ield strength expressed in decibels relative to 1 uV/m, as in equation 7.65, where D is the distance in kilometers and P is the ERP in watts. Chapter 7 Electromagnetic Waves Eu=76.9+10log10P-20log10D (7.65) Some workers prefer to base an equation on the isotropic transmitting antenna. The difference is that 10 log10 P becomes 2.1 dB greater, because EIRP is used, which is compen- sated for by the constant 76.9 becoming 74.8. Free-Space Path Loss transmission path can also be characterized by the free-space path loss, which is defined as the ratio of received power, P to transmitted power, P. Expressed in decibels, in a non- dissipative medium, this is given by equation 7.66. (7.66) Gt and Gr are the antenna gains relative to a half-wavelength dipole, and Lt and Lr are the antenna feeder losses (subscripts t and r being for transmitter and receiver). Again, some workers prefer to base an equation on the isotropic antenna, in which case the gains Gt and Gr are each increased by 2.1 dB to be relative to isotropic antennas instead of half-wavelength dipoles, which is compensated for by the constant 17.7 becoming 21.9. There can also be a preference to convert d in meters to D in kilometers, and a (meters) wavelength to frequency in megahertz, which must be matched by an appropriate change of constant. Variation from Free-Space Values Free-space values rarely occur in practice because factors such as the Earth's surface, build- ings, trees, and the atmosphere cause variations. It is, therefore, necessary to be able to predict an excess loss that is dependent on the particular propagation path under consider- ation. This loss prediction is complicated by the fact that the real world does not provide simple geometric shapes for the boundary conditions, and nearly all calculation systems have been based on such simple geometries. It is therefore convenient to stylize the real world into shapes that can be handled. The following sections discuss these shapes and the predictions based on them. Knife Edge Diffraction Fresne