A preliminary design procedure to find the aperture diameter and other basic parameters of a feed able to satisfy radiometric requirements

An analysis is presented which, starting from the radiometric requirements to be satisfied by a parabolic antenna, gives simple guidelines to properly size the primary feed. In particular the beam efficiency and the half power beamwidth of the antenna are taken into account. A loss budget for the beam efficiency is made giving in this way the feed aperture able with its primary pattern to provide the required beam efficiencies values. All the considerations contained in the paper can be qualitatively applied to whatever antenna at the condition that it is an offset parabolic reflector. Nevertheless a quantitative analysis is given taking as a practical case the MIMR (Multifrequency Imaging Microwave Radiometer) antenna optic. Moreover the higher frequency of this radiometer (90 GHz) is taken into account for different reasons. The required tolerances related to the manufacturing of a feed with small dimensions can be met with difficulties. For the above reasons the procedure described is conservative and can be applied without difficulties to lower frequencies. The beam efficiency requirements which give the radiometer temperature resolution are specified for 2 angular zones around the antenna beam peak. It is clear that it is not sufficient to increase the edge taper of the parabola illumination with the enlargement of the feed aperture but it is necessary to take great care to not exceed certain upper limits for the feed aperture diameter to avoid a parabola half power beamwidth larger than the required value of 0.174/spl deg/. It would be preferable to have a primary pattern with a shape similar to a gaussian function and with an adequate edge taper. The degradation factors which affect the beam efficiency are listed.<>