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The space shuttle Columbia's catastrophic failure has been attributed to a piece of external tank spray on foam insulation striking the left wing of the orbiter, causing significant damage to some of the reinforced carbon/carbon leading edge wing panels. Subsequently, several nondestructive testing (NDT) techniques have been considered for testing the external tank. One such technique involves using millimeter waves, which have been shown to easily penetrate he foam and provide high resolution images of its interior structures. This paper presents the results of testing three different spray on foam insulation covered panels by reflectometers at millimeter wave frequencies, specifically at 100 GHz. Each panel was fitted with various embedded discontinuities/inserts representing voids and unbonds of different shapes, sizes and locations within each panel. In conjunction with these reflectometers, radiators, including a focused lens antenna and a small horn antenna, were used. The focused lens antenna provided for a footprint diameter of approximately 12.5 mm (0.5 in.) at 254 mm (10 in.) away from the lens surface. The horn antenna was primarily operated in its near field for obtaining relatively high resolution images. These images were produced using two dimensional scanning mechanisms. Discussion of the difference between the capabilities of these two types of antennas (radiators) for the purpose of testing the spray on foam insulation as it relates to the produced images is also presented. [ABSTRACT FROM AUTHOR]