gation in the h.f. band from 3MHZ to 30MHz is probably the most variable and least predictable of all transmission modes, as it depends upon the height and intensity of the ionised layers in the 27.5 Low and medium frequency ground ionosphere. A wealth of data has been collected and incorporated in waves comprehensive computer programmes to enable predictions to be made with a reasonable degree of statistical accuracy, and modern In the low and medium frequency bands up to 3MHz, ground wave ionospheric sounders can evaluate conditions on a real time basis. (rather than sky wave) propagation is used, because sky wave is Unpredictable events such as solar flares do however mean that h.f. heavily absorbed in daytime by the D layer. L.F. and m.f. antennas propagation always contains some degree of uncertainty (as does are generally short in terms of wavelength so vertical mast radiators most radio wave propagation), and predictions can only be made on are used. These have the advantages of maximum radiation at low a statistical probability basis. angles to the horizon and also radiating vertically polarised trans- The ionosphere which is the primary influence on h.f. propagation missions. extends from approximately 80km to 300km above the Earth's The attenuation of vertically polarised ground wave transmissions surface, and divides into a number of distinct ionised layers. Vari- is very much less than for horizontal polarisation, for example the ations in the height and intensity of the layers occurs on a diurnal ground wave attenuation of a 2MHz vertically polarised trans- and seasonal basis due to the rotation and position of the earth in mission over medium soil is 45dB at 30km whereas a horizontally relation to the sun, and also on the longer term 11 year sunspot cycle. polarised signal would be attenuated by nearly 95dB. Vertical po- The diurnal variation is shown in Figure 27.7, and it will be seen larisation is therefore almost always employed. that at night time when incident radiation is at a minimum as the 