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.
... The left-wing of the SPA, or the Debs Caucus, including McReynolds, reconstituted the spirit of the SPA and Eugene V. Debs into the Socialist Party U.S.A. ...