A compact ultra-wideband antenna is presented for detecting malignant cells in the breast. The dimension of the proposed circular resonator-based antenna is 20 mm × 30 mm × 1.6 mm. The antenna sensor operates within the 3.1 GHz to 6.8 GHz (105.71%) range with peak gain 4.8 dB, radiation efficiency 89.2%, and an omnidirectional radiation pattern. Three types of breast phantoms (i.e., phantom without tumor, a phantom with a single tumor, and phantom with two tumors) are also fabricated. The electrical properties of the malignant cells differ from non-malignant breast cells. Sparameters have been measured with phantom, then with the help of Principal Component Analysis (PCA), and normal and malignant breast phantoms are identified. Further, the tumor's locations in the breast phantom are find out by using the specific absorption rate (SAR) values.