 Everest Cloud homogeneous terrain, but discontinuities occur in paths over land formations Tropospheric Troposphere and sea or over different types of soil, and the field strength over scatter region such a path can be calculated by a method developed by Millington, 1949. An example of the field strength over a land and sea path is shown in Figure 27.6 and it is interesting to note that the field strength rises after the wave front passes from land to sea. M.F. groundwave propagation offers the advantage of predictable but limited communications coverage, which is largely independent 10 10 of ionospheric conditions and diurnal or seasonal variations. Dis- Free electron density, m tances of up to 1000km are achievable over sea water, but in desert conditions this may be limited to tens of kilometres unless very high powers and directional antennas are used. A limitation that often Figure 27.3 The Earth's atmosphere; major features and occurs in medium frequency broadcasting is interference between ionisation groundwave and skywave from the same transmitter during dark- ness due to the absence of the absorbing D layer causing deep and Earth, where A is point A latitude in degrees; B is point B latitude rapid fading in received signals. The only practical solution is in degrees; L is the difference in longitude between A and B; C is limiting power radiated at night when the D layer is no longer the true bearing of the receiver from the transmitter, which may be present, and designing the antenna to minimise high angle radiation. 360° C if a negative value is calculated; D is the distance along the At I.f. and v.l.f., very long distance and global communications can path in degrees of arc, which may be converted to kilometres by be achieved, but this necessitates the use of high transmitter powers multiplying by 111.111 (i.e. 1 degree of arc = 111.111 kilometres). and very large antenna systems. cos D = sin A sin B + cos A cos B cos (27.10) High frequency skywave propagation cos C = sin B - sin A cos D (27.11) Propa