RADIO PROPAGATION FUNDAMENTALS 613 



shadow loss exceeded in only 10 per cent of the possible locations between 

 points A and B is about 20 + 15 = 35 db. It will be recognized that this 

 analysis is based on large-scale variations in field intensity, and does not 

 include the standing wave effects which sometimes cause the field inten- 

 sity to vary considerably within a few feet. 



Effects of Buildings and Trees 



The shadow losses resulting from buildings and trees follow somewhat 

 different laws from those caused by hills. Buildings may be more trans- 

 parent to radio waves than the solid earth, and there is ordinarily much 

 more back scatter in the city than in the open country. Both of these 

 factors tend to reduce the shadow losses caused by the buildings but, 

 on the other hand, the angles of diffraction over or around the buildings 

 are usually greater than for natural terrain. In other words, the artificial 

 canyons caused by buildings are considerably narrower than natural 

 valleys, and this factor tends to increase the loss resulting from the pres- 

 ence of buildings. The available quantitative data on the effects of build- 

 ings are confined primarily to New York City. These data indicate that 

 in the range of 40 to 450 mc there is no significant change with fre- 

 quency, or at least the variation with frequency is somewhat less than 

 that noted in the case of hills.^'^ The median field intensity at street level 

 for random locations in Manhattan (New York City) is about 25 db 

 below the corresponding plane earth value. The corresponding values 

 for the 10 per cent and 90 per cent points are about 15 and 35 db, re- 

 spectively. 



Typical values of attenuation through a brick wall, are from 2 to 5 

 db at 30 mc and 10 to 40 db at 3,000 mc, depending on whether the wall 

 is dry or wet. Consequently most buildings are rather opaque at fre- 

 quencies of the order of thousands of megacycles. 



When an antenna is surrounded by moderately thick trees and below 

 tree-top level, the average loss at 30 mc resulting from the trees is usually 

 2 or 3 db for vertical polarization and is negligible with horizontal polar- 

 ization. However, large and rapid variations in the received field inten- 

 sity may exist within a small area, resulting from the standing-wave 

 pattern set up by reflections from trees located at a distance of several 

 wavelengths from the antenna. Consequently, several near-by locations 

 should be investigated for best results. At 100 mc the average loss from 

 surrounding trees may be 5 to 10 db for vertical polarization and 2 or 3 

 db for horizontal polarization. The tree losses continue to increase as the 

 frequency in(?reases, and above 300 to 500 mc they tend to be inde- 

 pendent of the type of polarization. Above 1,000 mc, trees that are thick 



