Another algorithm for Phased Array Null formation
L. Kogan (NRAO)

The phased array beam pattern null towards the RI source can mitigate effect of the RFI. Several algorithms for creating null at the phased array beam pattern have been designed. See for example ATA memos of Geoffrey Bower (1) and G.R. Harp (2). Another algorithm for creating null at the phased array beam pattern is described here. This algorithm is based on modification of algorithm of array configuration optimization minimizing side lobes of an array (3). We tested this algorithm for possible SKA antenna station as a phased array of 19 antennas. The results are very similar to the simulation described by G. Bower and G.R Harp. The null can be created at one direction and at the square around the given direction. The narrow null (~0.1 lambda/D) at the level >100db can be easily created at the cost of loosing signal to noise ratio at the main direction at the level of 3%. Widening the null to 0.5 lambda/D leads to null level ~40db and loosing 13% of signal to noise ratio. The phase and amplitude correction to create the null have to be given with the very high accuracy which may be not practical. Rounding phase correction to 1 degree and weight to 0.1 aggravates the null from >100db to ~50db.
(1) Bower, Geoffrey C., Simulations of Narrow-Band Phased-Array Null Formation for the ATA, ATA memo #37, 2001
(2) Harp, G.R. Customized Beam Forming at the Allen Telescope Array, ATA memo #51, 2002
(3) L. Kogan, Mathematic basis of an array configuration optimization minimizing side lobes, EVLA memo #71, 2004