Bandwidth limits of beamforming networks for low-noise focal-plane arrays

Future radio telescopes will use arrays of closely-spaced antenna elements at the focal plane. The outputs from the elements will feed a beamforming network which will produce multiple beams. With a simple beamforming network the weights applied to the element outputs will be independent of frequency, resulting in a frequency-dependent feed pattern. We show, with results from numerical simulations, that the signal-to-noise ratio of a focal-plane array will be degraded with moderate bandwidths. We also briefly discuss ways to mitigate this effect.