11.3.4 Free-Space Loss
As we saw earlier, it is not possible for all of the power developed in the isotropic transmitting antenna to be incident on the receiving antenna. Since the power is radiated equally in all directions, only a fraction (1/4πd2) is incident on a receiving antenna. The power presented to the front end of the receiver is a function of the antenna aperture, which (with unity gain) will only collect λ2/4π of the incident power density. The combination of these two factors is the free-space loss mentioned before and is:
In radio path analysis, this equation is converted into frequency instead of wavelength and into decibel form for insertion into Equation (11.20).
where f is in MHz and d is in km.
The effect of free-space loss is dramatic and a great deal of effort in communications goes into antenna design so that as much of the radiated power as possible is directed towards the intended recipient, so that the effect of FSL is offset as much as possible.
In free-space communications, free-space loss is the only loss that would be encountered. Since the transmit and receive antennas are a finite height, however, the encroachment of the Earth into the radio path has a number of effects upon the received signal strength. The first of these is due to the interaction between the direct wave and the ground-reflected wave.
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