Directivity Pattern of 3 cm. Parabolic Reflector, Feed of Which is Displaced from the Focus

Abstract

Directivity pattern of a 3 cm. parabolic reflector has been studied, placing the dipole at various distances from the vertex. The antenna used here has an aperture diameter of 68 cm. and a focal length of 20·2 cm., thus having f/D=0·297. It was fed by a dipole disc feed at an operating wavelength 3·2 cm. Observations were taken in H plane by placing the feed at distances varying from 0·74 f to 0·99 f from the vertex of the paraboloid. It is observed that the main lobe of the directivity pattern gets bifurcated when the dipole is at distances lower than 0·84 f from the vertex. Side lobes are weaker when the dipole is at 0·89 f from the vertex. Half width of the main lobe goes on decreasing as the feed is brought nearer the focus.