A system and method for classifying body activities through an on-body antenna measuring changes in reflected power and the resulting reflection coefficients, in contrast to two antenna systems that measure transmission power therebetween. A single on-body antenna can be attached or held in close proximity to the body. The antenna can be calibrated based on changes in the reflection coefficients for various calibration activities for the location at which the antenna is placed on the body. Magnitude changes and patterns of the reflection coefficients at a given frequency for a given activity can be compared with calibrated data to correlate a given activity. Further accuracy can be gained by comparing both magnitude and phase changes and patterns at a given frequency with calibrated data for a given activity. The antenna can use power losses from a transmitted signal to measure the changes in the reflected power to minimize power requirements.
