tainable. To Of course, An is unknown, initially, for n greater than two, but it overcome this it is, first, possible to divide the distance R into N is solvable from Equation 6.104. 6/24 Electromagnetic waves 908kHz 1214kHz 1457kHz Distance from transmitter, KM Figure 6.28 Attenuations of m.f. signals through a built up area 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 1 n-1 attenuation in the clutter. In the open environment, the electric field is related to the mag- = 1 + Q,Rn netic field by the free space impedance. However, in the cluttered environment this is not the case. In fact, the impedance is less than (6.104) the free space value. Further details on this subject are available in Causebrook (1978b). This equation must be applied repeatedly from n = 3 until n = N, at which point the required attenuation at distance R is obtained 6.6.6 Curves for surface wave field strength 6.6.5 Electric to magnetic field ratio in cluttered prediction environments For many practical purposes it may be considered that calculating The attenuation, calculated by the method of the previous section, the field strength with mathematical techniques is too difficult, is only rigorously applicable just above the top of the clutter (build- cumbersome or time consuming. In this case a prediction may be ings, trees, etc.). However, most reception of m.f. and l.f. signals obtained from a set of published curves. These curves usually show takes place on antennas well in the clutter. the variation in surface wave field strength with distance as a For antennas like the ferrite rod, designed to respond to the function of both frequency and ground conductivity (relative per- magnetic field, the attenuation experienced should, on average, be mittivity is a less relevant parameter). The trend is for attenuation (n-1) Figure 6.29 Profile data (quantised form) References 6/25 with distance to be greater at higher frequencies and when ground CCIR