Robust transmit nulling in phased array antennas

The ability to create nulls in the transmit pattern of a phased array antenna has many applications for radar systems, including interference and clutter mitigation. Most nulling techniques introduce small perturbations in amplitude and phase, or phase-only, at each element of the phased array. For practical reasons, phase-only perturbations are desired and they create acceptable rms levels of null depth. However, the phase shift at each element will vary with the frequency of the transmitted signal. As a result, the depth and pointing accuracy of the transmit null will not be uniform over the bandwidth of the transmitted signal. This paper will describe some of the inherent limitations in achieving robust transmit nulling performance over a wide bandwidth when using phase-only, open-loop control on a typical phased array antenna. Some solutions to overcome these limitations and improve the performance of transmit nulls will be proposed.