 can be obtained by assuming that A(r) varies linearly within an interval, as in equation 7.103, where Sn and Rn can be derived from integrals that are amenable to analytic solution. In=SnAn-1+R,A, (7.103) Of course, An is initially unknown for n greater than 2, but it can be solved by equation 7.104. Ground Waves and Surface Waves (7.104) 1+QnRn This equation must be applied repeatedly from n=3 to n = N, at which point the required attenuation at distance R is obtained Ratio of Electric to Magnetic Field in Cluttered Environments The attenuation, calculated by the method of the previous section, is only rigorously appli- cable just above the top of the clutter (buildings, trees, etc.). However, most reception of MF and LF signals takes place on antennas well in the clutter. For antennas like the ferrite rod, designed to respond to the magnetic field, the attenu- ation experienced should, on average, be close to the calculation. This is not true for the electric field, like that received by a rod antenna on a car, because this can suffer more attenuation in the clutter. In the open environment, the electric field is related to the magnetic field by free-space impedance. However, in the cluttered environment this is not the case. In fact, the imped- ance is less than the free space value. Further details on this subject are available in Cause- brook (1978b). Curves for Surface Wave Field Strength Prediction For many practical purposes it can be considered that calculating the field strength with mathematical techniques is too difficult, cumbersome, or time consuming. In this case a prediction can be obtained from a set of published curves. These curves usually show the variation in surface wave field strength, with distance as a function of both frequency and ground conductivity (relative permittivity is a less relevant parameter). The trend is for attenuation with distance to be greater at higher frequencies and when ground conductivity is low. A comprehensive set of these curves is to be found in CCIR (1990d). G