Breast cancer, a common and deadly cancer for woman, is gradually increasing each year. Magnetic resonance imaging, mammography, tomography, ultrasound, and biopsy are some of the medical technologies that can be used to identify breast cancer. However, none of them are as simple and effective as a microwave imaging (MI) technique. MI is a non-ionizing, non-invasive, tumor-sensitive, low-cost approach. The performance of MI is primarily determined by the antenna employed in the system. Here, we propose a new partial ground plane and slots-based miniature patch antenna designed for breast tumor detection within the FCC's authorized range (3.1 GHz – 10.6 GHz). The dimension of this antenna is 30 × 20 mm2. High-frequency structure simulator (HFSS) software is used to design a breast phantom with and without a tumor, and the antenna is separately simulated on both the tumored and tumor-free breast phantoms. The presence of a tumor within the breast is clearly depicted by the changes in return loss, VSWR, current density, electric field, and magnetic field strengths. The findings demonstrate that the proposed antenna is a suitable sensor which can detect a very tiny breast tumor (2 mm) due to its compact size and broad bandwidth.