PERMANENT ECHOES 



iHl 



The angle between the two rays is then equal t(J 



\2ka d'd" d' d"/ 



where rf = d' + d" , and {ka)'^ = 1.18 • 10"' (meter) -\ 

 When d is measured in miles and h in feet, equation 

 (16) becomes 



/3i + /32 =1.89- 10"*d 



_hp_ 

 .d'd"' 



1 - 



d' 



hi 

 </"J 



fr 



Figure 7. Shielding between transmitter and receivor. 



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 ol)scure 

 1 he operating sector. 



5. Objects appear ■wider because of the antenna 

 l)eamwidth and of greater extent in range as a result 

 of the pulse width. 



(). 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 seriou.sly affects echoes from small 

 targets may not have any noticeable effect on the 

 response from large targets. An echo used for a. 

 standard target should be weak and near bv. 



"^ PERMANENT ECHOES 



"*' Introduction 



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 \vhere 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 nonstandard 

 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. 



10.4.2 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- 

 sj^stem, using a standai'd procedure for taking and 

 presenting the data. 



Permanent echo data should be taken under 

 a\'erage 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 Lsland 260- 

 miles away and appears only during times when 

 propagation is nonstandard. 



Care must be exercised in identif.ying 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 scope after the main pulse. These 

 latter echoes may be checked by changing the pulse- 

 repetition rate and observing the shift of the echo. 



