Customized Beam Forming at the Allen Telescope Array

One of the exciting prospects for large N arrays is the potential for custom beam forming when operating in phased array mode. Pattern nulls may be generated by properly weighting the signals from all antennas with only minor degradation of gain in the main beam. Here we explore the limits of beam shape manipulation using the parameters of the Allen Telescope Array. To generate antenna weights, we apply an iterative method that is particularly easy to understand yet is comparable to linearly-constrained methods. In particular, this method elucidates how narrow band nulls may be extended to wider bandwidth. In practical RFI mitigation, the gain in the synthetic beam is obviously affected by the number and bandwidth of nulls placed elsewhere. Here we show how to predict the impact of a set of nulls in terms of the area of sky covered and null bandwidth. Most critical for design of the ATA, we find that high-speed (~10 ms) amplitude control of each array element over the full range 0-1 is critically important to allow testing of wide area / wide bandwidth nulling.