 made noise may be similarly transmitted and corresponds to the normal room temperature of 17°C and B is Boltzmanns constant (1.38x10-23). emanates from power lines, industrial machinery and fluorescent tubes. Four standard levels for business, residential, rural and quite rural sites are defined in the CCIR report (CCIR, 1978). N = kTB (27.8) Analysis of a propagation path usually requires a calculation of distance from transmitter to receiver and it is useful to know the In communications links, noise is measured in terms of the noise great circle bearing. This information can be calculated from Equa- power available from a lossless antenna and may be expressed as in tions 27.10 and 27.11 for two points A and B on the surface of the 27/6 Radiowave propagation Because currents are induced in the ground by vertically polarised ground wave transmissions, attenuation is also dependent on ground conductivity and dielectric constant. Salt water offers low attenua- tion whereas desert sand or polar ice offer high attenuation, and consequent reduced communications coverage. Terrain irregu- larities such as hills and mountains, and also rough (as compared with calm) seas also reduce coverage. lonosphere Received field strength at any distance can be calculated theoreti- E layer Meteor scatter region cally (Braun, 1986; Maslin, 1987), however CCIR publish a series of graphs showing received field strength in microvolts per metre D layer and in dBs relative to 1 microvolt, for distances up to 1000km. Frequencies from 500khz to 10MHz, and homogeneous surface conditions from sea water to dry soil are shown. Some of the curves are shown in Figure 27.5 and are calculated on the basis of a 1kW Stratosphere transmitter working into a short omni-directional monopole an- tenna. The field strength values are proportional to the square root aircraft of the power and will have to be adjusted accordingly for different powers or for higher gain or directional antennas. Mount The CCIR curves shown in Figure 27.5 assume propagation over