received in given circumstances. Such calculations should allow for each of the ionospheric features, which means they are complicated and lengthy. It is therefore not practicable to give a comprehensive description of the calculations here, but references listed below can be used. It must also be realized that these predictions are largely empirically derived; it does not seem possible to be deterministic about such a problem. For HF predictions, CCIR (1970) is recommended. This, in its turn, has a comprehensive set of references and lists a lengthy computer program for completing the predictions described in the text. For MF and LF predictions, CCIR (1982a) is recommended. Of course, a problem exists because of the large differences associated with different geographic locations. For this reason it is also recommended that CCIR (1982b) be consulted. Ground Waves and Surface Waves The terminologies of ground wave and surface wave are often considered synonymous, but it is useful to make a distinction. The influence of the ground in producing a reflected ray was discussed previously for elevated antennas and high frequencies. This ray or wave can use- fully be described as the ground wave. With frequencies below about 1.5 MHz, and with antennas at ground level, another mechanism of propagation tends to dominate. This is a wave that is guided along the boundary between the surface of the Earth and the air above it. The best term to be applied to this mechanism is the surface wave. Reference Field Strength Equation 7.49 gave the field strength for a short monopole on a perfectly conducting plane. In practice, allowance must be made for 1. The power gain, g, of a particular antenna in a given direction, relative to the short monopole, assuming a perfect ground plane. 2. The imperfect conductivity of the ground plane, which causes a significant fraction of transmitter power to be lost as heat in the ground and not radiated as electromagnetic energy. This is taken into account by the transmitter antenna system efficie