In a reflector antenna[?], the relative reduction in the effective aperture of the antenna caused by the masking effect of other antenna parts, such as a feed horn or a secondary reflector, which parts obstruct the radiation path.
... (+ 1983)
April 22 - Robert Oppenheimer, physicist (+ 1967)
May 6 - Harry Martinson, swedish author, winner of the Nobel Prize in literature
May 11 - Salvador Dalí, ...