APPLICATIONS 



105 



4.2.5. Applications 



The communications engineer usually has available a small amount of 

 measured field-strength data from limited tests of a particular system. 

 He must then estimate the expected signal level or practical range of that 

 system, or other systems, for other tunes of the year, other years, and in 

 other areas. The variation of signal level from month to month and 

 climate to climate can be explained, in part, by its observed correlation 

 with Ns. 



Pickard and Stetson [10, 11] were among the first to note the correlation 

 of Ns and received field strengths. The correlation of Ns and field 

 strength over a particular path has been studied quantitatively [12, 13] 

 and found to be highest (correlation coefficients of 0.8 to 0.95) when the 

 variables are averaged over periods of a week to a month. This latter 

 study has shown that the regression coefficient (decibel change in field 

 strength per unit change in N s) varies diurnally from 0.14 dB in the after- 

 noon hours to 0.24 dB per unit change of N s iu the early morning hours. 

 This correlation is so sufficiently consistent that Gray [14] and Norton [15] 

 have utilized it in their recent prediction methods of transmission loss in 

 a band from 100 to 50,000 Mc/s. In addition, the coefficient 0.2 dB per 

 unit change in A''^ has been tentatively adopted by CCIR Study Group V 

 in their revision of the 30- to 300-Mc/s tropospheric-wave propagation 

 curves to account for the geographic and seasonal variations of field 

 strengths. The estimates of field strength variations attributed to N s 

 given in table 4.5 are based upon the CCIR coefficient. 



If one assumes, for comparison only, that the worldwide average value 

 of A^s is 330 and that one is able to estimate the field strength level of a 

 particular communications system at a given distance and for N s = 330, 

 then the above correlations would indicate that the climatic variations of 

 fields given in table 4.5 might be expected. 



Table 4.5. Climatic variation of hypothetical communications system relative to pre- 

 dicted value for Ns = 330, assuming a 0.2-dB variation per unit change in Ns 



•Climatic types are the same as those in table 4.3. 



