The present technology is directed to a system and method for implementing passive power harvesting from ambient electromagnetic emissions with a smart rectenna that incorporates automatic frequency response tuning features. The disclosed system incorporates a tunable High Pass Filter and voltage multiplier rectifier with a front-end ultra wide band antenna unit. The frequency response of tunable components can be actively adjusted to match the frequency band containing most of the energy in the incident electromagnetic emission. A look up table is used for determining the appropriate biasing levels of the tunable components for each frequency in a frequency band of interest. By tuning a frequency response of impedance matching, filtering and rectifying components to correspond to a frequency region of maximum power spectral density in the incident energy signal, the system facilitates the scavenging of ambient electromagnetic energy from the spectral region with the highest power spectral density.