282 BELL SYSTEM TECH NIC A L JOURNA L 



greater detail in section 14.8. Each element of this array consists of a fixed 

 vertical array of three polyrods. This elementary array provides the re- 

 quired vertical pattern and has appropriate horizontal characteristics. 

 Fourteen of these elements are arranged in a horizontal array with a spacing 

 between neighbors of about two wavelengths. Energy is distributed among 

 the elements with a system of branching waveguides. Thirteen rotary phase 

 changers are inserted strategically in the distributing system. Each phase 

 change is rotating continuously and shifts the phase linearly from 0° to 

 360° twice for each revolution. As the phase changers rotate the array 

 produces a beam which sweeps repeatedly linearly and continuously across 

 the scanning sector. 



When elements of a scanning array are spaced considerably less than one 

 wavelength it is a very simple matter to obtain a suitable elementary 

 pattern, for the array factor itself has only a single beam. This advantage 

 is offset by the greater number of elements and the consequent greater com- 

 plexity of distributing and phase shifting equipment. In one useful type of 

 scanning antenna however distributing and phase shifting is accomplished 

 in a particularly simple manner. Here the distributing system is merely a 

 waveguide which can transmit only the dominant mode. The wide dimen- 

 sion of the guide is varied to produce the phase shifts required for scanning. 

 The elements are dipoles. The center conductor of each dipole protrudes 

 just enough into the guide to pick up the required amount of energy. 



It is evident from the above discussion that such a waveguide fed dipole 

 array will produce a single beam in the normal direction only if the dipoles 

 are all fed in phase and are spaced less than a wavelength. It is therefore 

 not satisfactory to obtain constant phase excitation by tapping the dipoles 

 into the guide at successive guide wavelengths for these are greater than 

 free space wavelengths. Consequently the dipoles are tapped in at suc- 

 cessive half wavelengths in the guide and reversed successively in polarity 

 to compensate for the successive phase reversals due to their spacing. 



This type of array provides a line source which can be scanned by moving 

 the guide walls. In order to leave these mechanically free suitable wave 

 trapping slots are provided along the length of the array. 



A practical antenna of this type will be described in Sec. 16.3. 



12.3 Optical Scanning 



With a camera or telescope all parts of an angular sector or field are viewed 

 simultaneously. We would like to do the same thing by radar means, but 

 since this so far appears impossible we do the next best thing by looking 

 at the parts of the field in rapid succession. Nevertheless certain points of 

 similarity appear. These points are emphasized by a survey of the fixed 



