ted below may be used. It must also be realised that lation. All other waves are, therefore, modulated in amplitude by the these predictions are largely empirically derived, it does not seem strong wave. This is called ionospheric cross modulation. possible to be deterministic about such a problem. 6/22 Electromagnetic waves Ray escapes lonosphere (ref. index no) distance Multiple hop Surface wave Transmitter Figure 6.26 Sky wave and surface wave transmission showing skip distance For h.f. predictions CCIR (1970) is recommended. This, in its The quantity P' given by Equation 6.93 is called the effective turn, has a comprehensive set of references, and lists a lengthy monopole radiated power in kilowatts (E.M.R.P.), which is analo- computer program for completing the predictions, described in the gous to the E.R.P. defined previously for the space wave. text. For m.f. and 1.f. predictions CCIR (1982a) is recommended. Of P' = Pkwg = (6.93) course, a problem exists because of the large differences associated with different geographical locations. For this reason it is also recommended that CCIR (1982b) is consulted. Thus an E.M.R.P. of 1kW gives a field strength of 0.3volts/metre at 1km on a perfectly conducting ground. Monopole antennas that are longer than 0.1 wavelengths have The surface wave gains given by the graph of Figure 6.27. A practical antenna may obtain additional gain, in a given direction, by many means. This may be allowed for, as with the space wave, by the use of a 6.6.1 Ground wave and surface wave horizontal radiation pattern (H.R.P.). The field strength for the practical case now becomes as in The terminologies of ground wave and surface wave are often Equation 6.94. considered as 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 300 ( Pkw8t)" (6.94) wave may usefully be described as the ground wave. With frequen- volts/metre cies below about 1.5MHz, and wit