196 



SITING 



quencies based on ionosphere predictions. For 

 distances up to 100 or 200 miles, a half-wave hori- 

 zontal wire antenna should he used and the fre- 

 quency range is about 2 to 8 mc. The decrease in 

 gain for the short path up through the trees is 

 negligible at these frequencies. 



10.5.3 Yjjg Effect of Trees and Obstacles 

 on Microwaves 



At 10 cm, the absorption is so great with most 

 objects that the diffracted energj^ is the principal 

 portion transmitted. Only windows, light wooden 

 walls, or branches of leafless trees show less than 



10 db loss. Opaciue objects include: 



1. Rows of trees in leaf if more than two in depth. 



2. Screens of leafless trees if so dense that the 

 skyline is invisible through them. 



3. Trunks of trees. 



4. Walls of masonry. 



5. Any but the lightest wooden buildings, espe- 

 cially if there are partitions. 



Losses of a brick wall may be increased from 12 db 

 to 46 db by wetting. In computing diffraction over 

 treetops, the diffracting edge may be taken to be 

 5 feet or so less in height. In a 1.25-cm test, the 

 transmission loss through two medium-sized bare 

 trees increased 18 db after leaves appeared. 



