200 



REFLECTION AND TRANSMISSION OF RADIO WAVES 



15 



'T 10 



1 5 



^o o | I I t^Yt I I I °l f- l ° l °1 ^1 I L°h- I I h ^ 



D0_ - OAA _Oo 



2 -10 



-15 



300 320 340 10 20 30 40 50 60 70 80 90 100 120 140 



AZIMUTH ANGLE (deg) 



Fig. 4-22 Plot of Median Echo of F-51 Averaged over 5° of Azimuth. 



position where the signals are equal. Since the two samples are not received 

 at the same instant, any change in signal amplitude during the scanning 

 cycle, caused by target fluctuation, will lead to an angle error indication 

 even if the antenna is pointed correctly at the target. In order to make an 

 optimum choice of the parameters of a sequential lobing system, therefore, 

 it is necessary to have information on the amplitude fluctuation charac- 

 teristics of target aircraft. 



Some of the causes of amplitude fluctuations already have been men- 

 tioned in Paragraph 4-7, e.g. propeller modulation. A clear example of this 

 is shown in Figure 4-23, which is the spectrum of amplitude fluctuations in 



Fig. 4-23 Spectrum of Amplitude Noise of SNB (Two-Engine Transport) Aircraft 

 in an Approach Run, Showing Spectral Lines Due to Propeller Modulation (= 9400 



Mc). 



an approach run of an SNB (two-engine transport) taken at a radar 

 frequency around 9400 Mc. The repetition frequency of the radar was 

 1000 cycles, so spectral information up to 500 cycles is derivable. Here 

 peaks occur at multiples of about 58 cps. The remainder of the spectrum 

 consists of a continuous band'^ whose amplitude decreases with increasing 



l*The small ripples or scintillations are due to incomplete smoothing in the spectrum 

 analyzer. 



