202 



REFLECTION AND TRANSMISSION OF RADIO WAVES 



so that a line frequency spectrum similar to the propeller modulation 

 spectrum would not be expected. The amplitude modulation spectrum 

 observed in such cases would be due chiefly to beats between the doppler 

 frequencies of the salient reflection centers, as described in Paragraph 4-4. 

 The frequency of this type of modulation, therefore, would be proportional 

 to the radar frequency. Whether a continuous spectrum is obtained, or one 

 with spectral lines superposed on a continuous spectrum, depends on the 

 nature of the perturbations of the aircraft for straight-line flight and on the 

 duration of the observation (that is, on the length of sample). 



Figure 4-24 shows a spectrum^^ of the amplitude noise of the B-45 for 

 which the observation time was 5 seconds. The voltage-time plots from 

 which the spectrum was prepared are shown in Fig. 4-25. The 9380-Mc 



h 



B ■ 45 Run 10 1250 Mc 

 Range 13,600 ya 

 Azimuth 96°20'- 98°42' 

 Elevation 6''56'- 6°55' 



^ 



i:¥ 



ills 



V 



Fig. 4-25 Voltage-Time Plots of B-45 Fxho, Showing Low-Frequency Modulation. 



i^The resolving power of the spectrum analyzer used was about L5 cps. Also, its response 

 begins to drop off below about 6 cps. 



