We assess the effect that different antenna types and polarization may have on breast penetration depth, and consequently on tumor detection. To this end, we simulate a homogeneous breast model which is scanned by different broadband antennas operating in the 2-5 GHz band and aligned for vertical or horizontal polarization. The penetration depth is studied through the electric field frequency response calculated at 25 × 25 points inside the breast filled only with adipose tissue. To analyze the effect on tumor detection, we also included a 5-mm radius tumor inside the breast. Results show that the energy that enters the breast is mainly concentrated along the antenna axis, and the width of the illuminated region and the amplitude are highly dependent on the shape of the breast and antenna type. The horn antenna led to higher penetration values, followed by slot-based antenna, bowtie and Vivaldi, respectively. Vertical-oriented antennas led to better illumination. This effect translates into tumor detection.