Experimental and theoretical reradiation polar diagrams are compared for theH-plane of a rectangular pyramidal horn. A distinction is made between scattering, which is found to be due to the presence of higher modes, and reradiation, which is mainly responsible for the shadow. The distinction applies to receiving aerials in general, and a theory is presented in which every receiving aerial is regarded as a potential perfect absorber. The imperfection in normal operation is explained by the extent to which the incident field fails to correspond to the ‘complementary’ field that perfect absorption would require.