Cardiac output is an important hemodynamic parameter in critical care and has proven difficult to measure accurately, non-invasively, and without a highly skilled operator. Microwave technologies have been shown to be a possible alternative to current methods for cardiac output determination. However, previously reported microwave systems had limitations including low signal change over the cardiac cycle. In this thesis, we specifically address improving signal change over the cardiac cycle by introducing an ultra-wideband transmission system. We characterize the system using heart models and show that wave propagation through the chest is complex, making the system sensitive to antenna placement. Additionally, a semi-dynamic heart model for characterization of the system in hardware and in simulation is developed. This model shows comparable performance to a complex MRI based heart model. On a human test subject, ultra-wideband readings showed up to 6 dB of magnitude variation over the cardiac cycle.