A wireless body area network (WBAN) consists of several mobile devices worn on the human body. WBANs have enormous potential in health monitoring systems as it eliminates the inconvenience of having wires around the patient's body, offering more freedom of movement and comfort, enhanced monitoring, and the administration of at-home treatment. Low power consumption is crucial for such applications due to the limited capacity of portable batteries. The power consumption of wireless communication is especially important since the radio typically consumes the majority of the energy in such systems. There are two ways to solve this energy constraint. First, we reduce the RF communication power by optimizing the radio for the wireless communication channel. The RF communication channel is measured using custom hardware with UWB and narrowband signals, and it is observed that the channel has periods of time when it is good enough for a low sensitivity, low power receiver to be used. Second, we scavenge the energy from propagating radio waves with a subthreshold CMOS rectifier. Theoretical analysis of a subthreshold CMOS rectifier is presented, and used to maximize the sensitivity.