In order to dramatically reduce the size of energy harvesting sensor nodes, it's necessary for a receiver to share the same antenna with an energy harvester. However, the standard 50Ω power matched interface is only designed for the connection of the antenna and the LNA, rather than taking into account the presence of the energy harvester. This paper presents an optimally mismatched antenna interface and an LNA based on this interface for the combination of the energy harvester and the LNA. The interface offers large passive voltage boosting to improve the voltage gain and NF of the LNA in a low power environment, and increases the sensitivity of the energy harvester as well. The interface and LNA have been designed and verified with simulations in AMS 0.18μm technology. A comparison with another LNA design based on a standard 50Ω power matched interface shows the proposed LNA has 12.3dB higher voltage gain and 1.8dB lower NF at a power consumption of 50μW.