278 



BELL SYSTEM TECHNICAL JOURNAL 



minated by a liorn feed, a combination which produces excellent electrical 

 characteristics. The paraboloid and feed combination is made structurally 

 strong and is pivoted to permit rotational oscillation in a horizontal plane. 

 It is forced to oscillate by a rigid crank rod which is in turn driven by an 

 eccentric crank on a shaft. The shaft is belt driven by an electric motor and 

 its rotational rate is held nearly constant by a flywheel. The mechanical 

 arrangement described so far would oscillate rotationally in an approxi- 

 mately sinusoidal fashion. Since every action has an equal and opposite 

 reaction it would, however, react by producing an oscillatory torque on its 



Fig. 36 — Experimental Rocking Horse Antenna. 



mounting. Since the antenna is large and the oscillation rapid this would 

 J reduce a ssvere and undesirable vibration. To get around this difficulty 

 an opposite and balancing rotating moment is introduced into the mechan- 

 ical system. This appears in the form of a pivoted and weighted rod which 

 is driven from the same eccentric crank by another and almost parallel 

 crank arm. 



Although not theoretically perfect the rotational 'dynamic' balancing 

 described permits the antenna to scan without serious vibration. One form 

 of this antenna will be described in a later section. 



12.2 Array Scanning 



During our discussion of general principles in Part II, we saw that an 

 antenna wave front can be synthesized by assembling an array of radiating 



