Measured transmit nulling performance in wideband arrays

Modern radar systems often incorporate the ability to form nulls in the antenna transmit pattern. Transmit nulls can be used to suppress the returns from point clutter sources greatly and to reduce interference with neighboring emitters. To implement a transmit nulling capability, one practical technique is to provide independent phase control at each element of a phased array antenna. However, for legacy radar systems or for systems with reflector antennas, an alternative hardware approach is necessary. These architectures can be modified to provide a transmit nulling capability by placing an auxiliary antenna with independent phase and amplitude control in the vicinity of the main antenna. The auxiliary antenna forms a null in the far-field radiation pattern of both antennas by creating a signal equal in amplitude but 180 degrees out of phase with the main signal at the null location. The null depths achieved using the auxiliary antenna architecture are sufficient for narrowband applications but degrade when the transmit signal is wideband. This paper describes the measured improvements in wideband nulling performance made possible by an auxiliary antenna that has a tapped delay line (TDL) placed behind the antenna. The TDL affects the frequency response of the signal radiated by the auxiliary antenna and can be used to maintain a deep spatial null at a precise location over a wide signal bandwidth.