EFFECTS OF TREES, JUNGLE, ETC. 



Targets or mountains below and beyond the cover- 

 age contours will not be visible except by diffraction. 

 These contours may be drawn for several heights. 

 Where they are close together, the shielding is good 

 but the coverage is poor. Where the lines are widely 

 separated, as toward the sea, there is little or no 

 shielding except that due to earth curvature. With 

 the coverage contour diagram superimposed on a 

 map, the peaks exposed to radiation may be noted. 



The extent of the echoes due to these peaks de- 

 pends, besides the size of the peak, on the horizontal 

 radiation pattern, the pulse width, and the power and 

 sensitivity of the radar. It should be noted that the 

 half-power beamwdth is only a rough measure of the 

 ^^idth of an echo and some greater angle between 

 the half-power points and the nulls will usually be 

 obtained for the echoes. 



The extension of the echo in range « ill be at least 

 as great as the pulse width in miles as represented 

 on the scope. This is about 0.1 mile per micro- 

 second of pulse width. Actual echoes are thicker 

 than this, since all the exposed hill sends back echoes. 



After a careful inspection of the profiles, taking 

 into account the A'arious factors mentioned above, the 

 echoes are sketched in on the chart. In doing this, 

 judgment and experience are important factors, but 

 the following rules may be used as a guide. 



1. Shade in a circle for the main pulse several 

 miles wide, depending on the pulse ■\\ddth and local 

 return. 



2. Check each profile in turn and for each peak 

 or hillside in front of the shielding ridge or mountain 

 plot an echo for the main and all side lobes of the 

 antenna. 



3. A series of sharp hills within the shielding part 

 of the terrain should be plotted as a single large echo. 



4. The inner edge of an echo should be at the same 

 range as the hill. 



5. Peaks beyond the .shield may be in the diffrac- 

 tion region and the relative strength of the echo 

 may be estimated from a diffraction curve. 



6. In general, the echo strength varies as the 

 inverse square of the distance and is roughly pro- 

 portional to the target area. 



7. Where there is any doubt, the echo should be 

 plotted. 



Experience is an essential factor in peimanent 

 echo prediction, regardless of the method used. The 

 methods described here have been used successfully 

 in many areas and are capable of accuracy adequate 

 for most piu'poses. 



195 



'» ^ EFFECT OF TREES, JUNGLE, ETC. 



''■'■' The Eflfect of Trees 



Trees form very effective obstacles for high-fre- 

 ciuency radio waves. A single tree may cause a drop 

 in signal strength of .several decibels. The attenua- 

 tion is less for horizontal polarization than for 

 ^•ertical polarization for frequencies below 300 to 500 

 megacycles. For higher frequencies, the polarization 

 is not an important factor. With the transmitting 

 antenna sited in a moderately wooded area, repre- 

 sentative values for the losses are given in Table 1. 



Table 1 . Decrease in gain for transmitting antenna 

 situated in a moderately wooded area. 



Frequency 



Horizontal 

 polarization 



Vertical 

 polarization 



30 mc 

 100 mc 



Negligible 

 1-2 db 



2- 3db 

 5-10 db 



When both antennas are in the woods these losses 

 should be doubled. Measurements at 200 mc for 

 transmission through a grove of trees 100 feet ^ride 

 show losses of 21 db for vertical polarization and 

 6 db for horizontal polarization. 



When the antennas are in clearings, so that each is 

 more than 200 or 300 feet from the edge of the woods, 

 the decrease in gain is small. With vertical polariza- 

 tion, there may be large and rapid variations of field 

 intensity within a small area, due to reflections from 

 near-bv trees. 



10.5.2 



The Effect of Jungles 



In jungles or heavy undergrowth, an exponential 

 absorption is to be expected. Tests made of trans- 

 mission through heavy jungles, such as are found in 

 Panama or in New Guinea, show that the limit of 

 transmission for ordinary field sets is 1 mile. An 

 increase of power of several hundred fold is needed 

 for a range of 2 miles. The decreases in gain en- 

 countered are of the order of 50 to 60 db per mile. 



If the antennas are elevated above the jungle or 

 located in clearings, the effect of the jungle may be 

 minimized. Antennas should be 10 or more feet 

 away from trees to avoid a change in antenna 

 impedance. 



The best solution is skjMvave transmission even 

 for distances as short as 1 mile. Due consideration 

 should be given to the selection of optimum fre- 



