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We investigate the experimental feasibility of detecting small malignant breast tumors using a recently proposed method of microwave imaging via space-time beamforming. A microwave sensor comprised of a planar synthetic array of compact ultrawideband (1-11 GHz) antennas is placed above a breast tissue phantom - a tank of homogeneous normal breast tissue simulant covered by a thin layer of skin simulant. A small (< 0.5 cm) synthetic tumor is embedded in the breast phantom. At each position in the array, the antenna transmits a synthetically generated ultra-short pulse into the breast phantom. A robust data-adaptive algorithm removes the artifact caused by the dominant backscatter from the skin-breast interface. The signals are passed through a 3-D space-time beamformer designed to image backscattered energy as a function of location. Even millimeter-sized malignant tumors have relatively large microwave scattering cross-sections due to their significant dielectric-properties contrast with normal breast tissue. Therefore they produce localized regions of large backscatter energy levels in the beamformer image. The successful detection and localization of very small synthetic tumors embedded in the skin-breast tissue phantom is demonstrated.