 27.7) gives a free space loss of 72.4dB at 1km and 96.5dB at 16km, again a difference of 24dB. Figure 27.2 shows the free space Because of the very large differences in power density over long loss for frequency and distance based on this formula. propagation paths, particularly in the microwave bands, it is usual to measure power density in decibels (dB) relative to 1 watt i.e. dBW, or 1 milliwatt i.e. dBm. At frequencies below the microwave range, field strength in volts The propagation medium per metre is a more usual measurement than power density, partly because volts per metre has historically been used for the measure- Almost all propagation involves transmission through some of the ment of signal strength and partly because the thermal heating effect earth's atmosphere, and the structure of the atmosphere and features of power absorption is easier to use for microwave measurements. within it are shown in Figure 27.3. Most meteorological activity and The conversion can be made for power density to field strength cloud formation occur in the first 10km, and jet aircraft cruise at using the simple derivation of Ohm's law (Braun, 1986) as in between 10km and 15km. Air pressure falls with height and at Equation 27.3, where V is voltage, R is resistance in ohms and P is 30km, radiation from the sun is sufficient to generate some free power in watts. electrons, but the first distinct ionised layer, the D layer occurs at 70km. Above the D layer, temperature and incident radiation in- V2 = R P (27.3) crease and the E, F1 and F2 layers are formed between 120km and 450km. The ionosphere, which is the region in which the ionised In this case, R is substituted for Z the characteristic impedance layers are formed, spans the region from 50km to 600km. Ionised trails from meteors occur at around 100km and the lowest satellite of free space, which is a constant of 377 ohms. Field strength can orbit is at about 150km. therefore be converted to power density by (Maslin, 1987) Equation 27.4, where E is field strength in vol