174 



SITING AND COVERAGE OF GROUND RADARS 



41*000 



35,000 



RADIANS-*- 0.08 0.07 0.06 



0.03 



f =160 MC 



1^=500 FEET M 



SHORE LINE = 15840 FEET 

 d 110 MILES 



ANTENNA-VERTICAL HALF WAVE DIPOLE 

 VERTICAL POLARIZATION 



Figure 73. Coverage diagram for Example 21. 



lobe is equal to the free space field. At ranges shorter 

 than this the reflected wave opposes the direct wave. 

 Directly under the antenna the contour passes near 

 the surface so that the waves are very nearly in 

 opposition. Because of the variation in Dpz with y 

 the maxima will not occur exactly when the cosine 

 is unity, but this effect is generally negligible. 



15.7 



15.7.1 



CALIBRATION AND TESTING 

 Introduction 



It should not be inferred from Section 15.6 that a 

 reliable coverage diagram can be obtained by calcu- 

 lation alone. Under field conditions it is necessary 

 to make test flights and other checks before equip- 

 ment can be depended upon to meet a calculated 

 performance. On the other hand it is seldom possible 

 or desirable to obtain a satisfactory coverage diagram 

 from tests alone. Best results are attained when tests 

 and analysis supplement each other. 



Test flights are arduous, expensive in personnel 

 and materials, and time consuming. In most theaters 

 a number of agencies become involved, and careful 

 planning and organization are required to achieve a 

 useful result. For these reasons the amount of test 

 flying should be held to a minimum by intensive 

 analysis and equipment tests before and after the 

 test flights. "Calibration and testing" might well be 



a book in itself, but only a very brief discussion will 

 be given here for the sake of completeness. 



1072 Equipment Tests 



It is difficult to overemphasize the importance of 

 proper equipment maintenance. An unfortunate ten- 

 dency of inexperienced personnel is to maintain on an 

 emergency basis, rather than as a matter of system- 

 atic routine. In most cases the need is for a careful 

 check of all elements and restoration to as-good-as-new 

 condition, rather than a brilliant intuitive process 

 known as "trouble shooting." One survey of a large 

 number of systems disclosed an average reduction 

 from optimum performance of 13.5 db. This corre- 

 sponds to a maximum range of 50 per cent of normal. 

 Careful tests have shown the use of "standard tar- 

 gets" to be very misleading in many cases. Large 

 changes in the maximum ranges of small targets 

 were found without appreciable changes in the 

 strength of the permanent echoes used for checking 

 purposes. 



Full use of test instruments available should be 

 made in checking the equipment. Orientation should 

 be completed and the accuracy of range and azimuth 

 indicators checked. Tuning and modifications should 

 be done before the test flights are made, unless the 

 tests indicate poor performance. A great handicap 



