| The general discipline of phased array imaging presents a variety of problems which depend on the specific application. To date, phased array antennas have been used successfully for imaging in audio, ultrasonic and radar frequency ranges. Some of the problems that exist in these various disciplines are the presence of grating lobes and aperture side lobes, lack of depth resolution when imaging in the far field, narrow elemental angular beam-width, poor interelement isolation, and in some cases the complete loss of the image due to the presence of multiple echos in the signals received by the array elements.; Grating lobes can be eliminated by reducing the array element spacing to less than one half the wavelength of the travelling wave in the material being imaged. However, in designing multi-element phased arrays it is always desirable to keep the number of elements in the array to a minimum. This results in economic savings in the construction of the array and in computational savings in the processing of the collected data for the reconstruction of images. With this reduction in the number of elements comes an immediate trade-off in the array's performance between image resolution and the introduction of ambiguities into the image through the grating lobes. The negative effects of grating and side lobes and also the poor resolution in the near field can be reduced by optimally adjusting the interelement spacings, shading the element gains and varying the element phases of the array. The array element signals which contain echos (a problem arising in the nondestructive testing of solids) can be dereverberated through the use of homomorphic filtering before an image is made. This method works well when certain conditions are met.; When all these methods are applied simultaneously, significant improvements in the resolution of phased array images can be observed. This work examines the problems listed here and ascertains viable solutions which are demonstrated through simulations. |
