es by sky wave paths. Human-made noise can be similarly Low- and Medium-Frequency Ground Waves Maximum Atmospheric Minimum Atmospheric Urban Rural Human-made Galactic Heavy Rain Cosmic Background Fig. 8.4 Noise in the propagation 10 kHz 1 GHz 1 THz medium: typical figures. Frequency transmitted and emanates from power lines, industrial machinery, and fluorescent tubes. Four standard levels for business, residential, rural, and quite rural sites are defined in the ITU-R reports. Analysis of a propagation path usually requires calculation of distance from transmit- ter to receiver, and it is useful to know the great circle bearing. This information can be calculated from equations 8.10 and 8.11 for two points, A and B, on the surface of the Earth, where A is point-A latitude in degrees; B is point-B latitude in degrees; L is the difference in longitude between A and B; C is the true bearing of the receiver from the transmitter, which can be 360°; and D is the distance along the path in degrees of arc, which can be converted to kilometers by multiplying by 111.111 (i.e., 1 degree of arc = 111.111 kilometers). cosD = sin AsinB + COS AcosBcosL (8.10) cosc-sinB-sinAcosk (8.11) AsinD Low- and Medium-Frequency Ground Waves In the low- - and medium-frequency bands up to 3 MHz, ground wave (rather than sky wave) propagation is used because sky wave is heavily absorbed in daytime by the D layer. LF and MF antennas are generally short in terms of wavelength, SO vertical mast radiators are used. These have the advantages of maximum radiation at low angles to the horizon and also radiating vertically polarized transmissions. The attenuation of vertically polarized ground wave transmissions is very much less than for horizontal polarization-fo example, the ground wave attenuation of a 2-MHz Chapter 8 Radio Wave Propagation 500 kHz Desert 500 kHz 2 MHz Seawater Desert 2 MHz Moist 2 kHz Seawater 500 kHz Moist Soil Fig. 8.5 Ground wave propagation: field strength for different frequencies and surface 10 km 100 km 1000 km condition