The Open Cassegrain Antenna: Part I. Electromagnetic Design and Analysis

T h e open cassegrain antenna configuration is shown in Fig. 1. Its optical geometry is straight forward, consisting of hyperboloid and paraboloid surfaces, but it has the distinguishing characteristic that the axes of rotational symmetry of the sub- and main-reflector surfaces do not coincide. Fig. 2 shows a view of the antenna looking down the beam axis. T h e projected aperture is circular and no aperture blocking is introduced by the subreflcctor or its support structure (not shown in Fig. 2), hence the name open cassegrain. Non-orthogonal beam steering axes are used. T h e antenna is directed at zenith in Fig. 1(a) and at its minimum elevation of --5° in Fig. 1(b). The lowest elevation excursion is determined by the angle of the slant axis which is coincident with the "secondary optical" axis and is in1255 HYPERBOLOID SUBREFLECTOR~~-v Oi O S PARABOLOID ' AXIS to PARABOLOID MAIN REFLECTOR FEED HORN ROTARY JOINT SLANT -ELEVATION BEARING (a) M A X I M U M ELEVATION (ZENITH) Fig. 1 -- Open cassegrain antenna.