Sentence:  Very recently , observations have been made at 8 - mm wave length with a reflector 22 meters in diameter with a resultant <WSD>beam</WSD> width of only about 2 ' ( Amenitskii , Noskova , and Salomonovich , 1960 ) . 
Meaning of the word marked with <WSD> tags:  a signal transmitted along a narrow path; guides airplane pilots in darkness or bad weather