BROAD-BAND NEARFIELD BEAMFORMING USING A RADIAL BEAMPATTERN TRANSFORMATION

This paper presents a new method of designing a beamformer having a de sired nearfield broadband beampattern. The methodology uses the spheri cal harmonic solution to the wave equation to transform the desired ne arfield beampattern to an equivalent farfield beampattern. A farfield beamformer is then designed for a transformed farfield beampattern tha t, if achieved, gives the desired nearfield pattern exactly. Salient f eatures of the new method are as follows. i) The nearfield patterns ca n be achieved for all angles, not just the primary look direction. ii) There is no theoretical restriction on the bandwidth. iii) General ar ray geometries may be used. As an illustration, we apply the method to the problem of producing a practical array design that achieves a nea rfield beampattern that is frequency invariant over an octave bandwidt h, where at the lowest frequency, the array-source separation is three wavelengths.