FIRE-CONTROL RADARS FOR NAVAL VESSELS 3 



angles is very good provided the target can be seen clearly. This proviso is 

 a serious limitation under many typical operating conditions. It is fre- 

 quently difficult to see a target at a range of several miles on account of 

 haze even on a relatively clear day, and at night or in fog or smoke screen 

 the usefulness of a telescope is almost nil. The optical range finder is 

 subject to the same limitations as the telescope and in addition leaves 

 much to be desired in the matter of accuracy and continuity of data even 

 under the best visibility conditions. This is due to the fact that optical 

 range tinders are triangulation devices which inherently have accuracy 

 limitations. The need for a long and very stable base line between the 

 prisms of an optical range finder is difficult to meet aboard ship, and the 

 principle of operation makes inevitable a rapidly decreasing accuracy with 

 increasing range. Thus, as the effective range of guns increased, the need 

 for more accurate means for measuring range became more acute. 



In its earliest forms radar offered at once a potential means for measuring 

 range with much better accuracy than that of the optical range finder. 

 This was due to the different principle on which radar works. A pulse of 

 radio frequency energy is sent out to the target and the echo signal is re- 

 ceived back at the source. The velocity of the waves en route is the 

 same as that of light, and is one of the basic physical constants. To measure 

 range accurately with radar required only the development of techniques 

 for producing short transmitted pulses and for measuring accurately the 

 short intervals of time between the transmitted pulse and the returning 

 echo pulse. Both of these were the kind of problems which yield readily 

 to electronic solutions. The early work in Bell Telephone Laboratories 

 thus included the production of shorter transmitted pulses than were 

 being commonly used, and the development of improved range measuring 

 means. 



The second important general objective for the early work at Bell 

 Telephone Laboratories was to devise equipment which would operate at 

 frequencies much higher than had been previously used. The need for 

 higher-frequency operation arose from the fact that for a given size of 

 antenna the beam width decreases with increasing frequency while the 

 gain increases. Narrow beams are required to obtain accurate angular 

 data wiiile increased gain is desirable since it obviously provides increased 

 range for a given transmitter power and receiver noise figure. These factors 

 are illustrated by the curves of Figs. lA and IB which show the relationship 

 between beam width, antenna gain and antenna size expressed in wave- 

 lengths. The curve labeled "uniform illumination" yields maximum gain 

 and minimum beam width for a given antenna size but produces unwanted 

 side lobes of undesirable amplitude. For this reason the illumination is 

 usually graded over the antenna aperture to reduce minor lobes. The gain 



