SITING 
enn 
Ficure 7. Shielding between transmitter and receiver. 
When d is measured in miles and h in feet, equation 
(16) becomes 
Bi -+ B: = 1.89- 10a] 
hp hy i] 2 
1-— -- mGLZ 
Wait Ce (Le ) 
PERMANENT ECHOES 
Permanent echoes are caused by reflections from 
terrain features such as mountains or even smooth 
surfaces near the antenna (ground clutter). With 
radars, the indicator is obscured by the strong 
echoes from hills and the minimum detection range is 
increased by ground clutter. With direction finders, 
erroneous indications are caused by the spurious 
reflections. Permanent echoes are among the princi- 
pal problems involved in siting, as many otherwise 
excellent sites are rendered worthless by excessive 
fixed echoes. Several methods are available for 
determining the suitability of sites in this regard 
without actual field tests. 
A number of factors combine to make permanent 
echoes more troublesome than might be expected. 
1. Hills and land surfaces are so much greater in 
extent than the target which the equipment is de- 
signed to detect that strong echoes may be obtained 
from distances where an ordinary target would give 
an echo far below normal detection levels. 
2. The low elevation of the land surfaces places 
them in regions most subject to nonstandanl 
propagation effects where extreme ranges and large 
responses are frequently obtained. 
3. Side lobes of the horizontal pattern of the an- 
tenna cause permanent echoes to appear at several 
other azimuths in addition to that of the main lobe. 
4. Strong permanent echoes from mountains to 
the rear may be caused by back radiation from the 
antenna. The low intensity of the back radiation 
may be compensated by the size of the mountains. 
Such echoes are especially harmful as they obscure 
the operating sector, 
5. Objects appear wider because of the antenna 
heamwidth and of greater extent in range as a result 
of the pulse width. 
6. Diffraction over intervening ridges may be 
sufficient to nullify their screening action so that 
objects behind a ridge are visible. 
7. The use of a permanent echo as a standard 
target may be very misleading. A decrease in per- 
formance that seriously affects echoes from small 
targets may not have any noticeable effect on the 
477 
response from large targets. An echo used for a 
standard target should be weak and near by. 
Permanent Echo Diagrams 
The permanent echoes associated with a radar 
station may be plotted on a polar chart and their 
extent, location, and strength represented. Such 
diagrams should be prepared for each unit of a radar 
system, using a standard procedure for taking and 
presenting the data. 
Permanent echo data should be taken under 
average conditions with the gain set at some stand- 
ard level. At intervals of azimuth such as 5 de grees, 
the ranges of the permanent echoes are recorded. 
These data are then plotted on a polar chart and the 
points are connected to indicate obscured areas. 
The skill and judgment of the operator are important 
factors. In most cases the amplitudes of the echoes 
are so far above that of ordinary target echoes that 
the actual amplitudes need not be noted. 
In Figure 8 is shown an observed permanent echo 
diagram for a VHF radar. This was selected for 
purposes of illustration rather than as an example 
of a good site. The mountains to the north are un- 
shielded and cause extensive echoes. The large echo 
at 200 degrees is due to a mountainous island 260 
miles away and appears only during times when 
propagation is nonstandard. 
Care must be exercised in identifying the cause of 
an echo. Antenna side lobes cause spurious echoes 
and distant echoes may come in on the second or 
third sweep on the scone after the main pulse. These 
latter echoes may be checked by changing the pulse 
repetition rate and observing the shift of the echo. 
Permanent echo diagrams are useful for: 
1. Indicating blind areas in a station’s coverage. 
. Assigning the operating area of a station. 
. Checking the range and azimuth accuracy. 
. Checking the performance. 
. Estimating nonstandard propagation 
. Planning test flights. 
am kW tw 
Shielding 
The principal device in the field for the control of 
permanent echoes is shielding. This means that the 
antenna must be sited in such a way that distant 
hills are screened by a local obstruction. This local 
echo at, say, three miles, is combined with the main 
pulse or ground return and the distant echo is 
weakened or eliminated entirely. 
Shielding causes a loss of coverage, which in 
operating regions may be more serious than the 
permanent echoes. Rear areas which are not scanned 
should be well shielded so that back and side echoes 
do not interfere with targets in important tactical 
regions. Operation over such shielded sectors would 
be limited to high targets. 
