This paper presents the design and modeling of an ultra-wide band active antenna operating over 0.5 – 4 GHz for medical microwave radiometry. A single arm Archimedean spiral antenna integrated with a preamplifier in a multilayer structure is designed and optimized for use with a multi-frequency medical microwave radiometer. Numerical simulations of the active antenna on a multi-layer printed circuit board is compared with passive receive antenna on a two layer board. Simulations indicate that the radiation characteristics of the receive antenna with and without the pre-amplifier remains unchanged. © 2019 URSI. All rights reserved.