Sentence:  The antenna patterns and the power gain at the peak of the <WSD>beam</WSD> were both measured ( Mayer , McCullough , and Sloanaker , 1958 b ) , so that the absolute power sensitivity of the antenna beam over the solid angle of the moon was known . 
Meaning of the word marked with <WSD> tags:  a signal transmitted along a narrow path; guides airplane pilots in darkness or bad weather