We are developing a confocal microwave imaging system for the detection of early-stage breast cancer. Our proposed microwave sensor represents a novel adaptation and application of the principles of ultrawideband radar technology and confocal optical microscopy. The sensor is comprised of an electronically switched monostatic antenna array that, synthetically focuses a low-power pulsed microwave signal at a focal point within the breast and collects the backscattered signal. Malignant tumors have a significant scattering radar cross section due to the large dielectric contrast between malignant tumors and adjacent normal breast tissue. Therefore, the intensity of the backscattered signal increases dramatically when the focused transmitted signal encounters a malignant tumor. Two key performance specifications for the microwave sensor are the signal-to-clutter (S/C) ratio, defined as the ratio of the peak backscatter return from a tumor to the peak backscatter return from clutter, and the system dynamic range, defined as the ratio of the peak pulse power of the source to the system noise floor due to reverberations and thermal noise. Our two-dimensional FDTD simulations involving the computation of S/C ratios have demonstrated the feasibility of detecting lesions as small as 1 mm in diameter. In this paper, we highlight the results of our three-dimensional simulations of an antenna-array element placed at the surface of a breast tissue half-space.