This study conducts a technical comparison of monopole antenna-based sensors for near-field breast tumor detection at two resonant frequencies: 2 GHz and 915 MHz. The 2 GHz antenna configuration initially provides a gain of 2.13 dBi, which is significantly enhanced to 6.9 dBi by integrating a Partially Reflective Surface (PRS) comprising a 4 × 4 array of unit cells. However, due to limited penetration at this higher frequency, tumor detection is largely confined to the superficial adipose layer. To improve diagnostic depth and sensitivity, an alternative antenna is designed to operate at 915 MHz, achieving a baseline gain of 1. 8 6 dB i. The integration of a 5 × 5 PRS array further elevates the gain to 5.3 dBi. The increased aperture and improved directivity at this frequency support enhanced near-field sensing performance. S-parameter evaluation using a multilayer heterogeneous breast phantom demonstrates that the 9 1 5 MHz design facilitates the detection of tumors in both adipose and fibroglandular tissue layers. These findings highlight the superiority of lower-frequency antennas, especially when combined with PRS structures, for achieving deeper tissue penetration and improved sensitivity in breast tumor detection scenarios.