480 PROPAGATION THROUGH THE STANDARD ATMOSPHERE 
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 beamwidth is only a rough measure of the 
width 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 will 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 various factors mentioned above, the 
echoes are sketched in on the chart. In doing this 
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 width 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, plot the echo. 
Experience is an essential factor in permanent 
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 purposes. 
EFFECT GF TREES, JUNGLE, ETC. 
The Effect of Trees 
Trees form very effective obstacles for high-fre- 
quency 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 
vertical 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. 
When both antennas are in the woods these losses 
should be doubled. Measurements at 200 me for 
transmission through a grove of trees 100 feet wide 
show losses of 21 db for vertical polarization and 
6 db for horizontal polarization. 
TaBLE 1. Decrease in gain for transmitting antenna 
situated in a moderately wooded area. 
Horizontal Vertical 
Frequency polarization polarization 
30 mc Negligible 2- 3 db 
100 me 1-2 db 5-10 db 
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-by trees. 
The Effect of Jungles 
In jungles or heavy undergrowth, an exponential 
absorption is to be expected. Tests made of trans- 
mission through heavy jungles 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 sky-wave transmission even 
for distances as short as 1 mile. Due consideration 
should be given to the selection of optimum fre- 
quencies based on ionosphere predictions. For 
distances up to 100 or 200 miles, a half-wave hori- 
zontal wire antenna should be 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. ; 
Effect on Microwaves 
At 10 cm, the absorption is so great with most 
objects that the diffracted energy is the principal 
portion transmitted. Only windows, light wooden 
walls, or branches of leafless trees show less than 
10 db loss. Opaque 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. 
