ULTRASPARSE, ULTRAWIDEBAND ARRAYS

This paper investigates the properties of highly thinned ultrawideband (UWB) arrays. The design aim is high resolution and very low side rad iation levels (SL). One-and two-dimensional ultrasparse UWB arrays can be designed to achieve both. The minimum available pulse-echo SL is s hown to approach N-4 where N is the number of elements in the transmit and receive arrays. Periodic thinning is shown to be superior to rand om thinning, and amplitude taper is shown to raise the SL. Two-dimensi onal curvilinear deployment of elements are shown to outperform rectil inear designs, and different transmit and receive arrays in pulse-echo systems are shown to outperform systems that use the same array for t ransmit and receive. Very low SL is achievable in an ultrasparse UWB s ystem with so few elements that echo signal-to-noise ratio (SNR) rathe r than SL becomes the constraint on the minimum number of elements req uired by the system for the array to be useful for imaging. For exampl e, in ultrasonic pulse-echo breast imaging, SL approximate to -70 dB i s desired to distinguish small cysts from tumors. A 2-D randomly thinn ed array requires about 10,000 elements. A 2-D ultrasparse UWB periodi c array requires less than 100 to satisfy SL, a reduction of 100:1, bu t provides insufficient SNR. A 500-element, 7.5 MHz array operating wi th 4 cm penetration depth satisfies both. Experimental results demonst rate the theory.