High frequency skywave propagation 27/7 Maxdmum atmospheric Minimum almospheric Urban Rural Heavy rain Galactic Cosmic background 10kHz Frequency Figure 27.4 Noise in the propagation medium; typical figures 500kHz desert 2 MHz (sea water) 500kHz seawater desert moist 2 MHz seawater (land-sea-land) 500kHz moist soil 100km 1000km 100km 200km 300km Distance Distance from transmitter Figure 27.5 Ground wave propagation; field strength for Figure 27.6 Ground wave field strength curve for a different frequencies and surface conditions hypothetical non-homogeneous path 27/8 Radiowave propagation occur with the seasons so there is greater level of ionisation in northern latitudes during July (summer) than in January (winter), but the reverse applies in the southern hemisphere. The D layer actually spans an altitude from 50km to 90km and absorbs frequencies in the m.f. and lower h.f. bands. Higher fre- quencies will pass through the D layer suffering some attenuation E layer and may be reflected back to earth by the E layer at distances up to 2000km. Still higher frequencies within the h.f. band will pass through the E layer and may be reflected by one of the F layers, D layer providing very long distance communications. Frequencies in the v.h.f. bands and above (i.e. above 30MHz) will generally pass through all the layers and into space, except in unusual conditions of strong ionospheric activity. Although a radio wave can be visualised as being reflected by the ionosphere, the process is in fact one of refraction, and the angle of refraction is proportional to both the angle of incidence and the frequency. When the wave front enters an ionised layer, it excites Earth free electrons into oscillation which re-radiate electromagnetic en- ergy, and this re-radiation modifies the direction of the wavefront, tending to bend it back to earth. For a given frequency, this tendency Night increases as the angle of incidence is reduced, so at the critical angle, the wave will be refracted back to earth, but at greater angles (i.e. 