GENERAL PROBLEM 



161 



This effect can be counteracted in several ways. 

 One way is to employ microwaves whose inter- 

 ference lobes are narrow and close together. Vertical 

 polarization is another means of filling in the nulls 

 while gaining in maximum range at low angles. 

 Another device is to tilt the antenna beam upward 

 so that some radiation (but substantially less than 

 half) falls upon the sea. The result is a gain in low- 

 angle coverage while the high-angle coverage is that 

 of free space, without miirima. 



The effect of various percentages of specular 

 reflection in comparison with the free-space pattern 

 is shown in Figure 1 for various beam tilts of a 

 radar with a comparatively narrow beam width (11 

 degrees between half-power points). The experimen- 

 tal data, shown by small circles, illustrate the increase 

 in detection range at degree while at 5 degrees 

 elevation angle there is little gain over free space. 



The roll of a ship, by varying the beam tilt, results 

 in a shift in coverage, as can be seen from Figure 1. 



7.1.4 



Signal-to-Noise Ratio 



The visibility of a signal on a scope depends on its 

 relation to the noise. In early work with radar, the 

 maximum range was defined by a ratio of signal 

 voltage S to noise voltage N of unity, i.e., 



N 



= 1. 



(1) 



However, as pointed out in Section 2.3.5, the min- 

 imum detectable signal is greater than A^. Since the 

 pip on a scope includes noise, equation (1) is equiva- 

 lent to 



S+ N 

 N 



= 2. 



(2) 



On an A scope, this relation signifies that the height 

 of the signal is twice that of the noise grass. 



Since the visual signal in a set functioning properly 

 varies linearly with the signal voltage, the size of 

 targets can be estimated by means of the size of the 

 visual signal. The ratio S/N gives a means of meas- 

 uring a signal in terms of the noise. To change 

 (S + N)/N to S/N, the value of {S + N)/N is 

 expressed with unity as denominator. For example, 

 if {S + N)/N is estimated to be 8/2 from the scope, 

 the equivalent fraction is 4/1. The value of S/N is 

 (4 - 1)/1 = 3/1. 



The relation of receiver power, P^, to noise power, 

 NP, and the signal-to-noise ratio, <S/A'', is given by 



10 log -^ = 20 log - . (3) 



NP N 



^■'^ Calibration of an A Scope 



On an A scope, the ratio (S + N)/N can be 

 estimated roughly by eye. To improve upon this, a 

 cahbration is employed. One method is to mark the 

 A scope to facilitate the reading of heights. Another 

 method goes be.yond this and calibrates the gain 

 control. A turn of 5 db is equivalent to a raito of 

 1.8/1. A datum line 1 cm above the time line and 



Figure 2. Method of calibrating an A scope. 



Figure 3. Calibration of gain control of an A scope. 

 (Dotted lines are uncorrected calibration.) 



another at 1.8 cm are drawn on the A scope. The 

 noise is brought up to the datiun line by means of 

 the gain control. The position is marked on the 

 gain control (see Figure 2) . A steady signal is found 

 (permanent echo, large boat, or signal generator) 

 which produces a signal height of 1.8 cm above the 

 time line. The gain control is then turned until the 

 signal is reduced to the datum line. The position 



