250 TRANSHORIZON PARAMETERS 



Generally, the unst ratified -samples have the lowest overall field 

 strengths throughout the entire distribution range. The presence of any 

 layer (elevated or ground-based) tends to increase the field strength by 

 10-25 dB at any percentage level of the distribution. (The exception to 

 this observation, WXBQ-TV, is probably explained by the fact that the 

 observations were limited to six winter months, rather than the 2-year 

 period of the other stations.) Tilted elevated layers appear to produce 

 the greatest enchancement of signal strength, probably as a result of 

 focusing effects due to the layer tilt. 



6.2.4. Prediction of Field Strength for 

 Unstratified Conditions 



The field strengths recorded during the times when the radiosonde 

 ascents at both Joliet and Rantoul indicated nonstratified conditions were 

 compared with the values predicted by Norton, Rice, and others [31] for 

 the case of diffracted plus scattered radio waves. This particular pre- 

 diction process is adjustable for the average refractive conditions over the 

 path in that it adjusts the effective earth's radius factor to the initial 

 gradient of A^ for the calculation of diffracted field strengths. One also 

 needs the angular separation of the radio horizon rays at their intersection 

 near midpath. The average initial gradient of A^ was obtained for each 

 instance of unstratified profile by .simply averaging the initial gradients 

 from Rantoul and Joliet, while the angular separation was obtained by 

 determining the amount of radio ray refraction expected over each particu- 

 lar path in atmospheres of exponential decrease with height that closely 

 match the observed A^ conditions. 



Figure 6.14 illustrated a comparison of the predicted and observed field 

 strengths. For WNBQ and WENR there is approximate agreement be- 

 tween the two sets of data. However, the predicted values for WBKB 

 and WMBI are approximately 10 to 12 dB higher than the observed 

 values. 



This tendency to predict field in excess of the measured values suggests 

 that the empirical data, on which the predictions are based may include 

 meteorological conditions with some degree of stratification in the first 

 2 km or so, even though most of the empirical data refer to afternoon 

 hours in winter. It will be shown that elevated layers of moderate size 

 (say a few kilometers in horizontal dimensions), that may exist undetected 

 by the radiosonde, could produce field strengths on the Illinois paths com- 

 parable with the median values for "unstratified" conditions shown in 

 figure 6.13. Furthermore, the limits placed on the profile gradients 

 specifying unstratified conditions in table 6.5 are such that some layer 

 type profiles may be included in the unstratified category. Consequently, 

 it is important to study in more detail the properties of the elevated layer, 



