation is proportional to 20 times the loga- rithm of frequency and distance. So if the distance or frequency is doubled, the attenuation increases by 6 dB. If we want to maintain the received power, we have to increase the transmitted power by 6 dB, which requires a four times higher transmission power. This comes from the fact that the power ratio in decibels is 10 log10(P,IP) dB, as we saw in Chapter 3. The free-space loss shown in (4.4) may give results that are too optimis- tic by as much as 30 dB in actual conditions. Additional attenuation is intro- duced if there is a hill, a building, or a wall on or close to the straight line between the transmitting and receiving antennas. This is most often the case Transmission in mobile radio communication where actual attenuation may be of the order of 30 dB higher than free-space loss. To estimate the impact of the environment, several propagation models have been developed for cellular network planning. However, free-space loss in (4.4) clearly explains the impact of frequency and distance on radio wave attenuation. 4.2.7 Antennas Link loss was calculated assuming that antennas are isotropic, which means that they transmit and receive equally to and from all directions. This assumption keeps the attenuation independent of the antennas in use. How- ever, practical antennas have a focusing effect that acts like amplification, compensating for some of the propagation loss. This focusing effect can be expressed as a gain of an antenna, although a passive antenna cannot actually amplify the signal. The maximum transmitting and receiving gain (to direc- tion of maximum radiation or sensitivity) of an antenna with effective aper- ture area A is [1] (4.5) The value of A for a dish or horn antenna approximately equals its physical area and large parabolic dishes may provide gains in excess of 60 dBi, where dBi stands for gain in decibels co