114 



RADAR SCOPES 



cause lobe patterns typically are rather 

 irregularly "pear-shaped" and will cause an 

 echo to show up as wider at medium than at 

 short or long distances. Beam^\ddth in the 

 present context will refer only to actual 

 beamwidth of the pip on the scope, i.e., the 

 length of the arc segment (cf. Fig, 1). In 



small objects, and therefore require maximal 

 vie'\\"ing conditions. 



The effect on visibility of changing pulse 

 length and beamwidth can be seen in Figs, 

 5 and 6. These data were taken on a noise- 

 free scope but very similar results have been 

 obtained with noise. The total range of 



-2 -4 -6 -8 -10 



CRT BIAS IN VOLTS FROM VRI 



Fig. 4. Positive versus negative modulation of beam 

 Positive modulation is normal, producing a brighter-than-background pip. Negative modulation 

 produces a pip darker than its background. Large pip is 7| jus x 10°; small pip, 1 )us x 1°. No video 

 noise. Otherwise conditions same as in Fig. 2. Broken lines indicate invisibility. 



Hnear measure, the angular width of a pip 

 varies as its distance from the center of the 

 scope. In angular subtense at the observer's 

 eye, a very small pip | microsecond x 1 

 degree would, if seen from 12 inches at half- 

 radius on a 6-inch scope and a 20-mile range 

 scale, measure 1'48" x 7'31". An "average" 

 size pip might be 2 microseconds x 10 de- 

 grees, which would subtend an area about 

 1%" X 1°15'. Pips, on the whole, are rather 



pip sizes is very large compared to those 

 encountered in operational conditions. Ex- 

 cept in the case of the dim screen, a differ- 

 ent law is seen to hold for beamwidth than 

 for pulse length. For small pips, visibility 

 increases more rapidly with pulse length 

 than with beamwidth, even when the latter 

 are equated in retinal subtense. This effect 

 is presumably due to the time difference in 

 rotary presentation: all points on a radial 



