nearer the vertical) only partial or no refraction will occur. This is illustrated in Figure 27.8. Similarly, for a given angle of in- cidence, refraction will decrease with increase in frequency, so at a critical frequency the wavefront will pass through the layer. As the wavefront passes through an ionised layer, it imparts energy to the electrons and a small amount of this energy is lost as heat. In the D layer, this transfer of energy is sufficient to completely absorb medium frequencies during daylight but at higher frequencies it causes some attenuation in the refraction process. Refracted waves also undergo a change of polarisation due to the complex movement of the free electrons, and this phenomenon is called Faraday rota- Figure 27.7 The diurnal variation in the Earth's ionised tion. layers In planning an h.f. skywave link, it will be necessary to know the distance between the transmitter and receiver, or the area to be ionosphere is in the earths shadow, the ionosphere comprises two covered, and this can be calculated by Equations 27.10 and 27.11. comparatively thin layers at 110km and 210km, called the E and F It is desirable to minimise the number of 'hops' i.e. refractions from layer respectively. During daylight, these two layers increase in the ionosphere and reflections from the earth's surface, to minimise thickness and intensity, and the F layer divides into the two separate path attenuation and the inherent variability associated with ionos- F. and F2 layers at 210km and 300km. In addition, the D layer forms pheric refraction. Maximum radiation from the antenna should at 80km during daylight but disappears at night. Similar variations therefore be at an angle (the take off angle) that results in refraction lonosphere Critical angle Skip zone Earth Figure 27.8 The critical angle of ionospheric refraction High frequency skywave propagation 27/9 Take off angle (degrees) Great circle distance Figure 27.9 Take off angle for single hop sky wave paths from the ionosphere onto the target area. Figure 