Focal-Plane Array Architectures: Horn Clusters vs. Phased-Array Techniques

The science goals of the SKA specify a field of view which is far greater than what current radio telescopes provide. Two possible feed architectures for reflector antennas are clusters of horns or phased-array feeds. This memo compares these two alternatives and finds that the beams produced by horn clusters fall short of fully sampling the sky and require interleaved pointings, whereas phased-array feeds can provide complete sampling with a single pointing. Thus for a given focal-plane area horn clusters incur an equivalent system temperature penalty of ∼ 2× or more. The situation is worse for wide-band feeds since the spacing of the beams is constant while the beamwidth is inversely proportional to frequency, increasing the number of pointings for a fully-sampled map at the high-end of an operating band. These disadvantages, along with adaptive beamforming capabilities, provide a strong argument for the development of phased-array technology for wide-field and wide-band feeds.