Broadband Null-Steering for Signal-Matched Antenna Arrays

A method for creating and steering a pattern null for a broadband signal-matched array antenna is discussed. This null-steering capability is of great importance in avoiding interference and jamming. Typical pattern responses to impulse noise are given and transient effects are examined. Introduction An important advantage of an array antenna sys­ tem over a reflector antenna is the former's ability to create a pattern null in a specified direction. This null-steering capability is of great value in a jamming environment and in locations where unwanted interfering signals may exist. Adaptive control net­ works can be designed to steer the nulls of a CW single-frequency array onto jamming and/or interfer­ ing sources on a real-time basis so that the proba­ bility of detecting the desired signal is enhanced.^ However, little information is available on the feasibility of null-steering for broadband operations for which narrow-band filters and phase-shifters will not suffice. This paper proposes a method for cre­ ating and steering a pattern null for a linear array with a matched filter which is designed to match radar pulses with a linear frequency modulation. The basic principle lies in the use of doublet elements. Each doublet consists of two elements separated by an appropriate distance. The output of one of the elements is delayed in time before it is combined with that of the other in order to create a null in the array pattern. A jamming interference impinging on the array at the designed null direction will yield no output under ideal conditions. The null direction can be relocated by changing the time delay between the elements of the doublets. Typical array patterns in response to an impulse noise are plotted for a signal-matched Chebyshev array consist­ ing of 40 doublet elements in the transient state. Linear Array with Matched Filter Consider a linear array of 2N isotropic elements spaced at distances + x^, +"X2 >..., + from the array center 0, as shown in Fig. 1. The array is in a receiving mode with amplitude weighting factors I , n = + 1, +2,..., + N. The incoming signal s(t) is assumed to have a rectangular frequency spectrum centered at w with a bandwidth 2yw and the transfer o o function, H(jw), of the matched filter is the complex conjugate of signal spectrum S(jw). The time delays, T , are provided to steer the mainbeam direction of n the array pattern. If the signal is a pulse with a linear frequency modulation, we have In Eq. (1), to + (J) . . H(jto) = S*(jco) = Rect ( 2 ■ °)e~~|Ct ' o Rect (y) = l , f < y < \